ViewGroup源码解读

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我们之前刚刚分析完事件传递机制和view的源码，如果没有看过的，建议看完View的事件拦截机制浅析以及View的事件源码解析。这次我们来分析下viewgroup的。

可能有人会想，怎么又是源码分析，肯定又是一大通。其实没你想的那么复杂。仔细分析一波就行了。

解读ViewGroup

我们都知道，一个事件完整的流程是从dispatchTouchevent–>onInterceptTouchevent–>onTouchEvent。我们先不说事件监听的问题。上述三个步骤就是正常一个点击的流程。前面我们分析view的时候发现它并没有onInterceptTouchevent这个方法。这个我之前有提到，view已经是最底层了，所以就不需要拦截了。而这一整套的机制就是在ViewGroup中体现出来的。我们先来看一张图：

触摸事件发生后，在Activity内最先接收到事件的是Activity自身的dispatchTouchEvent，然后Activity传递给Activity的Window。接着Window传递给最顶端的View，也就是DecorView。接下来才是我们熟悉的触摸事件流程：首先是最顶端的ViewGroup(这边便是DecorView)的dispatchTouchEvent接收到事件。并通过onInterceptTouchEvent判断是否需要拦截。如果拦截则分配到ViewGroup自身的onTouchEvent，如果不拦截则查找位于点击区域的子View(当事件是ACTION_DOWN的时候，会做一次查找并根据查找到的子View设定一个TouchTarget，有了TouchTarget以后，后续的对应id的事件如果不被拦截都会分发给这一个TouchTarget)。查找到子View以后则调用dispatchTransformedTouchEvent把MotionEvent的坐标转换到子View的坐标空间，这不仅仅是x，y的偏移，还包括根据子View自身矩阵的逆矩阵对坐标进行变换(这就是使用setTranslationX,setScaleX等方法调用后，子View的点击区域还能保持和自身绘制内容一致的原因。使用Animation做变换点击区域不同步是因为Animation使用的是Canvas的矩阵而不是View自身的矩阵来做变换)。

