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 downfinal 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 indexfor (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 indexfor (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。