Android中自定义view中事件分发机制与处理的示例分析
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题引
事件只有一个，多个人想要处理，处理的对象不是我们想给的对象就是事件冲突。
如上图，RecyclerView 的父布局是ViewPager，左右滑动时没问题，上下滑动时RecyclerView好像没收到滑动事件一样，无法达到我们预期的效果。我们的触摸被封装成MotionEvent事件传递，在多个层级中它是如何传递的呢？又是根据什么来确定哪个view处理这个事件的呢，咱们抽丝剥茧一步步揭开她的面纱！
Activity对事件的分发过程
追溯本源，寻找事件分发的开始。
当一个点击操作发生时，事件最先传递给当前的Activity，由Activity的dispatchTouchEvent进行分发
public boolean dispatchTouchEvent(MotionEvent ev) { if (ev.getAction() == MotionEvent.ACTION_DOWN) { onUserInteraction(); } if (getWindow().superDispatchTouchEvent(ev)) { return true; } return onTouchEvent(ev); }这里的getWindow返回的window类只有一个实现，PhoneWindow
private DecorView mDecor public boolean superDispatchTouchEvent(MotionEvent event) { return mDecor.superDispatchTouchEvent(event); }我们继续看 DecorView的superDispatchTouchEvent方法实现
public boolean superDispatchTouchEvent(MotionEvent event) { return super.dispatchTouchEvent(event); }DecorView 继承于 ViewGroup。此时应该理解了，Activity 的 事件分发交给了 DecorView 处理，而 DecorView 又是什么
DecorView是activity窗口的根视图，是一个FrameLayout，DecorView内部又分为两部分，一部分是ActionBar，另一部分是ContentParent，即activity在setContentView对应的布局。如此一来，事件分发从系统层面开始向我们写的布局分发事件！
事件分发是一个递归的过程，主要涉及三个函数
dispatchTouchEvent
onInterceptTouchEvent
onTouchEvent
三者关系
public boolean dispatchTouchEvent(MotionEvent ev){ boolean result = false; if(onInterceptTouchEvent(ev)){ // 如果拦截则交给自己的 onTouchEvent 处理事件 result = onTouchEvent(ev); }else{ // 如果不拦截，交给子布局分发，这是个层层递归过程 result = chlid.dispatchTouchEvent(ev); } return result;}直接撸源码是一件很痛苦的事情，多种可能的发生让源码可读性很差。下面我们会从某一种特定逻辑下分析，这样会清晰很多。每次只分析一种情境！
父布局拦截的分发处理过程
父布局拦截我们分两步，ACTION_DOWN、ACTION_MOVE
ACTION_DOWN 事件
进入 ViewGroup 的 dispatchTouchEvent 方法内
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(); }因为是 ACTION_DOWN 事件，先清空状态，一个是TouchTarget的状态，一个是 mGroupFlags。这个用不到继续走
// Check for interception.final boolean intercepted;// 因为是第一次过来 mFirstTouchTarget = null ，且是 ACTION_DOWN 事件，走入 if 内if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { // 咱们走的是父布局拦截事件，子布局用尚方宝剑，disallowIntercept =false final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { // 咱们在这拦截， intercepted = true 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; }代码注释的比较全，这边主要是判断本view是否拦截，如果拦截 intercepted = true 。所以后面的遍历子view分发都进不去
// intercepted = true , 进不去if (!canceled && !intercepted) { // 这是一个遍历子 view 接盘的故事 for (int i = childrenCount - 1; i >= 0; i--) { }}一直往下走
// 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);}符合这个条件，深入 dispatchTransformedTouchEvent 函数，第三个参数是 null
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, View child, int desiredPointerIdBits) { final boolean handled; if (child == null) { // ===== 执行位置 ==== 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); }}第三个参数传的是 null ，即 child = null 。调用 super 的 dispatchTouchEvent 。ViewGroup 的 super 即是 View。
handled = View.dispatchTouchEvent(event);
深入 View 的 dispatchTouchEvent 方法，主要处理逻辑是下面两段代码
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; }我们可以得出结论：onTouch 比 onTouchEvent 优先级高，如果onTouch 拦截事件则 onTouchEvent 无法接到事件。这也是为什么我们在onTouch方法返回true后onClick事件失效的原因。onTouchEvent 的逻辑比较简单，此处不做分析
这里要说明一点，事件分发机制的分发其实有两种含义。一是事件在不同view之间的分发，父布局到子布局的分发；二是事件在view中对不同监听的分发，onTouch、onClick、onLongClick 在分发时也是有顺序的。
到这里父布局拦截的DOWN事件算结束了，下面是MOVE事件，继DOWN后的滑动，这是个连续的过程
ACTION_MOVE 事件
手指点击后开始滑动，继续分发move事件
