本文由 Luzhuo 编写,转发请保留该信息. 原文: http://blog.csdn.net/Rozol/article/details/78161840
ListView 作为界面展示的容器控件必然会直接或者间接的继承ViewGroup, 现在看看源代码的继承结构
public class ListView extends AbsListView { } public abstract class AbsListView extends AdapterView<ListAdapter> implements TextWatcher, ViewTreeObserver.OnGlobalLayoutListener, Filter.FilterListener, ViewTreeObserver.OnTouchModeChangeListener, RemoteViewsAdapter.RemoteAdapterConnectionCallback { } public abstract class AdapterView<T extends Adapter> extends ViewGroup { }现在知道ListView确实是继承ViewGroup的,那么就会重写 onMeasure() onLayout() onDraw() 这三个基本的方法, 大家是否注意到继承ViewGroup的后,onMeasure() onLayout()会多次执行的问题(执行了4次onMeasure(), 2次onLayout()),以下是log.
从上面的使用代码可以看出,要让ListView正常工作,就要设置Adapter,Adapter就是适配器
public class MyAdapter extends BaseAdapter { @Override public int getCount() { return 0; } @Override public Object getItem(int position) { return null; } @Override public long getItemId(int position) { return 0; } @Override public View getView(int position, View convertView, ViewGroup parent) { return null; } }继承Adapter会被要求必须重写上述4个方法. 数据是不尽相同的, ListView只关心交互和展示的工作,不关心你数据是什么样的,从哪来的. 而Adapter的统一接口就解决的数据适配的问题.
示范图:为了简洁说明ListView是怎么工作的,先讲下 AbsListView 类里的内部类 RecycleBin
public abstract class AbsListView extends AdapterView<ListAdapter> implements TextWatcher, ViewTreeObserver.OnGlobalLayoutListener, Filter.FilterListener, ViewTreeObserver.OnTouchModeChangeListener, RemoteViewsAdapter.RemoteAdapterConnectionCallback { final RecycleBin mRecycler = new RecycleBin(); class RecycleBin { private RecyclerListener mRecyclerListener; /** * The position of the first view stored in mActiveViews. */ private int mFirstActivePosition; /** * Views that were on screen at the start of layout. This array is populated at the start of * layout, and at the end of layout all view in mActiveViews are moved to mScrapViews. * Views in mActiveViews represent a contiguous range of Views, with position of the first * view store in mFirstActivePosition. */ // ↓↓↓ private View[] mActiveViews = new View[0]; /** * Unsorted views that can be used by the adapter as a convert view. */ // ↓↓↓ private ArrayList<View>[] mScrapViews; private int mViewTypeCount; // ↓↓↓ private ArrayList<View> mCurrentScrap; private ArrayList<View> mSkippedScrap; private SparseArray<View> mTransientStateViews; private LongSparseArray<View> mTransientStateViewsById; // ↓↓↓ public void setViewTypeCount(int viewTypeCount) { if (viewTypeCount < 1) { throw new IllegalArgumentException("Can't have a viewTypeCount < 1"); } //noinspection unchecked ArrayList<View>[] scrapViews = new ArrayList[viewTypeCount]; for (int i = 0; i < viewTypeCount; i++) { scrapViews[i] = new ArrayList<View>(); } mViewTypeCount = viewTypeCount; mCurrentScrap = scrapViews[0]; mScrapViews = scrapViews; } /** * Fill ActiveViews with all of the children of the AbsListView. * * @param childCount The minimum number of views mActiveViews should hold * @param firstActivePosition The position of the first view that will be stored in * mActiveViews */ // ↓↓↓ void fillActiveViews(int childCount, int firstActivePosition) { if (mActiveViews.length < childCount) { mActiveViews = new View[childCount]; } mFirstActivePosition = firstActivePosition; //noinspection MismatchedReadAndWriteOfArray final View[] activeViews = mActiveViews; for (int i = 0; i < childCount; i++) { View child = getChildAt(i); AbsListView.LayoutParams lp = (AbsListView.LayoutParams) child.getLayoutParams(); // Don't put header or footer views into the scrap heap if (lp != null && lp.viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { // Note: We do place AdapterView.ITEM_VIEW_TYPE_IGNORE in active views. // However, we will NOT place them into scrap views. activeViews[i] = child; // Remember the position so that setupChild() doesn't reset state. lp.scrappedFromPosition = firstActivePosition + i; } } } /** * Get the view corresponding to the specified position. The view will be removed from * mActiveViews if it is found. * * @param position The position to look up in mActiveViews * @return The view if it is found, null otherwise */ // ↓↓↓ View getActiveView(int position) { int index = position - mFirstActivePosition; final View[] activeViews = mActiveViews; if (index >=0 && index < activeViews.length) { final View match = activeViews[index]; activeViews[index] = null; return match; } return null; } /** * @return A view from the ScrapViews collection. These are unordered. */ // ↓↓↓ View getScrapView(int position) { final int whichScrap = mAdapter.getItemViewType(position); if (whichScrap < 0) { return null; } if (mViewTypeCount == 1) { return retrieveFromScrap(mCurrentScrap, position); } else if (whichScrap < mScrapViews.length) { return retrieveFromScrap(mScrapViews[whichScrap], position); } return null; } /** * Puts a view into the list of scrap views. * <p> * If the list data hasn't changed or the adapter has stable IDs, views * with transient state will be preserved for later retrieval. * * @param scrap The view to add * @param position The view's position within its parent */ // ↓↓↓ void addScrapView(View scrap, int position) { final AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams(); if (lp == null) { // Can't recycle, but we don't know anything about the view. // Ignore it completely. return; } lp.scrappedFromPosition = position; // Remove but don't scrap header or footer views, or views that // should otherwise not be recycled. final int viewType = lp.viewType; if (!shouldRecycleViewType(viewType)) { // Can't recycle. If it's not a header or footer, which have // special handling and should be ignored, then skip the scrap // heap and we'll fully detach the view later. if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { getSkippedScrap().add(scrap); } return; } scrap.dispatchStartTemporaryDetach(); // The the accessibility state of the view may change while temporary // detached and we do not allow detached views to fire accessibility // events. So we are announcing that the subtree changed giving a chance // to clients holding on to a view in this subtree to refresh it. notifyViewAccessibilityStateChangedIfNeeded( AccessibilityEvent.CONTENT_CHANGE_TYPE_SUBTREE); // Don't scrap views that have transient state. final boolean scrapHasTransientState = scrap.hasTransientState(); if (scrapHasTransientState) { if (mAdapter != null && mAdapterHasStableIds) { // If the adapter has stable IDs, we can reuse the view for // the same data. if (mTransientStateViewsById == null) { mTransientStateViewsById = new LongSparseArray<>(); } mTransientStateViewsById.put(lp.itemId, scrap); } else if (!mDataChanged) { // If the data hasn't changed, we can reuse the views at // their old positions. if (mTransientStateViews == null) { mTransientStateViews = new SparseArray<>(); } mTransientStateViews.put(position, scrap); } else { // Otherwise, we'll have to remove the view and start over. getSkippedScrap().add(scrap); } } else { if (mViewTypeCount == 1) { mCurrentScrap.add(scrap); } else { mScrapViews[viewType].add(scrap); } if (mRecyclerListener != null) { mRecyclerListener.onMovedToScrapHeap(scrap); } } } } } 以上代码是从源码中拷贝出来,并删掉了一些不重要的方法成员变量:
View[] mActiveViews: 用于存放活动View(也就是在屏幕上展示的View) int mFirstActivePosition: 存放mActiveViews中第一个View的Position(也就是第几个Item)ArrayList<View>[] mScrapViews: 废弃的View,可通过Adapter转为convertView继续使用(看看基本使用Adapter代码,我们就是将这些废弃的view重复使用的)ArrayList<View> mCurrentScrap: ViewTypeCount == 1 的废弃View会被存在这个集合里方法:public void setViewTypeCount(int viewTypeCount) {}: 该方法会根据传入的类型数量初始化 mScrapViews 和 mCurrentScrap; mScrapViews 存了不同类型View的集合, mCurrentScrap是mScrapViews的第一个集合; 看来ListView是可以传入多个类型的View的void fillActiveViews(int childCount, int firstActivePosition) {}: 主要是将mActiveViews填满子View (保存屏幕上展示的View)View getActiveView(int position) {}: 根据position获取mActiveViews里的ViewView getScrapView(int position) {}: 源码主要调用了retrieveFromScrap(mCurrentScrap, position)方法,现在看看这个方法是干吗的?
