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网上看到过很多人写的事件分发机制解析, 感觉表述都不是很清楚, 也可能没有看到写得好的文章, 所以自己重新看了一遍源码, 来彻底搞清楚 Android 事件分发机制.
触摸事件有哪些以及怎么从 Activity 传递到 DecorView 大家可以上网查下，几个重要方法的基本调用顺序, 这些很容易搜到, 我们重点关注事件从 ViewGroup 到 View 的事件具体的执行过程.
1 Android 事件分发涉及到的方法主要有
dispatchTouchEvent()事件分发
onInterceptTouchEvent() 拦截事件
onTouchEvent() 处理事件
requestDisallowInterceptTouchEvent() 请求不要拦截事件
2 测试类介绍
BigGroup 类型 ViewGroup
SmallGroup 类型 ViewGroup
TestView 类型 View*(clickable 默认 false)
3 默认情况下的事件分发流程 (View.onTouchEvent() 返回 super.onTouchEvent())
log 如下所示：
BigGroup: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
BigGroup: onInterceptTouchEvent() called with: ev = [ACTION_DOWN]
SmallGroup: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
SmallGroup: onInterceptTouchEvent() called with: ev = [ACTION_DOWN]
TestView: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
TestView: onTouchEvent() called with: event = [ACTION_DOWN]
SmallGroup: onTouchEvent() called with: event = [ACTION_DOWN]
BigGroup: onTouchEvent() called with: event = [ACTION_DOWN]
我们看到不管怎么点击滑动，都只会触发 DOWN 事件的流程，我们来看下这是为什么，首先我们来看下 ViewGroup 的 dispatchTouchEvent()
| ///////////ViewGroup | |
| @Override | |
| public boolean dispatchTouchEvent(MotionEvent ev) { | |
| ... 省略部分代码 | |
| boolean handled = false; | |
| if (onFilterTouchEventForSecurity(ev)) {final int action = ev.getAction(); | |
| final int actionMasked = action & MotionEvent.ACTION_MASK; | |
| // 如果是 DOWN 事件, 清除标记, 恢复一些标记状态 | |
| if (actionMasked == MotionEvent.ACTION_DOWN) {cancelAndClearTouchTargets(ev); | |
| resetTouchState();} | |
| // Check for interception. | |
| // 判断是否拦截 | |
| final boolean intercepted; | |
| // 触发 DOWN 事件或者 mFirstTouchTarget 不是 Null, 才会走 if 里面的逻辑 | |
| 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. | |
| // 不是 DOWN 事件 mFirstTouchTarget 为空 拦截后续事件 | |
| intercepted = true; | |
| } | |
| ... 省略部分代码 | |
| // 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; | |
| // 事件没有被取消或者没有被拦截执行 if 中逻辑 | |
| if (!canceled && !intercepted) {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 (childWithAccessibilityFocus != null) {if (childWithAccessibilityFocus != child) {continue;} | |
| childWithAccessibilityFocus = null; | |
| i = childrenCount - 1; | |
| } | |
| // 判断 child 是否可以响应事件 是否点击在了 View 的范围之内 | |
| if (!canViewReceivePointerEvents(child) | |
| || !isTransformedTouchPointInView(x, y, child, null)) {ev.setTargetAccessibilityFocus(false); | |
| continue; | |
| } | |
| // 获取上次响应事件的 View,DOWN 事件时候 newTouchTarget 为 Null | |
| 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); | |
| // 很重要的一个方法, 在下面我们会看这个方法做了什么 | |
| 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(); | |
| // 在这个方法中把 child 赋值给了 mFirstTouchTarget | |
| newTouchTarget = addTouchTarget(child, idBitsToAssign); | |
| alreadyDispatchedToNewTouchTarget = true; | |
| break; | |
| } | |
| ev.setTargetAccessibilityFocus(false); | |
| } | |
| if (preorderedList != null) preorderedList.clear();} | |
| ... | |
| } | |
| } | |
| // 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; | |
| } | |
| } | |
| ... | |
| } | |
| ... | |
| return handled; | |
| } | |
| // 最重要的几行代码的逻辑 | |
| private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, | |
| View child, int desiredPointerIdBits) { | |
| final boolean handled; | |