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;}actionMasked = ACTION_MOVE ，mFirstTouchTarget = null ，直接走 else 模块，即 intercepted = true。
// intercepted = true , 进不去if (!canceled && !intercepted) { for (int i = childrenCount - 1; i >= 0; i--) { }}同样的分发子view的模块我们依旧进不去
// 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);}到这里就跟之前的逻辑完全一样了，至此父布局的拦截过程结束
总结：
ViewGroup 的 onTouchEvent 方法直接调用父类（View）的实现
父布局一旦拦截Down事件，后续的move事件都直接由父布局执行
这么分析的好处是咱们的状态是确定的，分析代码不会有太多可能性搞乱逻辑，下面是父布局不拦截的情况下事件分发
父布局不拦截时的分发处理过程
父布局不拦截，咱们按照正常流程走一遍，还是按上面那个思路，先 down 后 move
ACTION_DOWN
进入 GroupView 的 dispatchTouchEvent 方法后依旧西先是清空状态，然后判断当前布局是否拦截
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;}咱们的设定是不拦截，所以 intercepted = false。下面是遍历子view的代码
final View[] children = mChildren;for (int i = childrenCount - 1; i >= 0; i--) { final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder); // 逆序拿到一个 child ，即从最上层的子view开始往内层遍历 final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex); // 判断触点的位置是否在view的范围之内或者view是否在播放动画，如果都不满足则直接遍历下一个 if (!child.canReceivePointerEvents()|| !isTransformedTouchPointInView(x, y, child, null)) { 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); // dispatchTransformedTouchEvent 函数是处理分发的函数，父布局处理用的也是这个 // 如果子view消费了事件则给标志位赋值，并 break 结束循环，如果没有消费则继续循环寻找分发 if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) { 注释1 // 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(); // 如果子view消费了事件则给 alreadyDispatchedToNewTouchTarget 和 mFirstTouchTarget 赋值 // 保存 child newTouchTarget = addTouchTarget(child, idBitsToAssign); 注释2 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);}分析上面干了啥
从最上层的子view开始往内层遍历
判断当前的view在位置上是否满足触点位置
调用 dispatchTransformedTouchEvent 判断是否子view消费了事件
如果消费了事件则记录 mFirstTouchTarget 和标志位，并跳出循环
如果没有没有消费事件则继续循环
注释1的逻辑 dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, View child, int desiredPointerIdBits) { final boolean handled; 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()); } // ===== 执行位置 ==== handled = child.dispatchTouchEvent(transformedEvent); }}这次过来 child != null ，调用的是 child.dispatchTouchEvent(event) 。child 可能是View，也可能是 ViewGroup。如果是 ViewGroup 又是一个递归的过程 。层层的递归返回 handled 告诉父布局是否消费了事件！
再看注释2的逻辑
private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) { final TouchTarget target = TouchTarget.obtain(child, pointerIdBits); // 此时 mFirstTouchTarget = null target.next = mFirstTouchTarget; mFirstTouchTarget = target; return target; }给 mFirstTouchTarget 赋值，下次 move 事件过来时 mFirstTouchTarget 就是有值的了！！即
target.next = null
mFirstTouchTarget = newTouchTarget
保存 child 在 target 中
至此 ACTION_DOWN 事件结束
ACTION_MOVE
继上面点击后开始滑动
if (actionMasked == MotionEvent.ACTION_DOWN) { cancelAndClearTouchTargets(ev); resetTouchState(); }move事件不会重置，继续走
if (actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != null)
记得down事件中给mFirstTouchTarget 赋过值嘛，虽然不是down事件依旧可以进入此方法。也就是说这里依旧会判断父布局是否要拦截子view，这里也是以后咱们处理事件冲突的重点。当前的逻辑是不拦截，所以 intercepted = false
if (actionMasked == MotionEvent.ACTION_DOWN || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN) || actionMasked == MotionEvent.ACTION_HOVER_MOVE)
只有ACTION_DOWN事件才会进行分发，所以不会进入遍历子view的逻辑代码！MOVE事件不会分发事件！
// mFirstTouchTarget 有值，走else模块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; // alreadyDispatchedToNewTouchTarget 是 false if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) { handled = true; } else { // 此处的结果是 false final boolean cancelChild = resetCancelNextUpFlag(target.child)|| intercepted; // 在这里被分发处理 child就是我们要分发的对象 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; }}alreadyDispatchedToNewTouchTarget 在每次进来时都会重置为 false ，最后又会调用 dispatchTransformedTouchEvent 处理分发