private View retrieveFromScrap(ArrayList<View> scrapViews, int position) { final int size = scrapViews.size(); if (size > 0) { // See if we still have a view for this position or ID. for (int i = 0; i < size; i++) { final View view = scrapViews.get(i); final AbsListView.LayoutParams params = (AbsListView.LayoutParams) view.getLayoutParams(); if (mAdapterHasStableIds) { final long id = mAdapter.getItemId(position); if (id == params.itemId) { return scrapViews.remove(i); } } else if (params.scrappedFromPosition == position) { final View scrap = scrapViews.remove(i); clearAccessibilityFromScrap(scrap); return scrap; } } final View scrap = scrapViews.remove(size - 1); clearAccessibilityFromScrap(scrap); return scrap; } else { return null; } } 看来getScrapView(int position)是获取废弃的View, 如果能获取到就返回View,获取不到就返回nullvoid addScrapView(View scrap, int position) {}: 把废弃的View添加到mCurrentScrap里, 把具有过渡效果的废弃View添加到mTransientStateViews里(带有过渡效果的View这里不做讲解)可见recycleBin主要工作就是填满和获取展示View,添加和获取缓存View.onMeasure() 主要是测量ListView的大小; onLayout()用于确定子View的布局, 这才是核心, 该方法并没有在ListView中实现, 而是在抽象父类AbsListView中实现.
protected void onLayout(boolean changed, int l, int t, int r, int b) { super.onLayout(changed, l, t, r, b); mInLayout = true; final int childCount = getChildCount(); // ↓↓↓ 1. 如果change == true, ListView的大小和位置发生变化 if (changed) { for (int i = 0; i < childCount; i++) { // ↓↓↓ 2. 那么就把所有子布局强制重绘 getChildAt(i).forceLayout(); } mRecycler.markChildrenDirty(); } // ↓↓↓ 3. 调用子类ListView的layoutChildren()方法 layoutChildren(); mInLayout = false; mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR; // TODO: Move somewhere sane. This doesn't belong in onLayout(). if (mFastScroll != null) { mFastScroll.onItemCountChanged(getChildCount(), mItemCount); } }接着看看layoutChildren()做了什么
protected void layoutChildren() { final boolean blockLayoutRequests = mBlockLayoutRequests; if (blockLayoutRequests) { return; } mBlockLayoutRequests = true; try { super.layoutChildren(); invalidate(); if (mAdapter == null) { resetList(); invokeOnItemScrollListener(); return; } final int childrenTop = mListPadding.top; final int childrenBottom = mBottom - mTop - mListPadding.bottom; // ↓↓↓ 1. ListView中还未填充任务子View, 得到结果为0 final int childCount = getChildCount(); int index = 0; int delta = 0; View sel; View oldSel = null; View oldFirst = null; View newSel = null; // Remember stuff we will need down below switch (mLayoutMode) { case LAYOUT_SET_SELECTION: index = mNextSelectedPosition - mFirstPosition; if (index >= 0 && index < childCount) { newSel = getChildAt(index); } break; case LAYOUT_FORCE_TOP: case LAYOUT_FORCE_BOTTOM: case LAYOUT_SPECIFIC: case LAYOUT_SYNC: break; case LAYOUT_MOVE_SELECTION: default: // Remember the previously selected view index = mSelectedPosition - mFirstPosition; if (index >= 0 && index < childCount) { oldSel = getChildAt(index); } // Remember the previous first child oldFirst = getChildAt(0); if (mNextSelectedPosition >= 0) { delta = mNextSelectedPosition - mSelectedPosition; } // Caution: newSel might be null newSel = getChildAt(index + delta); } boolean dataChanged = mDataChanged; if (dataChanged) { handleDataChanged(); } // Handle the empty set by removing all views that are visible // and calling it a day if (mItemCount == 0) { resetList(); invokeOnItemScrollListener(); return; } else if (mItemCount != mAdapter.getCount()) { throw new IllegalStateException("The content of the adapter has changed but " + "ListView did not receive a notification. Make sure the content of " + "your adapter is not modified from a background thread, but only from " + "the UI thread. Make sure your adapter calls notifyDataSetChanged() " + "when its content changes. [in ListView(" + getId() + ", " + getClass() + ") with Adapter(" + mAdapter.getClass() + ")]"); } setSelectedPositionInt(mNextSelectedPosition); AccessibilityNodeInfo accessibilityFocusLayoutRestoreNode = null; View accessibilityFocusLayoutRestoreView = null; int accessibilityFocusPosition = INVALID_POSITION; // Remember which child, if any, had accessibility focus. This must // occur before recycling any views, since that will clear // accessibility focus. final ViewRootImpl viewRootImpl = getViewRootImpl(); if (viewRootImpl != null) { final View focusHost = viewRootImpl.getAccessibilityFocusedHost(); if (focusHost != null) { final View focusChild = getAccessibilityFocusedChild(focusHost); if (focusChild != null) { if (!dataChanged || isDirectChildHeaderOrFooter(focusChild) || focusChild.hasTransientState() || mAdapterHasStableIds) { // The views won't be changing, so try to maintain // focus on the current host and virtual view. accessibilityFocusLayoutRestoreView = focusHost; accessibilityFocusLayoutRestoreNode = viewRootImpl .getAccessibilityFocusedVirtualView(); } // If all else fails, maintain focus at the same // position. accessibilityFocusPosition = getPositionForView(focusChild); } } } View focusLayoutRestoreDirectChild = null; View focusLayoutRestoreView = null; // Take focus back to us temporarily to avoid the eventual call to // clear focus when removing the focused child below from messing // things up when ViewAncestor assigns focus back to someone else. final View focusedChild = getFocusedChild(); if (focusedChild != null) { // TODO: in some cases focusedChild.getParent() == null // We can remember the focused view to restore after re-layout // if the data hasn't changed, or if the focused position is a // header or footer. if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild) || focusedChild.hasTransientState() || mAdapterHasStableIds) { focusLayoutRestoreDirectChild = focusedChild; // Remember the specific view that had focus. focusLayoutRestoreView = findFocus(); if (focusLayoutRestoreView != null) { // Tell it we are going to mess with it. focusLayoutRestoreView.dispatchStartTemporaryDetach(); } } requestFocus(); } // Pull all children into the RecycleBin. // These views will be reused if possible final int firstPosition = mFirstPosition; final RecycleBin recycleBin = mRecycler; // ↓↓↓ 2. 一旦数据源有变化, dataChanged == true if (dataChanged) { for (int i = 0; i < childCount; i++) { recycleBin.addScrapView(getChildAt(i), firstPosition+i); } } else { // ↓↓↓ 2.2 如果数据源, 执行该方法, 调用RecycleBin的fillActiveViews()缓存展示的所有view, 但是现在并没有展示的View recycleBin.fillActiveViews(childCount, firstPosition); } // Clear out old views // ↓↓↓ 2.3 从父容器中清除所有view, 现在没有可清理的view detachAllViewsFromParent(); recycleBin.removeSkippedScrap(); // ↓↓↓ 3. 判断布局模式, 默认布局模式为LAYOUT_NORMAL switch (mLayoutMode) { case LAYOUT_SET_SELECTION: if (newSel != null) { sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom); } else { sel = fillFromMiddle(childrenTop, childrenBottom); } break; case LAYOUT_SYNC: sel = fillSpecific(mSyncPosition, mSpecificTop); break; case LAYOUT_FORCE_BOTTOM: sel = fillUp(mItemCount - 1, childrenBottom); adjustViewsUpOrDown(); break; case LAYOUT_FORCE_TOP: mFirstPosition = 0; sel = fillFromTop(childrenTop); adjustViewsUpOrDown(); break; case LAYOUT_SPECIFIC: final int selectedPosition = reconcileSelectedPosition(); sel = fillSpecific(selectedPosition, mSpecificTop); /** * When ListView is resized, FocusSelector requests an async selection for the * previously focused item to make sure it is still visible. If the item is not * selectable, it won't regain focus so instead we call FocusSelector * to directly request focus on the view after it is visible. */ if (sel == null && mFocusSelector != null) { final Runnable focusRunnable = mFocusSelector .setupFocusIfValid(selectedPosition); if (focusRunnable != null) { post(focusRunnable); } } break; case LAYOUT_MOVE_SELECTION: sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom); break; default: // ↓↓↓ 3.1 于是会执行该部分代码 if (childCount == 0) { if (!mStackFromBottom) { final int position = lookForSelectablePosition(0, true); setSelectedPositionInt(position); // ↓↓↓ 3.2 执行方法 sel = fillFromTop(childrenTop); } else { final int position = lookForSelectablePosition(mItemCount - 1, false); setSelectedPositionInt(position); sel = fillUp(mItemCount - 1, childrenBottom); } } else { if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) { sel = fillSpecific(mSelectedPosition, oldSel == null ? childrenTop : oldSel.getTop()); } else if (mFirstPosition < mItemCount) { sel = fillSpecific(mFirstPosition, oldFirst == null ? childrenTop : oldFirst.getTop()); } else { sel = fillSpecific(0, childrenTop); } } break; } // Flush any cached views that did not get reused above recycleBin.scrapActiveViews(); // remove any header/footer that has been temp detached and not re-attached removeUnusedFixedViews(mHeaderViewInfos); removeUnusedFixedViews(mFooterViewInfos); if (sel != null) { // The current selected item should get focus if items are // focusable. if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) { final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild && focusLayoutRestoreView != null && focusLayoutRestoreView.requestFocus()) || sel.requestFocus(); if (!focusWasTaken) { // Selected item didn't take focus, but we still want to // make sure something else outside of the selected view // has focus. final View focused = getFocusedChild(); if (focused != null) { focused.clearFocus(); } positionSelector(INVALID_POSITION, sel); } else { sel.setSelected(false); mSelectorRect.setEmpty(); } } else { positionSelector(INVALID_POSITION, sel); } mSelectedTop = sel.getTop(); } else { final boolean inTouchMode = mTouchMode == TOUCH_MODE_TAP || mTouchMode == TOUCH_MODE_DONE_WAITING; if (inTouchMode) { // If the user's finger is down, select the motion position. final View child = getChildAt(mMotionPosition - mFirstPosition); if (child != null) { positionSelector(mMotionPosition, child); } } else if (mSelectorPosition != INVALID_POSITION) { // If we had previously positioned the selector somewhere, // put it back there. It might not match up with the data, // but it's transitioning out so it's not a big deal. final View child = getChildAt(mSelectorPosition - mFirstPosition); if (child != null) { positionSelector(mSelectorPosition, child); } } else { // Otherwise, clear selection. mSelectedTop = 0; mSelectorRect.setEmpty(); } // Even if there is not selected position, we may need to // restore focus (i.e. something focusable in touch mode). if (hasFocus() && focusLayoutRestoreView != null) { focusLayoutRestoreView.requestFocus(); } } // Attempt to restore accessibility focus, if necessary. if (viewRootImpl != null) { final View newAccessibilityFocusedView = viewRootImpl.getAccessibilityFocusedHost(); if (newAccessibilityFocusedView == null) { if (accessibilityFocusLayoutRestoreView != null && accessibilityFocusLayoutRestoreView.isAttachedToWindow()) { final AccessibilityNodeProvider provider = accessibilityFocusLayoutRestoreView.getAccessibilityNodeProvider(); if (accessibilityFocusLayoutRestoreNode != null && provider != null) { final int virtualViewId = AccessibilityNodeInfo.getVirtualDescendantId( accessibilityFocusLayoutRestoreNode.getSourceNodeId()); provider.performAction(virtualViewId, AccessibilityNodeInfo.ACTION_ACCESSIBILITY_FOCUS, null); } else { accessibilityFocusLayoutRestoreView.requestAccessibilityFocus(); } } else if (accessibilityFocusPosition != INVALID_POSITION) { // Bound the position within the visible children. final int position = MathUtils.constrain( accessibilityFocusPosition - mFirstPosition, 0, getChildCount() - 1); final View restoreView = getChildAt(position); if (restoreView != null) { restoreView.requestAccessibilityFocus(); } } } } // Tell focus view we are done mucking with it, if it is still in // our view hierarchy. if (focusLayoutRestoreView != null && focusLayoutRestoreView.getWindowToken() != null) { focusLayoutRestoreView.dispatchFinishTemporaryDetach(); } mLayoutMode = LAYOUT_NORMAL; mDataChanged = false; if (mPositionScrollAfterLayout != null) { post(mPositionScrollAfterLayout); mPositionScrollAfterLayout = null; } mNeedSync = false; setNextSelectedPositionInt(mSelectedPosition); updateScrollIndicators(); if (mItemCount > 0) { checkSelectionChanged(); } invokeOnItemScrollListener(); } finally { if (mFocusSelector != null) { mFocusSelector.onLayoutComplete(); } if (!blockLayoutRequests) { mBlockLayoutRequests = false; } } } private View fillFromTop(int nextTop) { mFirstPosition = Math.min(mFirstPosition, mSelectedPosition); mFirstPosition = Math.min(mFirstPosition, mItemCount - 1); if (mFirstPosition < 0) { mFirstPosition = 0; } return fillDown(mFirstPosition, nextTop); } private View fillDown(int pos, int nextTop) { View selectedView = null; int end = (mBottom - mTop); if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { end -= mListPadding.bottom; } // ↓↓↓ 3.2 循环计算子布局位置, 然后调用makeAndAddView() while (nextTop < end && pos < mItemCount) { // is this the selected item? boolean selected = pos == mSelectedPosition; View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected); nextTop = child.getBottom() + mDividerHeight; if (selected) { selectedView = child; } pos++; } setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1); return selectedView; } private View makeAndAddView(int position, int y, boolean flow, int childrenLeft, boolean selected) { View child; if (!mDataChanged) { // Try to use an existing view for this position // ↓↓↓ 3.3 从RecycleBin里获取一个缓存的View, 但是缓存为空, 返回null child = mRecycler.getActiveView(position); if (child != null) { // Found it -- we're using an existing child // This just needs to be positioned setupChild(child, position, y, flow, childrenLeft, selected, true); return child; } } // Make a new view for this position, or convert an unused view if possible // ↓↓↓ 3.4 然而从RecycleBin里获取缓存的View失败, 则试图通过obtainView()方法获取一个View child = obtainView(position, mIsScrap); // This needs to be positioned and measured // ↓↓↓ 3.7 将适配器获得的View, 交给该方法 setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]); return child; } View obtainView(int position, boolean[] isScrap) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView"); isScrap[0] = false; // ... // ↓↓↓ 3.5 从RecycleBin里获取一个缓存的废弃的View, 现在RecycleBin里没有缓存任何View, 所以返回null final View scrapView = mRecycler.getScrapView(position); // ↓↓↓ 3.6 并且从适配器里获取一个View, 这个只要我们给了数据, 一定能获取到 final View child = mAdapter.getView(position, scrapView, this); if (scrapView != null) { if (child != scrapView) { // Failed to re-bind the data, return scrap to the heap. mRecycler.addScrapView(scrapView, position); } else { if (child.isTemporarilyDetached()) { isScrap[0] = true; // Finish the temporary detach started in addScrapView(). child.dispatchFinishTemporaryDetach(); } else { // we set isScrap to "true" only if the view is temporarily detached. // if the view is fully detached, it is as good as a view created by the // adapter isScrap[0] = false; } } } if (mCacheColorHint != 0) { child.setDrawingCacheBackgroundColor(mCacheColorHint); } if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) { child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES); } setItemViewLayoutParams(child, position); if (AccessibilityManager.getInstance(mContext).isEnabled()) { if (mAccessibilityDelegate == null) { mAccessibilityDelegate = new ListItemAccessibilityDelegate(); } if (child.getAccessibilityDelegate() == null) { child.setAccessibilityDelegate(mAccessibilityDelegate); } } Trace.traceEnd(Trace.TRACE_TAG_VIEW); return child; } private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft, boolean selected, boolean recycled) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem"); final boolean isSelected = selected && shouldShowSelector(); final boolean updateChildSelected = isSelected != child.isSelected(); final int mode = mTouchMode; final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL && mMotionPosition == position; final boolean updateChildPressed = isPressed != child.isPressed(); final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested(); // Respect layout params that are already in the view. Otherwise make some up... // noinspection unchecked AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams(); if (p == null) { p = (AbsListView.LayoutParams) generateDefaultLayoutParams(); } p.viewType = mAdapter.getItemViewType(position); p.isEnabled = mAdapter.isEnabled(position); if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) { attachViewToParent(child, flowDown ? -1 : 0, p); } else { p.forceAdd = false; if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { p.recycledHeaderFooter = true; } // ↓↓↓ 3.8 把获得的View添加到ListView中, 这样就完成了一次onLayout的加载 addViewInLayout(child, flowDown ? -1 : 0, p, true); } if (updateChildSelected) { child.setSelected(isSelected); } if (updateChildPressed) { child.setPressed(isPressed); } if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) { if (child instanceof Checkable) { ((Checkable) child).setChecked(mCheckStates.get(position)); } else if (getContext().getApplicationInfo().targetSdkVersion >= android.os.Build.VERSION_CODES.HONEYCOMB) { child.setActivated(mCheckStates.get(position)); } } if (needToMeasure) { final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec, mListPadding.left + mListPadding.right, p.width); final int lpHeight = p.height; final int childHeightSpec; if (lpHeight > 0) { childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY); } else { childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(), MeasureSpec.UNSPECIFIED); } child.measure(childWidthSpec, childHeightSpec); } else { cleanupLayoutState(child); } final int w = child.getMeasuredWidth(); final int h = child.getMeasuredHeight(); final int childTop = flowDown ? y : y - h; if (needToMeasure) { final int childRight = childrenLeft + w; final int childBottom = childTop + h; child.layout(childrenLeft, childTop, childRight, childBottom); } else { child.offsetLeftAndRight(childrenLeft - child.getLeft()); child.offsetTopAndBottom(childTop - child.getTop()); } if (mCachingStarted && !child.isDrawingCacheEnabled()) { child.setDrawingCacheEnabled(true); } if (recycled && (((AbsListView.LayoutParams)child.getLayoutParams()).scrappedFromPosition) != position) { child.jumpDrawablesToCurrentState(); } Trace.traceEnd(Trace.TRACE_TAG_VIEW); }第一次onLayout, 由于第一次, 所以屏幕上和RecycleBin的缓存里并没有View, 只有通过适配器获取View, 直至将屏幕填满.