dispatchTouchevent分析

我们先放上dispatchTouchevent的源码，然后一步一步来分析：

public boolean dispatchTouchEvent(MotionEvent ev) { if (mInputEventConsistencyVerifier != null) { mInputEventConsistencyVerifier.onTouchEvent(ev, 1); } // If the event targets the accessibility focused view and this is it, start // normal event dispatch. Maybe a descendant is what will handle the click. if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) { ev.setTargetAccessibilityFocus(false); } boolean handled = false; if (onFilterTouchEventForSecurity(ev)) { final int action = ev.getAction(); final int actionMasked = action & MotionEvent.ACTION_MASK; // Handle an initial down. if (actionMasked == MotionEvent.ACTION_DOWN) { // Throw away all previous state when starting a new touch gesture. // The framework may have dropped the up or cancel event for the previous gesture // due to an app switch, ANR, or some other state change. cancelAndClearTouchTargets(ev); resetTouchState(); } // Check for interception. final boolean intercepted; if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { intercepted = onInterceptTouchEvent(ev); ev.setAction(action); // restore action in case it was changed } else { intercepted = false; } } else { // There are no touch targets and this action is not an initial down // so this view group continues to intercept touches. intercepted = true; } // If intercepted, start normal event dispatch. Also if there is already // a view that is handling the gesture, do normal event dispatch. if (intercepted || mFirstTouchTarget != null) { ev.setTargetAccessibilityFocus(false); } // Check for cancelation. final boolean canceled = resetCancelNextUpFlag(this) || actionMasked == MotionEvent.ACTION_CANCEL; // Update list of touch targets for pointer down, if needed. final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0; TouchTarget newTouchTarget = null; boolean alreadyDispatchedToNewTouchTarget = false; if (!canceled && !intercepted) { // If the event is targeting accessiiblity focus we give it to the // view that has accessibility focus and if it does not handle it // we clear the flag and dispatch the event to all children as usual. // We are looking up the accessibility focused host to avoid keeping // state since these events are very rare. View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus() ? findChildWithAccessibilityFocus() : null; if (actionMasked == MotionEvent.ACTION_DOWN || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN) || actionMasked == MotionEvent.ACTION_HOVER_MOVE) { final int actionIndex = ev.getActionIndex(); // always 0 for down final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex) : TouchTarget.ALL_POINTER_IDS; // Clean up earlier touch targets for this pointer id in case they // have become out of sync. removePointersFromTouchTargets(idBitsToAssign); final int childrenCount = mChildrenCount; if (newTouchTarget == null && childrenCount != 0) { final float x = ev.getX(actionIndex); final float y = ev.getY(actionIndex); // Find a child that can receive the event. // Scan children from front to back. final ArrayList<View> preorderedList = buildTouchDispatchChildList(); final boolean customOrder = preorderedList == null && isChildrenDrawingOrderEnabled(); final View[] children = mChildren; for (int i = childrenCount - 1; i >= 0; i--) { final int childIndex = getAndVerifyPreorderedIndex( childrenCount, i, customOrder); final View child = getAndVerifyPreorderedView( preorderedList, children, childIndex); // If there is a view that has accessibility focus we want it // to get the event first and if not handled we will perform a // normal dispatch. We may do a double iteration but this is // safer given the timeframe. if (childWithAccessibilityFocus != null) { if (childWithAccessibilityFocus != child) { continue; } childWithAccessibilityFocus = null; i = childrenCount - 1; } if (!canViewReceivePointerEvents(child) || !isTransformedTouchPointInView(x, y, child, null)) { ev.setTargetAccessibilityFocus(false); continue; } newTouchTarget = getTouchTarget(child); if (newTouchTarget != null) { // Child is already receiving touch within its bounds. // Give it the new pointer in addition to the ones it is handling. newTouchTarget.pointerIdBits |= idBitsToAssign; break; } resetCancelNextUpFlag(child); if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) { // Child wants to receive touch within its bounds. mLastTouchDownTime = ev.getDownTime(); if (preorderedList != null) { // childIndex points into presorted list, find original index for (int j = 0; j < childrenCount; j++) { if (children[childIndex] == mChildren[j]) { mLastTouchDownIndex = j; break; } } } else { mLastTouchDownIndex = childIndex; } mLastTouchDownX = ev.getX(); mLastTouchDownY = ev.getY(); newTouchTarget = addTouchTarget(child, idBitsToAssign); alreadyDispatchedToNewTouchTarget = true; break; } // The accessibility focus didn't handle the event, so clear // the flag and do a normal dispatch to all children. ev.setTargetAccessibilityFocus(false); } if (preorderedList != null) preorderedList.clear(); } if (newTouchTarget == null && mFirstTouchTarget != null) { // Did not find a child to receive the event. // Assign the pointer to the least recently added target. newTouchTarget = mFirstTouchTarget; while (newTouchTarget.next != null) { newTouchTarget = newTouchTarget.next; } newTouchTarget.pointerIdBits |= idBitsToAssign; } } } // Dispatch to touch targets. if (mFirstTouchTarget == null) { // No touch targets so treat this as an ordinary view. handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS); } else { // Dispatch to touch targets, excluding the new touch target if we already // dispatched to it. Cancel touch targets if necessary. TouchTarget predecessor = null; TouchTarget target = mFirstTouchTarget; while (target != null) { final TouchTarget next = target.next; if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) { handled = true; } else { final boolean cancelChild = resetCancelNextUpFlag(target.child) || intercepted; if (dispatchTransformedTouchEvent(ev, cancelChild, target.child, target.pointerIdBits)) { handled = true; } if (cancelChild) { if (predecessor == null) { mFirstTouchTarget = next; } else { predecessor.next = next; } target.recycle(); target = next; continue; } } predecessor = target; target = next; } } // Update list of touch targets for pointer up or cancel, if needed. if (canceled || actionMasked == MotionEvent.ACTION_UP || actionMasked == MotionEvent.ACTION_HOVER_MOVE) { resetTouchState(); } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) { final int actionIndex = ev.getActionIndex(); final int idBitsToRemove = 1 << ev.getPointerId(actionIndex); removePointersFromTouchTargets(idBitsToRemove); } } if (!handled && mInputEventConsistencyVerifier != null) { mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1); } return handled; }

是不是整个人都蒙蔽了，这么长一串。其实整段代码可以缩减成几句话，就是这样：

public boolean dispatchTouchEvent(MotionEvent ev) { boolean result = false; if (!onInterceptTouchEvent(ev)) { result = child.dispatchTouchEvent(ev); } if (!result) { result = onTouchEvent(ev); } return result; }

默认不消耗事件，如果本身没有拦截，就交给子类的dispatch事件，如果事件没有消费，就调用自身的onTouchEvent事件。你们仔细想想，流程是不是这样的？

好了，我们现在开始分析整个dispatch事件。具体说明和代码，你们自己对应= =因为太长了。

对action_down的处理： 我们发现，刚进方法的时候有个判断，第一次按下的时候，他会通过 cancelAndClearTouchTargets(ev)取消并且清除所有的手势操作，并且通过resetTouchState()把手势状态设置成默认状态。

接下来的操作，当然就是检查是否需要拦截事件拉。既然是拦截，当然就会走onInterceptTouchEvent这个方法了。我们来看看，viewgroup的onInterceptTouchEvent方法是怎么处理的。

public boolean onInterceptTouchEvent(MotionEvent ev) { if (ev.isFromSource(InputDevice.SOURCE_MOUSE) && ev.getAction() == MotionEvent.ACTION_DOWN && ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY) && isOnScrollbarThumb(ev.getX(), ev.getY())) { return true; } return false; }

