Android事件分发机制源码分析

xiaoxiao2021-03-01  22

前面写过一篇文章,说了下事件分发机制的方法和大致流程,本文尝试从源码的角度一层一层的看下分发机制。

源码的查看:https://www.androidos.net.cn/sourcecode(可能是我下的源码有问题,部分方法我是在线查看的)

Activity的事件分发机制

我们从activity的dispatchTouchEvent方法进行源码分析:

public boolean dispatchTouchEvent(MotionEvent ev) { if (ev.getAction() == MotionEvent.ACTION_DOWN) { onUserInteraction(); } if (getWindow().superDispatchTouchEvent(ev)) { return true; } return onTouchEvent(ev); }

点进去看到,onUserInteraction是个空方法,这里是用来实现屏保功能的,当activity位于栈顶时,触屏点击home、menu、back会触发。

public void onUserInteraction() { }

然后我们来看第二个判断

if (getWindow().superDispatchTouchEvent(ev)) { return true; }

继续往下看,

public abstract boolean superDispatchTouchEvent(MotionEvent event);

这玩意是个抽象方法,对应的Window也是个抽象类,我们能找到PhoneWindow,

@Override public boolean superDispatchTouchEvent(MotionEvent event) { return mDecor.superDispatchTouchEvent(event); }

然后我们在看下这个mDecor是啥,

找到这个类

public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks

他是继承FrameLayout的,也就是mDecor.SuperDispatchTouchEvent即等同于Viegroup的分发机制。(即事件从activity传递到了viewgroup)

再看最后一个onTouch方法,

public boolean onTouchEvent(MotionEvent event) { if (mWindow.shouldCloseOnTouch(this, event)) { finish(); return true; } return false; }

这里默认返回false,就是不处理,向下层分发,那我们来看下这个判断里面。

public boolean shouldCloseOnTouch(Context context, MotionEvent event) { final boolean isOutside = event.getAction() == MotionEvent.ACTION_DOWN && isOutOfBounds(context, event) || event.getAction() == MotionEvent.ACTION_OUTSIDE; if (mCloseOnTouchOutside && peekDecorView() != null && isOutside) { return true; } return false; }

这里主要判断是否在边界外,在即消费事件,返回true,分发结束,反之返回false,activity层的分发也结束,扔给viewgroup继续分发,直到被消费。

viewgroup的分发机制

我们从ViewGroup的dispatchTouchEvent方法进行源码分析:

if (!canceled && !intercepted) { // If the event is targeting accessibility 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);

代码很长,挑一点看看(不完整,感兴趣的自己查看),就是判断事件没有取消也没有被拦截,然后给viewgroup内的子view进行遍历,继续看重点

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; }

我们给这个dispatchTransformedTouchEvent点进去,

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; }

这里面进行了判断,事件传递给了view(即child)进行分发,或者给他的上一层viewgroup进行分发,直到事件分发结束,即被消费。

回过头来,我们看拦截的判断

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; }

这里面的事件被拦截即 mFirsTouchTarget!=null不成立,即不拦截,mFirsTouchTarget!=null,disallowIntercept表示是否允许被拦截,是可以用代码来控制的,经过判断,允许被拦截再调用拦截的方法,

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; }

这里面进行多个判断,默认返回false,即不拦截。拦截的要求大致上,这个事件是来自使用者的输入、是down事件、这个是按钮、点击的事件只在viewgroup内。一旦拦截了,它就会向它的父类分发,也就是view的分发,但执行ViewGroup的onTouch() ->> onTouchEvent() ->> performClick() ->> onClick(),即自己处理该事件,事件不会往下传递(具体请参考View事件的分发机制中的View.dispatchTouchEvent())。(可以理解为虽然是个viewgroup,但里面并没有子view,所以事件分发相当于view的事件分发),这个地方要注意。

View的事件分发机制

我们从View的dispatchTouchEvent方法进行源码分析:(源码可能下的有点问题,我们在线看下)

boolean result = false; if (mInputEventConsistencyVerifier != null) { mInputEventConsistencyVerifier.onTouchEvent(event, 0); } final int actionMasked = event.getActionMasked(); if (actionMasked == MotionEvent.ACTION_DOWN) { // Defensive cleanup for new gesture stopNestedScroll(); } if (onFilterTouchEventForSecurity(event)) { if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) { result = true; } //noinspection SimplifiableIfStatement ListenerInfo li = mListenerInfo; if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED && li.mOnTouchListener.onTouch(this, event)) { result = true; } if (!result && onTouchEvent(event)) { result = true; } }

它返回的是result,即决定是否继续分发,返回true,即事件被消费,false即要调用onTouch方法。返回true有三种情况

1. view是可点击的且handleScrollBarDragging

   看下源码(太长,看下什么时候返回true)

* @return true if the event was handled as a scroll bar dragging, false otherwise. */ protected boolean handleScrollBarDragging(MotionEvent event) {

翻译:如果作为一个滚动条拖动事件处理返回true

2. view可点击且onTouch事件不为空

3. onTouch事件返回true

下面看下onTouchEvent方法

if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) { switch (action) { case MotionEvent.ACTION_UP: mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN; if ((viewFlags & TOOLTIP) == TOOLTIP) { handleTooltipUp(); } if (!clickable) { removeTapCallback(); removeLongPressCallback(); mInContextButtonPress = false; mHasPerformedLongPress = false; mIgnoreNextUpEvent = false; break; } boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0; if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) { // take focus if we don't have it already and we should in // touch mode. boolean focusTaken = false; if (isFocusable() && isFocusableInTouchMode() && !isFocused()) { focusTaken = requestFocus(); }

即view可点击进入switch判断具体DOWN、UP等事件,重点看下这个

public boolean performClick() { final boolean result; final ListenerInfo li = mListenerInfo; if (li != null && li.mOnClickListener != null) { playSoundEffect(SoundEffectConstants.CLICK); li.mOnClickListener.onClick(this); result = true; } else { result = false; } sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED); notifyEnterOrExitForAutoFillIfNeeded(true); return result; }

根据代码我们知道只要我们通过setOnClickListener()为控件View注册1个点击事件,那么就会给mOnClickListener变量赋值(即不为空),则会往下回调onClick() & performClick()返回true。即调用onTouch事件要调用performClick事件,当这些执行完才能执行我们常见的onClick事件,至此,事件分发结束。

核心结论

事件逐层分发,判断是否拦截,拦截就本层消费,否则向下分发,直至被消费。

写在最后

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