接下来我们来看看第二次onLayout, 与第一次的差别是, 这一次屏幕上已经有View了
protected void onLayout(boolean changed, int l, int t, int r, int b) { super.onLayout(changed, l, t, r, b); mInLayout = true; final int childCount = getChildCount(); if (changed) { for (int i = 0; i < childCount; i++) { getChildAt(i).forceLayout(); } mRecycler.markChildrenDirty(); } // ↓↓↓ 1.第二次的onLayout执行还是从这里开始 layoutChildren(); mInLayout = false; mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR; // TODO: Move somewhere sane. This doesn't belong in onLayout(). if (mFastScroll != null) { mFastScroll.onItemCountChanged(getChildCount(), mItemCount); } } protected void layoutChildren() { final boolean blockLayoutRequests = mBlockLayoutRequests; if (blockLayoutRequests) { return; } mBlockLayoutRequests = true; try { super.layoutChildren(); invalidate(); if (mAdapter == null) { resetList(); invokeOnItemScrollListener(); return; } final int childrenTop = mListPadding.top; final int childrenBottom = mBottom - mTop - mListPadding.bottom; // 2. 获取子View的数量, 由于屏幕已经填充了View, 所以这里返回的数字 > 0 final int childCount = getChildCount(); int index = 0; int delta = 0; View sel; View oldSel = null; View oldFirst = null; View newSel = null; // Remember stuff we will need down below switch (mLayoutMode) { case LAYOUT_SET_SELECTION: index = mNextSelectedPosition - mFirstPosition; if (index >= 0 && index < childCount) { newSel = getChildAt(index); } break; case LAYOUT_FORCE_TOP: case LAYOUT_FORCE_BOTTOM: case LAYOUT_SPECIFIC: case LAYOUT_SYNC: break; case LAYOUT_MOVE_SELECTION: default: // Remember the previously selected view index = mSelectedPosition - mFirstPosition; if (index >= 0 && index < childCount) { oldSel = getChildAt(index); } // Remember the previous first child oldFirst = getChildAt(0); if (mNextSelectedPosition >= 0) { delta = mNextSelectedPosition - mSelectedPosition; } // Caution: newSel might be null newSel = getChildAt(index + delta); } boolean dataChanged = mDataChanged; if (dataChanged) { handleDataChanged(); } // Handle the empty set by removing all views that are visible // and calling it a day if (mItemCount == 0) { resetList(); invokeOnItemScrollListener(); return; } else if (mItemCount != mAdapter.getCount()) { throw new IllegalStateException("The content of the adapter has changed but " + "ListView did not receive a notification. Make sure the content of " + "your adapter is not modified from a background thread, but only from " + "the UI thread. Make sure your adapter calls notifyDataSetChanged() " + "when its content changes. [in ListView(" + getId() + ", " + getClass() + ") with Adapter(" + mAdapter.getClass() + ")]"); } setSelectedPositionInt(mNextSelectedPosition); AccessibilityNodeInfo accessibilityFocusLayoutRestoreNode = null; View accessibilityFocusLayoutRestoreView = null; int accessibilityFocusPosition = INVALID_POSITION; // Remember which child, if any, had accessibility focus. This must // occur before recycling any views, since that will clear // accessibility focus. final ViewRootImpl viewRootImpl = getViewRootImpl(); if (viewRootImpl != null) { final View focusHost = viewRootImpl.getAccessibilityFocusedHost(); if (focusHost != null) { final View focusChild = getAccessibilityFocusedChild(focusHost); if (focusChild != null) { if (!dataChanged || isDirectChildHeaderOrFooter(focusChild) || focusChild.hasTransientState() || mAdapterHasStableIds) { // The views won't be changing, so try to maintain // focus on the current host and virtual view. accessibilityFocusLayoutRestoreView = focusHost; accessibilityFocusLayoutRestoreNode = viewRootImpl .getAccessibilityFocusedVirtualView(); } // If all else fails, maintain focus at the same // position. accessibilityFocusPosition = getPositionForView(focusChild); } } } View focusLayoutRestoreDirectChild = null; View focusLayoutRestoreView = null; // Take focus back to us temporarily to avoid the eventual call to // clear focus when removing the focused child below from messing // things up when ViewAncestor assigns focus back to someone else. final View focusedChild = getFocusedChild(); if (focusedChild != null) { // TODO: in some cases focusedChild.getParent() == null // We can remember the focused view to restore after re-layout // if the data hasn't changed, or if the focused position is a // header or footer. if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild) || focusedChild.hasTransientState() || mAdapterHasStableIds) { focusLayoutRestoreDirectChild = focusedChild; // Remember the specific view that had focus. focusLayoutRestoreView = findFocus(); if (focusLayoutRestoreView != null) { // Tell it we are going to mess with it. focusLayoutRestoreView.dispatchStartTemporaryDetach(); } } requestFocus(); } // Pull all children into the RecycleBin. // These views will be reused if possible final int firstPosition = mFirstPosition; final RecycleBin recycleBin = mRecycler; if (dataChanged) { for (int i = 0; i < childCount; i++) { recycleBin.addScrapView(getChildAt(i), firstPosition+i); } } else { // ↓↓↓ 3. 将屏幕上的View全部添加到RecycleBin里 recycleBin.fillActiveViews(childCount, firstPosition); } // Clear out old views // ↓↓↓ 4. 从父容器中清除所有的view detachAllViewsFromParent(); recycleBin.removeSkippedScrap(); switch (mLayoutMode) { case LAYOUT_SET_SELECTION: if (newSel != null) { sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom); } else { sel = fillFromMiddle(childrenTop, childrenBottom); } break; case LAYOUT_SYNC: sel = fillSpecific(mSyncPosition, mSpecificTop); break; case LAYOUT_FORCE_BOTTOM: sel = fillUp(mItemCount - 1, childrenBottom); adjustViewsUpOrDown(); break; case LAYOUT_FORCE_TOP: mFirstPosition = 0; sel = fillFromTop(childrenTop); adjustViewsUpOrDown(); break; case LAYOUT_SPECIFIC: final int selectedPosition = reconcileSelectedPosition(); sel = fillSpecific(selectedPosition, mSpecificTop); /** * When ListView is resized, FocusSelector requests an async selection for the * previously focused item to make sure it is still visible. If the item is not * selectable, it won't regain focus so instead we call FocusSelector * to directly request focus on the view after it is visible. */ if (sel == null && mFocusSelector != null) { final Runnable focusRunnable = mFocusSelector .setupFocusIfValid(selectedPosition); if (focusRunnable != null) { post(focusRunnable); } } break; case LAYOUT_MOVE_SELECTION: sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom); break; default: // ↓↓↓ 5. 这里的childCount > 0 if (childCount == 0) { if (!mStackFromBottom) { final int position = lookForSelectablePosition(0, true); setSelectedPositionInt(position); sel = fillFromTop(childrenTop); } else { final int position = lookForSelectablePosition(mItemCount - 1, false); setSelectedPositionInt(position); sel = fillUp(mItemCount - 1, childrenBottom); } } else { // ↓↓↓ 6. 所以执行以下判断代码 // ↓↓↓ 6.1 由于是新加载的, 所以不会有被选择的条目, 所以 mSelectedPosition = -1, 改判断不成立 if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) { sel = fillSpecific(mSelectedPosition, oldSel == null ? childrenTop : oldSel.getTop()); // ↓↓↓ 6.2 所以这 mFirstPosition = 0, mItemCount >= 0, 判断成立 } else if (mFirstPosition < mItemCount) { // ↓↓↓ 7. 