| ... 省略部分代码 | |
| // Perform any necessary transformations and dispatch. | |
| //child 为空调用父 View 的 dispatchTouchEvent | |
| 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()); | |
| } | |
| //child 不为空调用 child 的 dispatchTouchEvent 方法 | |
| handled = child.dispatchTouchEvent(transformedEvent); | |
| } | |
| // Done. | |
| transformedEvent.recycle(); | |
| return handled; | |
| } | |
| // 默认返回 false | |
| 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; | |
| } | |
| //////////View | |
| public boolean dispatchTouchEvent(MotionEvent event) { | |
| ... | |
| boolean result = false; | |
| ... | |
| if (onFilterTouchEventForSecurity(event)) {if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {result = true;} | |
| //noinspection SimplifiableIfStatement | |
| ListenerInfo li = mListenerInfo; | |
| // 我们看到有 mOnTouchListener, 并且 onTouch 返回 true, 就没有 onTouchEvent 啥事了 | |
| if (li != null && li.mOnTouchListener != null | |
| && (mViewFlags & ENABLED_MASK) == ENABLED | |
| && li.mOnTouchListener.onTouch(this, event)) {result = true;} | |
| if (!result && onTouchEvent(event)) {result = true;} | |
| } | |
| ... | |
| return result; | |
| } | |
| public boolean onTouchEvent(MotionEvent event) {final float x = event.getX(); | |
| final float y = event.getY(); | |
| final int viewFlags = mViewFlags; | |
| final int action = event.getAction(); | |
| final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE | |
| || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) | |
| || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE; | |
| ... | |
| if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {switch (action) { | |
| case MotionEvent.ACTION_UP: | |
| ... | |
| break; | |
| case MotionEvent.ACTION_DOWN: | |
| ... | |
| break; | |
| case MotionEvent.ACTION_CANCEL: | |
| ... | |
| break; | |
| } | |
| return true; | |
| } | |
| return false; | |
| } |
我们梳理下 DOWN 事件的执行流程:
1 触发 BigGroup 的 dispatchTouchEvent()方法
2 给 intercepted 赋值,disallowIntercept 默认为 false, 这是就会触发 onInterceptTouchEvent() 方法,onInterceptTouchEvent 默认返回 false.intercepted 赋值为 false
3 接下来走到 if (!canceled && !intercepted)的逻辑里面去了, 注意注意 intercepted 为 true 的话这个方法直接就跳过去了. 在这段逻辑里遍历所有的子 View, 接下来进入淘汰机制,child 不能获得焦点, 下一个, 然后看看 View 不可见, 下一个, 没有点到 child 范围之内, 下一个. 剩下来的娃会执行 dispatchTransformedTouchEvent 方法
4 接下来 dispatchTransformedTouchEvent 中调用了 child 的 dispatchTouchEvent 方法
5 执行 dispatchTouchEvent 的 child 就是我们的 SmallGroup, 执行逻辑同 2 -4
6 这次执行 dispatchTouchEvent 的 child 就是我们的 TestView 了
7 TestView dispatchTouchEvent 中首先判断了有没有 onTouchListener, 判断 onTouch 方法的返回值. 我们肯定没写的了, 接下来会执行 onTouchEvent
8 在 TestView onTouchEvent 中我们看到如果这个 View 可点击就返回 true, 不可点击就返回 false.TestView 不可点击, 所以返回了 false
9 触发了连锁反应,TestView dispatchTouchEvent 返回 false
10SmallGroup dispatchTransformedTouchEvent 返回了 false, mFirstTouchTarget 为 null
11SmallGroup 再次执行 dispatchTransformedTouchEvent child 参数为 null, 执行 super.dispatchTouchEvent(),ViewGroup 的 super 最后就是 View, 所以会执行逻辑 7 -9, 不同的事主角事 SmallGroup
12BigGroup dispatchTransformedTouchEvent 返回了 false, mFirstTouchTarget 为 null, 重复步骤 11, 主角 BigGroup,DOWN 事件结束
mFirstTouchTarget 为 null 会有什么影响呢? 后续事件中 intercepted 为 true, 执行 dispatchTransformedTouchEvent child 参数为 null, 执行 super.dispatchTouchEvent(), 事件直接传递不下来了, 后续事件都接收不到了
所以 log 只能看到有关于 DOWN 事件的．