我们可以发现，他默认就是false的。那么我们继续回到dispatch看。判断是否拦截后，我们发现他还执行了一句话ev.setAction(action) 官方说明是恢复操作，防止被更改。

事件处理 接下来就是检查事件是否取消咯。如果没有取消并且没有拦截就执行正常的事件处理。

如果事件是针对可访问性焦点视图，我们将其提供给具有可访问性焦点的视图。如果它不处理它，我们清除该标志并像往常一样将事件分派给所有的 ChildView。我们检测并避免保持这种状态，因为这些事非常罕见。这段是官方的解释。我们继续向下看，他执行这样一个方法removePointersFromTouchTargets(idBitsToAssign)。是为了防止指针不同步，清除之前的触摸标识。自我认为可能会和多指触控有关，先不管他，我们继续向下分析。

接下来就是打造了，他会先得到触摸点的坐标位置，然后在当前位置查找可接触的ChildView。然后重点！！！他的查找顺序是从后向前查找。什么意思呢？就是如果A和B有重叠的部分，并且B在A的上面，那么他处理的便是B的事件了。而不处理A的事件。

如果子View可以接受事件，那么我们就给他一个触摸的标识。接下来他会通过调用dispatchTransformedTouchEvent把事件分配给子View。

最后他会判断是否有touchtarget。如果没有的话，那就处理子view的事件。否则就会遍历touchtarget处理事件，也就是之前说的多点触控。在往后就是对action_up和cancel做的一些处理了，譬如：重置手势状态，移除多指操作等等。

dispatchTransformedTouchEvent分析

前面我们说到了，会通过这个方法把事件分发给子view。我们还是先来看代码：

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, View child, int desiredPointerIdBits) { final boolean handled; // Canceling motions is a special case. We don't need to perform any transformations // or filtering. The important part is the action, not the contents. final int oldAction = event.getAction(); if (cancel || oldAction == MotionEvent.ACTION_CANCEL) { event.setAction(MotionEvent.ACTION_CANCEL); if (child == null) { handled = super.dispatchTouchEvent(event); } else { handled = child.dispatchTouchEvent(event); } event.setAction(oldAction); return handled; } // Calculate the number of pointers to deliver. final int oldPointerIdBits = event.getPointerIdBits(); final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits; // If for some reason we ended up in an inconsistent state where it looks like we // might produce a motion event with no pointers in it, then drop the event. if (newPointerIdBits == 0) { return false; } // If the number of pointers is the same and we don't need to perform any fancy // irreversible transformations, then we can reuse the motion event for this // dispatch as long as we are careful to revert any changes we make. // Otherwise we need to make a copy. final MotionEvent transformedEvent; if (newPointerIdBits == oldPointerIdBits) { if (child == null || child.hasIdentityMatrix()) { if (child == null) { handled = super.dispatchTouchEvent(event); } else { final float offsetX = mScrollX - child.mLeft; final float offsetY = mScrollY - child.mTop; event.offsetLocation(offsetX, offsetY); handled = child.dispatchTouchEvent(event); event.offsetLocation(-offsetX, -offsetY); } return handled; } transformedEvent = MotionEvent.obtain(event); } else { transformedEvent = event.split(newPointerIdBits); } // Perform any necessary transformations and dispatch. if (child == null) { handled = super.dispatchTouchEvent(transformedEvent); } else { final float offsetX = mScrollX - child.mLeft; final float offsetY = mScrollY - child.mTop; transformedEvent.offsetLocation(offsetX, offsetY); if (! child.hasIdentityMatrix()) { transformedEvent.transform(child.getInverseMatrix()); } handled = child.dispatchTouchEvent(transformedEvent); } // Done. transformedEvent.recycle(); return handled; }

这段代码就比之前简单很多了。我们会发现，他先判断状态是否取消，如果取消了，把当前事件变成取消状态，然后在判断是否有子view。如果有子view的话直接调用子view的dispatch事件。下面就是多指了，一个pointer对应一个ID，防止处理冲突。我印象中能简单粗暴的处理多指，应该是ViewDragHelper了。具体，你们可以自己去看。后面就如之前一样，判断child是否为null。然后得到是执行自身的事件还是child的事件。

总结

1.ViewGroup包涵多个子view的时候，我们是从后遍历，判断当前view是否可以点击，然后分发给需要处理的子view。 2.我们可以在onInterceptTouchEvent中进行事件拦截。 3.我们可以发现ViewGroup没有onTouchEvent事件，说明他的处理逻辑和View是一样的。 4.子view如果消耗了事件，那么ViewGroup就不会在接受到事件了。

最后我放上ViewGroup源码，你们可以自己去了解。