可见该方法会去获取缓存View,然后优先填充指定位置的View, 然后填满其他位置的View, 第二次onLayout结束 sel = fillSpecific(mFirstPosition, oldFirst == null ? childrenTop : oldFirst.getTop()); } else { sel = fillSpecific(0, childrenTop); } } break; } // Flush any cached views that did not get reused above recycleBin.scrapActiveViews(); // remove any header/footer that has been temp detached and not re-attached removeUnusedFixedViews(mHeaderViewInfos); removeUnusedFixedViews(mFooterViewInfos); if (sel != null) { // The current selected item should get focus if items are // focusable. if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) { final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild && focusLayoutRestoreView != null && focusLayoutRestoreView.requestFocus()) || sel.requestFocus(); if (!focusWasTaken) { // Selected item didn't take focus, but we still want to // make sure something else outside of the selected view // has focus. final View focused = getFocusedChild(); if (focused != null) { focused.clearFocus(); } positionSelector(INVALID_POSITION, sel); } else { sel.setSelected(false); mSelectorRect.setEmpty(); } } else { positionSelector(INVALID_POSITION, sel); } mSelectedTop = sel.getTop(); } else { final boolean inTouchMode = mTouchMode == TOUCH_MODE_TAP || mTouchMode == TOUCH_MODE_DONE_WAITING; if (inTouchMode) { // If the user's finger is down, select the motion position. final View child = getChildAt(mMotionPosition - mFirstPosition); if (child != null) { positionSelector(mMotionPosition, child); } } else if (mSelectorPosition != INVALID_POSITION) { // If we had previously positioned the selector somewhere, // put it back there. It might not match up with the data, // but it's transitioning out so it's not a big deal. final View child = getChildAt(mSelectorPosition - mFirstPosition); if (child != null) { positionSelector(mSelectorPosition, child); } } else { // Otherwise, clear selection. mSelectedTop = 0; mSelectorRect.setEmpty(); } // Even if there is not selected position, we may need to // restore focus (i.e. something focusable in touch mode). if (hasFocus() && focusLayoutRestoreView != null) { focusLayoutRestoreView.requestFocus(); } } // Attempt to restore accessibility focus, if necessary. if (viewRootImpl != null) { final View newAccessibilityFocusedView = viewRootImpl.getAccessibilityFocusedHost(); if (newAccessibilityFocusedView == null) { if (accessibilityFocusLayoutRestoreView != null && accessibilityFocusLayoutRestoreView.isAttachedToWindow()) { final AccessibilityNodeProvider provider = accessibilityFocusLayoutRestoreView.getAccessibilityNodeProvider(); if (accessibilityFocusLayoutRestoreNode != null && provider != null) { final int virtualViewId = AccessibilityNodeInfo.getVirtualDescendantId( accessibilityFocusLayoutRestoreNode.getSourceNodeId()); provider.performAction(virtualViewId, AccessibilityNodeInfo.ACTION_ACCESSIBILITY_FOCUS, null); } else { accessibilityFocusLayoutRestoreView.requestAccessibilityFocus(); } } else if (accessibilityFocusPosition != INVALID_POSITION) { // Bound the position within the visible children. final int position = MathUtils.constrain( accessibilityFocusPosition - mFirstPosition, 0, getChildCount() - 1); final View restoreView = getChildAt(position); if (restoreView != null) { restoreView.requestAccessibilityFocus(); } } } } // Tell focus view we are done mucking with it, if it is still in // our view hierarchy. if (focusLayoutRestoreView != null && focusLayoutRestoreView.getWindowToken() != null) { focusLayoutRestoreView.dispatchFinishTemporaryDetach(); } mLayoutMode = LAYOUT_NORMAL; mDataChanged = false; if (mPositionScrollAfterLayout != null) { post(mPositionScrollAfterLayout); mPositionScrollAfterLayout = null; } mNeedSync = false; setNextSelectedPositionInt(mSelectedPosition); updateScrollIndicators(); if (mItemCount > 0) { checkSelectionChanged(); } invokeOnItemScrollListener(); } finally { if (mFocusSelector != null) { mFocusSelector.onLayoutComplete(); } if (!blockLayoutRequests) { mBlockLayoutRequests = false; } } } private View fillSpecific(int position, int top) { boolean tempIsSelected = position == mSelectedPosition; // ↓↓↓ 6.3 获取缓存View并显示 View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected); // Possibly changed again in fillUp if we add rows above this one. mFirstPosition = position; View above; View below; final int dividerHeight = mDividerHeight; if (!mStackFromBottom) { // ↓↓↓ 6.8 向上填满view above = fillUp(position - 1, temp.getTop() - dividerHeight); // This will correct for the top of the first view not touching the top of the list adjustViewsUpOrDown(); // ↓↓↓ 6.9 向下填满view below = fillDown(position + 1, temp.getBottom() + dividerHeight); int childCount = getChildCount(); if (childCount > 0) { correctTooHigh(childCount); } } else { below = fillDown(position + 1, temp.getBottom() + dividerHeight); // This will correct for the bottom of the last view not touching the bottom of the list adjustViewsUpOrDown(); above = fillUp(position - 1, temp.getTop() - dividerHeight); int childCount = getChildCount(); if (childCount > 0) { correctTooLow(childCount); } } if (tempIsSelected) { return temp; } else if (above != null) { return above; } else { return below; } } private View makeAndAddView(int position, int y, boolean flow, int childrenLeft, boolean selected) { View child; if (!mDataChanged) { // Try to use an existing view for this position // ↓↓↓ 6.4 从RecycleBin里获取缓存的View child = mRecycler.getActiveView(position); if (child != null) { // Found it -- we're using an existing child // This just needs to be positioned // ↓↓↓ 6.5 获取到view后执行该方法 setupChild(child, position, y, flow, childrenLeft, selected, true); return child; } } // Make a new view for this position, or convert an unused view if possible child = obtainView(position, mIsScrap); // This needs to be positioned and measured setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]); return child; } private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft, boolean selected, boolean recycled) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem"); final boolean isSelected = selected && shouldShowSelector(); final boolean updateChildSelected = isSelected != child.isSelected(); final int mode = mTouchMode; final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL && mMotionPosition == position; final boolean updateChildPressed = isPressed != child.isPressed(); final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested(); // Respect layout params that are already in the view. Otherwise make some up... // noinspection unchecked AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams(); if (p == null) { p = (AbsListView.LayoutParams) generateDefaultLayoutParams(); } p.viewType = mAdapter.getItemViewType(position); p.isEnabled = mAdapter.isEnabled(position); // ↓↓↓ 6.6 recycled = true 这是参数传进来的, 第一次onLayout时p.forceAdd被标记为false. (p.forceAdd = false;) if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) { // ↓↓↓ 6.7 因此执行这句, 让所有View都处于attach状态, 即显示 attachViewToParent(child, flowDown ? -1 : 0, p); } else { p.forceAdd = false; if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { p.recycledHeaderFooter = true; } addViewInLayout(child, flowDown ? -1 : 0, p, true); } if (updateChildSelected) { child.setSelected(isSelected); } if (updateChildPressed) { child.setPressed(isPressed); } if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) { if (child instanceof Checkable) { ((Checkable) child).setChecked(mCheckStates.get(position)); } else if (getContext().getApplicationInfo().targetSdkVersion >= android.os.Build.VERSION_CODES.HONEYCOMB) { child.setActivated(mCheckStates.get(position)); } } if (needToMeasure) { final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec, mListPadding.left + mListPadding.right, p.width); final int lpHeight = p.height; final int childHeightSpec; if (lpHeight > 0) { childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY); } else { childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(), MeasureSpec.UNSPECIFIED); } child.measure(childWidthSpec, childHeightSpec); } else { cleanupLayoutState(child); } final int w = child.getMeasuredWidth(); final int h = child.getMeasuredHeight(); final int childTop = flowDown ? y : y - h; if (needToMeasure) { final int childRight = childrenLeft + w; final int childBottom = childTop + h; child.layout(childrenLeft, childTop, childRight, childBottom); } else { child.offsetLeftAndRight(childrenLeft - child.getLeft()); child.offsetTopAndBottom(childTop - child.getTop()); } if (mCachingStarted && !child.isDrawingCacheEnabled()) { child.setDrawingCacheEnabled(true); } if (recycled && (((AbsListView.LayoutParams)child.getLayoutParams()).scrappedFromPosition) != position) { child.jumpDrawablesToCurrentState(); } Trace.traceEnd(Trace.TRACE_TAG_VIEW); } 第二次onLayout主要做的是: 将屏幕上的View全部缓存到RecycleBin里, 然后将他们从父容器中清除(detach);然后从RecycleBin里获取缓存的View, 并将他们添加到父容器(attach), 然后依次向上, 向下填充满屏幕另外 attachViewToParent 和 detachAllViewsFromParent 是组合使用的ListView初始化过程图
两次的onLayout只是初始化了第一屏的view, 当我们手指上下滑的时候, ListView会重复利用滑出屏幕的条目, 这才是ListView的神奇之处. 想要了解滑动事件是如何处理的, 那么就要从 onTouchEvent() 入手.