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()); } // 递归调用它来分发 handled = child.dispatchTouchEvent(transformedEvent);}至此move事件也结束，做个总结
DOWN 事件是事件分发，寻找接盘的 child 并保存在 mFirstTouchTarget 中
MOVE 事件虽然不需要遍历寻找接盘的view，但还可以被ViewGroup拦截的（比如ViewPager包裹着RecyclerView，DOWN事件时被RecyclerView拦截，横向滑动时被抛弃，这时候ViewPager是可以拦截横向滑动接盘的）
解决冲突方案
滑动冲突解决方案有两种：内部拦截、外部拦截。顾名思义，内部拦截是在子View中写逻辑拦截，外部拦截则是从父布局下手解决问题
都以ViewPager包裹RecyclerView滑动冲突为例
外部拦截
public class BadViewPager extends ViewPager { private int mLastX, mLastY; public BadViewPager(@NonNull Context context) { super(context); } public BadViewPager(@NonNull Context context, @Nullable AttributeSet attrs) { super(context, attrs); } // 外部拦截法：父容器处理冲突 // 我想要把事件分发给谁就分发给谁 @Override public boolean onInterceptTouchEvent(MotionEvent event) { int x = (int) event.getX(); int y = (int) event.getY(); switch (event.getAction()) { case MotionEvent.ACTION_DOWN: { mLastX = (int) event.getX(); mLastY = (int) event.getY(); break; } case MotionEvent.ACTION_MOVE: { int deltaX = x - mLastX; int deltaY = y - mLastY; if (Math.abs(deltaX) > Math.abs(deltaY)) { // 横向滑动时拦截 return true; } break; } case MotionEvent.ACTION_UP: { break; } default: break; } return super.onInterceptTouchEvent(event); }}内部拦截
ViewPager 代码
public class BadViewPager extends ViewPager { private int mLastX, mLastY; public BadViewPager(@NonNull Context context) { super(context); } public BadViewPager(@NonNull Context context, @Nullable AttributeSet attrs) { super(context, attrs); } @Override public boolean onInterceptTouchEvent(MotionEvent event) { if (event.getAction() == MotionEvent.ACTION_DOWN){ super.onInterceptTouchEvent(event); // 此处是重点 return false; } return true; }}RecyclerView 代码
public class MyListView extends ListView { public MyListView(Context context) { super(context); } public MyListView(Context context, AttributeSet attrs) { super(context, attrs); } // 内部拦截法：子view处理事件冲突 private int mLastX, mLastY; @Override public boolean dispatchTouchEvent(MotionEvent event) { int x = (int) event.getX(); int y = (int) event.getY(); switch (event.getAction()) { case MotionEvent.ACTION_DOWN: { getParent().requestDisallowInterceptTouchEvent(true); break; } case MotionEvent.ACTION_MOVE: { int deltaX = x - mLastX; int deltaY = y - mLastY; if (Math.abs(deltaX) > Math.abs(deltaY)) { getParent().requestDisallowInterceptTouchEvent(false); } break; } case MotionEvent.ACTION_UP: { break; } default: break; } mLastX = x; mLastY = y; return super.dispatchTouchEvent(event); }}此处一定要注意，父布局在 ACTION_DOWN 时一定要返回false。原因如下：
当分发DOWN事件时，执行了 resetTouchState(); 函数
private void resetTouchState() { clearTouchTargets(); resetCancelNextUpFlag(this); mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT; mNestedScrollAxes = SCROLL_AXIS_NONE; }mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT
在判断父布局拦截时
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;}即 mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT & FLAG_DISALLOW_INTERCEPT ！= 0 ==》false
使用 if 语句永远是true，在这里ViewPager会拦截事件，所以RecyclerView无法上下滑动。所以内部拦截时要修改父布局的 onInterceptTouchEvent 函数！
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