android开发分享Android 事件分化机制，看这一篇就够了

争取事无巨细的把事件分发机制搞明白。

1、四个主要方法
dispatchTouchEvent，onInterceptTouchEvent，dispatchTransformedTouchEvent，onTouchEvent。

dispatchTouchEvent：此为最先调用的方法，从最外层的ViewGroup开始调用。实际上从最最开始的ViewGroup开始调用的。从Activity的方法开始调用。

public boolean dispatchTouchEvent(MotionEvent ev) {     if (ev.getAction() == MotionEvent.ACTION_DOWN) {  //没啥用，给了我们一个加入执行代码的方法。 onUserInteraction（）可重写。         onUserInteraction();     }     if (getWindow().superDispatchTouchEvent(ev)) {         return true;     }     return onTouchEvent(ev); }

getWindow().superDispatchTouchEvent(ev)  一切故事开始的地方。后面的逻辑从此展开，非常之长。需要耐心细心。

首先 getWindow()，获取的是window的实例。目前android 框架只有 android.view.PhoneWindow 。如何初始化赋值的暂时不说。后续写 android的布局系统再说。这篇主要集中事件分化机制。

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

mDecor DecorView属于一个页面的最外层容器，这个是系统给你添加的。

  @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        final Window.Callback cb = mWindow.getCallback();
        return cb != null && !mWindow.isDestroyed() && mFeatureId < 0
                ? cb.dispatchTouchEvent(ev) : super.dispatchTouchEvent(ev);
    }

   /** The feature ID of the panel, or -1 if this is the application's DecorView */
    private final int mFeatureId;

从这个解释可以看出来，除开系统的window。其余的要走super.dispatchTouchEvent(ev)。也就是ViewGroup 的dispatchTouchEvent。现在终于到关键的方法了。前面走了这么多，从Activity的dispatchTouchEvent开始，其实从最初的MotionEvent 开始到Actvitiy的 dispatchTouchEvent也有不少逻辑，等我们有空再慢慢琢磨吧。

我不打算把这个方法进行精简了，因为简洁的代码会少了需要原汁原味。开始。

@Override 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 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);                          // 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; }

看起来有点长实际上有好多代码逻辑并不是需要我们重点关注的，我们只需要粗略浏览一下。

if (mInputEventConsistencyVerifier != null) {     mInputEventConsistencyVerifier.onTouchEvent(ev, 1); }

  /**
     * Consistency verifier for debugging purposes.
     * @hide
     */
    protected final InputEventConsistencyVerifier mInputEventConsistencyVerifier =
            InputEventConsistencyVerifier.isInstrumentationEnabled() ?
                    new InputEventConsistencyVerifier(this, 0) : null;

此方法貌似是用于调试的目的，跟我们分析的程序逻辑不产生影响。此为View中的变量。

  // 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);
        }

已经有注释了，无障碍辅助服务 这个特性的产生导致好多源码都有这块处理逻辑。等有时间也好好扒拉一下吧。

boolean handled = false;

//此变量对逻辑理解很重要。这个是标识能够消费MotionEvent的View的布尔值。

首先判断onFilterTouchEventForSecurity(ev)，就是判断事件是否合法。我们分析的都是合法的事件，所以进入。

 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();
            }

//down事件的地位比较特殊，代表一个事件的开头，主要作用是将mFirstTouchTarget 置空，对于ViewGroup和View来说是个非常重要的变量。down事件可以说主要是生成mFirstTouchTarget。后面几个关键逻辑是围绕mFirstTouchTarget 的next，child变量做文章。了解了这个基本后续很容易弄懂了。

// 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; }

//这里主要是看下是否拦截down事件。这里我分析下。如果拦截和不拦截的区别。

如果拦截，down事件不会向下面分发了。直接走到这里。直接交给了他的super去分发。所以我们理解拦截并不是直接就给自己了，还是给到自己去分发这个也很关键。每个ViewGroup的分化都是走这个过程的。

if (mFirstTouchTarget == null) {     // No touch targets so treat this as an ordinary view.     handled = dispatchTransformedTouchEvent(ev, canceled, null,             TouchTarget.ALL_POINTER_IDS); }

//如果不拦截，那么逻辑就比较复杂了。先是down事件，向children进行分化，我们看下关键的方法。

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; }

这个方法里面的if，dispatchTransformedTouchEvent这个方法实际上是对子类进行进一步分化。可以看下。这个方法的具体实现，这两个方法组成了事件分发机制。我去掉了非主干逻辑。

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,         View child, int desiredPointerIdBits) {     final boolean handled;      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);     }      return handled; }

//可以看到， handled = child.dispatchTouchEvent(event);具体看这句，如果这个child任然是个ViewGroup会继续向

children分发，如果是View，那么就要看这个View是否消费，这里有个很重要的概念。消费事件的首先都是给自己的子类View，实在子类View没有消费者。那么就给自己消费。这个逻辑后面有代码论证。

如果返回true，那么newTouchTarget = addTouchTarget(child, idBitsToAssign); alreadyDispatchedToNewTouchTarget = true;然后父类层层返回，直至最终的父类，也就是说被消费down事件这条线的mFirstTouchEvent都不会为空。并且chid指向被消费的child。这个概念非常重要，因为后续up事件就是根据这个来分化的。我们来看看up事件分发。

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;             }

}

我已经去掉了不重要的逻辑。第一个mFirstTouchTarget==null是什么情况呢，就是我刚说的子类View都不消费，那么给到自己的父类super来分发。

如果是找到了mFirstTouchTarget，那么

if (dispatchTransformedTouchEvent(ev, cancelChild, target.child, target.pointerIdBits)) { handled = true; }

就会对child进行分发，看下child是否消费up事件。刚刚说了。如果down事件你不被消费，不好意思，up事件你也没戏。原因为在于dispatchTransformedTouchEvent，里面的分发逻辑就是根据mFirstTouch的child来分发的。每个child有个mFirstTouch,每个mFirstTouch有个child，这个是一条链条，这也是down事件消费的链条，通过这个链条来传递up和move等事件。

好了，到这里，Android事件分发机制就讲完了，需要对里面的几个核心概念仔细琢磨，才能体会到上面两个方法组合的妙处。不然你光看if和esle肯定会头晕。