public boolean onTouchEvent(MotionEvent ev) { if (!isEnabled()) { // A disabled view that is clickable still consumes the touch // events, it just doesn't respond to them. return isClickable() || isLongClickable(); } if (mPositionScroller != null) { mPositionScroller.stop(); } if (mIsDetaching || !isAttachedToWindow()) { // Something isn't right. // Since we rely on being attached to get data set change notifications, // don't risk doing anything where we might try to resync and find things // in a bogus state. return false; } startNestedScroll(SCROLL_AXIS_VERTICAL); if (mFastScroll != null && mFastScroll.onTouchEvent(ev)) { return true; } initVelocityTrackerIfNotExists(); final MotionEvent vtev = MotionEvent.obtain(ev); final int actionMasked = ev.getActionMasked(); if (actionMasked == MotionEvent.ACTION_DOWN) { mNestedYOffset = 0; } vtev.offsetLocation(0, mNestedYOffset); switch (actionMasked) { case MotionEvent.ACTION_DOWN: { onTouchDown(ev); break; } // ↓↓↓ 1. 各种的触摸事件, 我们只需关心移动事件Move即可 case MotionEvent.ACTION_MOVE: { onTouchMove(ev, vtev); break; } case MotionEvent.ACTION_UP: { onTouchUp(ev); break; } case MotionEvent.ACTION_CANCEL: { onTouchCancel(); break; } case MotionEvent.ACTION_POINTER_UP: { onSecondaryPointerUp(ev); final int x = mMotionX; final int y = mMotionY; final int motionPosition = pointToPosition(x, y); if (motionPosition >= 0) { // Remember where the motion event started final View child = getChildAt(motionPosition - mFirstPosition); mMotionViewOriginalTop = child.getTop(); mMotionPosition = motionPosition; } mLastY = y; break; } case MotionEvent.ACTION_POINTER_DOWN: { // New pointers take over dragging duties final int index = ev.getActionIndex(); final int id = ev.getPointerId(index); final int x = (int) ev.getX(index); final int y = (int) ev.getY(index); mMotionCorrection = 0; mActivePointerId = id; mMotionX = x; mMotionY = y; final int motionPosition = pointToPosition(x, y); if (motionPosition >= 0) { // Remember where the motion event started final View child = getChildAt(motionPosition - mFirstPosition); mMotionViewOriginalTop = child.getTop(); mMotionPosition = motionPosition; } mLastY = y; break; } } if (mVelocityTracker != null) { mVelocityTracker.addMovement(vtev); } vtev.recycle(); return true; } private void onTouchMove(MotionEvent ev, MotionEvent vtev) { if (mHasPerformedLongPress) { // Consume all move events following a successful long press. return; } int pointerIndex = ev.findPointerIndex(mActivePointerId); if (pointerIndex == -1) { pointerIndex = 0; mActivePointerId = ev.getPointerId(pointerIndex); } if (mDataChanged) { // Re-sync everything if data has been changed // since the scroll operation can query the adapter. layoutChildren(); } final int y = (int) ev.getY(pointerIndex); // ↓↓↓ 2. 这里的 mTouchMode = TOUCH_MODE_SCROLL switch (mTouchMode) { case TOUCH_MODE_DOWN: case TOUCH_MODE_TAP: case TOUCH_MODE_DONE_WAITING: // Check if we have moved far enough that it looks more like a // scroll than a tap. If so, we'll enter scrolling mode. if (startScrollIfNeeded((int) ev.getX(pointerIndex), y, vtev)) { break; } // Otherwise, check containment within list bounds. If we're // outside bounds, cancel any active presses. final View motionView = getChildAt(mMotionPosition - mFirstPosition); final float x = ev.getX(pointerIndex); if (!pointInView(x, y, mTouchSlop)) { setPressed(false); if (motionView != null) { motionView.setPressed(false); } removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ? mPendingCheckForTap : mPendingCheckForLongPress); mTouchMode = TOUCH_MODE_DONE_WAITING; updateSelectorState(); } else if (motionView != null) { // Still within bounds, update the hotspot. final float[] point = mTmpPoint; point[0] = x; point[1] = y; transformPointToViewLocal(point, motionView); motionView.drawableHotspotChanged(point[0], point[1]); } break; case TOUCH_MODE_SCROLL: case TOUCH_MODE_OVERSCROLL: // ↓↓↓ 3. 于是也就执行该分支的代码 scrollIfNeeded((int) ev.getX(pointerIndex), y, vtev); break; } } private void scrollIfNeeded(int x, int y, MotionEvent vtev) { int rawDeltaY = y - mMotionY; int scrollOffsetCorrection = 0; int scrollConsumedCorrection = 0; if (mLastY == Integer.MIN_VALUE) { rawDeltaY -= mMotionCorrection; } if (dispatchNestedPreScroll(0, mLastY != Integer.MIN_VALUE ? mLastY - y : -rawDeltaY, mScrollConsumed, mScrollOffset)) { rawDeltaY += mScrollConsumed[1]; scrollOffsetCorrection = -mScrollOffset[1]; scrollConsumedCorrection = mScrollConsumed[1]; if (vtev != null) { vtev.offsetLocation(0, mScrollOffset[1]); mNestedYOffset += mScrollOffset[1]; } } final int deltaY = rawDeltaY; int incrementalDeltaY = mLastY != Integer.MIN_VALUE ? y - mLastY + scrollConsumedCorrection : deltaY; int lastYCorrection = 0; if (mTouchMode == TOUCH_MODE_SCROLL) { if (PROFILE_SCROLLING) { if (!mScrollProfilingStarted) { Debug.startMethodTracing("AbsListViewScroll"); mScrollProfilingStarted = true; } } if (mScrollStrictSpan == null) { // If it's non-null, we're already in a scroll. mScrollStrictSpan = StrictMode.enterCriticalSpan("AbsListView-scroll"); } if (y != mLastY) { // We may be here after stopping a fling and continuing to scroll. // If so, we haven't disallowed intercepting touch events yet. // Make sure that we do so in case we're in a parent that can intercept. if ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) == 0 && Math.abs(rawDeltaY) > mTouchSlop) { final ViewParent parent = getParent(); if (parent != null) { parent.requestDisallowInterceptTouchEvent(true); } } final int motionIndex; if (mMotionPosition >= 0) { motionIndex = mMotionPosition - mFirstPosition; } else { // If we don't have a motion position that we can reliably track, // pick something in the middle to make a best guess at things below. motionIndex = getChildCount() / 2; } int motionViewPrevTop = 0; View motionView = this.getChildAt(motionIndex); if (motionView != null) { motionViewPrevTop = motionView.getTop(); } // No need to do all this work if we're not going to move anyway boolean atEdge = false; // ↓↓↓ 4. 如果移动增量不为0, 说明用户进行了移动操作, 执行该方法 if (incrementalDeltaY != 0) { // ↓↓↓ 4.1 执行该方法, 传递按下的Y坐标, 和Y轴移动增量 atEdge = trackMotionScroll(deltaY, incrementalDeltaY); } // Check to see if we have bumped into the scroll limit motionView = this.getChildAt(motionIndex); if (motionView != null) { // Check if the top of the motion view is where it is // supposed to be final int motionViewRealTop = motionView.getTop(); if (atEdge) { // Apply overscroll int overscroll = -incrementalDeltaY - (motionViewRealTop - motionViewPrevTop); if (dispatchNestedScroll(0, overscroll - incrementalDeltaY, 0, overscroll, mScrollOffset)) { lastYCorrection -= mScrollOffset[1]; if (vtev != null) { vtev.offsetLocation(0, mScrollOffset[1]); mNestedYOffset += mScrollOffset[1]; } } else { final boolean atOverscrollEdge = overScrollBy(0, overscroll, 0, mScrollY, 0, 0, 0, mOverscrollDistance, true); if (atOverscrollEdge && mVelocityTracker != null) { // Don't allow overfling if we're at the edge mVelocityTracker.clear(); } final int overscrollMode = getOverScrollMode(); if (overscrollMode == OVER_SCROLL_ALWAYS || (overscrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && !contentFits())) { if (!atOverscrollEdge) { mDirection = 0; // Reset when entering overscroll. mTouchMode = TOUCH_MODE_OVERSCROLL; } if (incrementalDeltaY > 0) { mEdgeGlowTop.onPull((float) -overscroll / getHeight(), (float) x / getWidth()); if (!mEdgeGlowBottom.isFinished()) { mEdgeGlowBottom.onRelease(); } invalidateTopGlow(); } else if (incrementalDeltaY < 0) { mEdgeGlowBottom.onPull((float) overscroll / getHeight(), 1.f - (float) x / getWidth()); if (!mEdgeGlowTop.isFinished()) { mEdgeGlowTop.onRelease(); } invalidateBottomGlow(); } } } } mMotionY = y + lastYCorrection + scrollOffsetCorrection; } mLastY = y + lastYCorrection + scrollOffsetCorrection; } } else if (mTouchMode == TOUCH_MODE_OVERSCROLL) { // ... } } boolean trackMotionScroll(int deltaY, int incrementalDeltaY) { final int childCount = getChildCount(); if (childCount == 0) { return true; } final int firstTop = getChildAt(0).getTop(); final int lastBottom = getChildAt(childCount - 1).getBottom(); final Rect listPadding = mListPadding; // "effective padding" In this case is the amount of padding that affects // how much space should not be filled by items. If we don't clip to padding // there is no effective padding. int effectivePaddingTop = 0; int effectivePaddingBottom = 0; if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { effectivePaddingTop = listPadding.top; effectivePaddingBottom = listPadding.bottom; } // FIXME account for grid vertical spacing too? final int spaceAbove = effectivePaddingTop - firstTop; final int end = getHeight() - effectivePaddingBottom; final int spaceBelow = lastBottom - end; final int height = getHeight() - mPaddingBottom - mPaddingTop; if (deltaY < 0) { deltaY = Math.max(-(height - 1), deltaY); } else { deltaY = Math.min(height - 1, deltaY); } if (incrementalDeltaY < 0) { incrementalDeltaY = Math.max(-(height - 1), incrementalDeltaY); } else { incrementalDeltaY = Math.min(height - 1, incrementalDeltaY); } final int firstPosition = mFirstPosition; // Update our guesses for where the first and last views are if (firstPosition == 0) { mFirstPositionDistanceGuess = firstTop - listPadding.top; } else { mFirstPositionDistanceGuess += incrementalDeltaY; } if (firstPosition + childCount == mItemCount) { mLastPositionDistanceGuess = lastBottom + listPadding.bottom; } else { mLastPositionDistanceGuess += incrementalDeltaY; } final boolean cannotScrollDown = (firstPosition == 0 && firstTop >= listPadding.top && incrementalDeltaY >= 0); final boolean cannotScrollUp = (firstPosition + childCount == mItemCount && lastBottom <= getHeight() - listPadding.bottom && incrementalDeltaY <= 0); if (cannotScrollDown || cannotScrollUp) { return incrementalDeltaY != 0; } // ↓↓↓ 5. 当 incrementalDeltaY < 0 时, 表示向下滑动, 否则为向上滑动 final boolean down = incrementalDeltaY < 0; final boolean inTouchMode = isInTouchMode(); if (inTouchMode) { hideSelector(); } final int headerViewsCount = getHeaderViewsCount(); final int footerViewsStart = mItemCount - getFooterViewsCount(); int start = 0; int count = 0; // ↓↓↓ 6. 当向下滑动时 if (down) { int top = -incrementalDeltaY; if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { top += listPadding.top; } for (int i = 0; i < childCount; i++) { // ↓↓↓ 6.1 获取子View final View child = getChildAt(i); if (child.getBottom() >= top) { break; } else { // ↓↓↓ 6.2 如果子View的bottom < top 时, 则说明子View已经滑出屏幕了 count++; int position = firstPosition + i; if (position >= headerViewsCount && position < footerViewsStart) { // The view will be rebound to new data, clear any // system-managed transient state. child.clearAccessibilityFocus(); // ↓↓↓ 6.3 将其添加到RecycleBin的废弃View里, 并count++ mRecycler.addScrapView(child, position); } } } } else { int bottom = getHeight() - incrementalDeltaY; if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { bottom -= listPadding.bottom; } for (int i = childCount - 1; i >= 0; i--) { final View child = getChildAt(i); if (child.getTop() <= bottom) { break; } else { start = i; count++; int position = firstPosition + i; if (position >= headerViewsCount && position < footerViewsStart) { // The view will be rebound to new data, clear any // system-managed transient state. child.clearAccessibilityFocus(); mRecycler.addScrapView(child, position); } } } } mMotionViewNewTop = mMotionViewOriginalTop + deltaY; mBlockLayoutRequests = true; if (count > 0) { // ↓↓↓ 7. 如果有需要被回收的View, 则将这些View从父容器中清除 (detach) detachViewsFromParent(start, count); mRecycler.removeSkippedScrap(); } // invalidate before moving the children to avoid unnecessary invalidate // calls to bubble up from the children all the way to the top if (!awakenScrollBars()) { invalidate(); } // ↓↓↓ 8. 然后根据手指滑动的增量incrementalDeltaY, 移动这些View, 也就产生了listview里的itemView的移动效果 offsetChildrenTopAndBottom(incrementalDeltaY); if (down) { mFirstPosition += count; } final int absIncrementalDeltaY = Math.abs(incrementalDeltaY); // ↓↓↓ 9. 如果一个View的底部移入了屏幕, 或者 一个View的顶部移入了屏幕, 则执行该方法 if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) { fillGap(down); } mRecycler.fullyDetachScrapViews(); if (!inTouchMode && mSelectedPosition != INVALID_POSITION) { final int childIndex = mSelectedPosition - mFirstPosition; if (childIndex >= 0 && childIndex < getChildCount()) { positionSelector(mSelectedPosition, getChildAt(childIndex)); } } else if (mSelectorPosition != INVALID_POSITION) { final int childIndex = mSelectorPosition - mFirstPosition; if (childIndex >= 0 && childIndex < getChildCount()) { positionSelector(INVALID_POSITION, getChildAt(childIndex)); } } else { mSelectorRect.setEmpty(); } mBlockLayoutRequests = false; invokeOnItemScrollListener(); return false; } abstract void fillGap(boolean down); void fillGap(boolean down) { final int count = getChildCount(); // ↓↓↓ 9.1 如果向下滑 if (down) { int paddingTop = 0; if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { paddingTop = getListPaddingTop(); } final int startOffset = count > 0 ? getChildAt(count - 1).getBottom() + mDividerHeight : paddingTop; // ↓↓↓ 9.2 则执行该方法, 来填充View fillDown(mFirstPosition + count, startOffset); correctTooHigh(getChildCount()); } else { int paddingBottom = 0; if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { paddingBottom = getListPaddingBottom(); } final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight : getHeight() - paddingBottom; fillUp(mFirstPosition - 1, startOffset); correctTooLow(getChildCount()); } } private View fillDown(int pos, int nextTop) { // ... while (nextTop < end && pos < mItemCount) { // is this the selected item? boolean selected = pos == mSelectedPosition; // ↓↓↓ 9.3 就需要去创建或者获取需要的View View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected); nextTop = child.getBottom() + mDividerHeight; if (selected) { selectedView = child; } pos++; } setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1); return selectedView; } private View makeAndAddView(int position, int y, boolean flow, int childrenLeft, boolean selected) { View child; if (!mDataChanged) { // Try to use an existing view for this position // ↓↓↓ 9.4 获取缓存的View, 既然View都已经展示在屏幕上了, 那么就没有可用的缓存View 了 child = mRecycler.getActiveView(position); if (child != null) { // Found it -- we're using an existing child // This just needs to be positioned setupChild(child, position, y, flow, childrenLeft, selected, true); return child; } } // Make a new view for this position, or convert an unused view if possible // ↓↓↓ 9.5 于是就会调用该方法去获取 child = obtainView(position, mIsScrap); // This needs to be positioned and measured // ↓↓↓ 9.8 获取到View后, 执行该方法 setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]); return child; } View obtainView(int position, boolean[] isScrap) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView"); isScrap[0] = false; // ... // ↓↓↓ 9.6 获取一个废弃的View, 可能获取到, 可能获取不到 final View scrapView = mRecycler.getScrapView(position); // ↓↓↓ 9.7 从适配器里获取一个View, 这是必然能获取到的, 因为getView参数收了scrapView, 如果该scrapView的不为null, 我们将赋值, 为null, 我们将创建 final View child = mAdapter.getView(position, scrapView, this); if (scrapView != null) { if (child != scrapView) { // Failed to re-bind the data, return scrap to the heap. mRecycler.addScrapView(scrapView, position); } else { if (child.isTemporarilyDetached()) { isScrap[0] = true; // Finish the temporary detach started in addScrapView(). child.dispatchFinishTemporaryDetach(); } else { // we set isScrap to "true" only if the view is temporarily detached. // if the view is fully detached, it is as good as a view created by the // adapter isScrap[0] = false; } } } if (mCacheColorHint != 0) { child.setDrawingCacheBackgroundColor(mCacheColorHint); } if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) { child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES); } setItemViewLayoutParams(child, position); if (AccessibilityManager.getInstance(mContext).isEnabled()) { if (mAccessibilityDelegate == null) { mAccessibilityDelegate = new ListItemAccessibilityDelegate(); } if (child.getAccessibilityDelegate() == null) { child.setAccessibilityDelegate(mAccessibilityDelegate); } } Trace.traceEnd(Trace.TRACE_TAG_VIEW); return child; } private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft, boolean selected, boolean recycled) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem"); final boolean isSelected = selected && shouldShowSelector(); final boolean updateChildSelected = isSelected != child.isSelected(); final int mode = mTouchMode; final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL && mMotionPosition == position; final boolean updateChildPressed = isPressed != child.isPressed(); final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested(); // Respect layout params that are already in the view. Otherwise make some up... // noinspection unchecked AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams(); if (p == null) { p = (AbsListView.LayoutParams) generateDefaultLayoutParams(); } p.viewType = mAdapter.getItemViewType(position); p.isEnabled = mAdapter.isEnabled(position); if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) { // 9.9 然后就将View添加到父容器中 (attach), 基本流程就到这里结束了 attachViewToParent(child, flowDown ? -1 : 0, p); } else { p.forceAdd = false; if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { p.recycledHeaderFooter = true; } addViewInLayout(child, flowDown ? -1 : 0, p, true); } if (updateChildSelected) { child.setSelected(isSelected); } if (updateChildPressed) { child.setPressed(isPressed); } if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) { if (child instanceof Checkable) { ((Checkable) child).setChecked(mCheckStates.get(position)); } else if (getContext().getApplicationInfo().targetSdkVersion >= android.os.Build.VERSION_CODES.HONEYCOMB) { child.setActivated(mCheckStates.get(position)); } } if (needToMeasure) { final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec, mListPadding.left + mListPadding.right, p.width); final int lpHeight = p.height; final int childHeightSpec; if (lpHeight > 0) { childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY); } else { childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(), MeasureSpec.UNSPECIFIED); } child.measure(childWidthSpec, childHeightSpec); } else { cleanupLayoutState(child); } final int w = child.getMeasuredWidth(); final int h = child.getMeasuredHeight(); final int childTop = flowDown ? y : y - h; if (needToMeasure) { final int childRight = childrenLeft + w; final int childBottom = childTop + h; child.layout(childrenLeft, childTop, childRight, childBottom); } else { child.offsetLeftAndRight(childrenLeft - child.getLeft()); child.offsetTopAndBottom(childTop - child.getTop()); } if (mCachingStarted && !child.isDrawingCacheEnabled()) { child.setDrawingCacheEnabled(true); } if (recycled && (((AbsListView.LayoutParams)child.getLayoutParams()).scrappedFromPosition) != position) { child.jumpDrawablesToCurrentState(); } Trace.traceEnd(Trace.TRACE_TAG_VIEW); } 可见ListView在向下滑动时, 会将移出屏幕的View添加到RecycleBin的废弃View里, 然后将其标记为detach状态, 然后移动屏幕上的View, 让他们产生对应的滑动同时, 屏幕上方的View会进入屏幕, 首先从RecycleBin里获取缓存View, 获取不到就到Adapter里去获取ListView滑动过程图
案例:
方式一实现代码:
Adapter代码, 跟往常一样
public class Case1Adapter extends BaseAdapter { private LayoutInflater inflater; private List<String> mDatas; public Case1Adapter(Context context, List<String> datas) { this.inflater = LayoutInflater.from(context); this.mDatas = datas; } @Override public int getCount() { return mDatas == null ? 0 : mDatas.size(); } @Override public Object getItem(int i) { return mDatas.get(i); } @Override public long getItemId(int i) { return i; } @Override public View getView(int i, View convertView, ViewGroup viewGroup) { ViewHolder viewHolder; if(convertView == null){ convertView = inflater.inflate(R.layout.item, null); viewHolder = new ViewHolder(); viewHolder.sync = (TextView)convertView.findViewById(R.id.sync); viewHolder.async = (TextView)convertView.findViewById(R.id.async); convertView.setTag(viewHolder); }else{ viewHolder = (ViewHolder) convertView.getTag(); } viewHolder.sync.setText("当前位置为: " + i); if (mDatas.size() != 0) { AsyncUtil.ayncLoaderData((ListView) viewGroup, viewHolder.async, mDatas.get(i)); } return convertView; } class ViewHolder{ TextView sync; TextView async; } }这里主要看DataTask线程池里的操作
public class AsyncUtil { public static void ayncLoaderData(ListView listView, TextView textView, String s) { if (textView == null || s == null) return; textView.setEnabled(false); new DataTask(listView, textView, s).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR); } /** * 线程池 * @author Luzhuo */ private static class DataTask extends AsyncTask<Object, Void, Void>{ private ListView mListView; private TextView mTextView; private String mData; public DataTask(ListView listView, TextView textView, String s) { this.mListView = listView; this.mTextView = textView; this.mData = s; } @Override protected void onPreExecute() { // 设置Tag mTextView.setTag(mData); } /** * 设置延迟, 模拟图片的网络请求耗时 */ @Override protected Void doInBackground(Object... params) { SystemClock.sleep(500); return null; } /** * 设置文字, 模拟图片设置 * @param result */ @Override protected void onPostExecute(Void result) { // 获取有该Tag的View TextView tv = (TextView) mListView.findViewWithTag(mData); if (tv == null) return; mTextView.setText("异步数据: " + mData); mTextView.setEnabled(true); } } }效果图:
未做处理的效果 使用方式一处理后的效果