view事件分发源码学习和分析
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activity是承载视图和事件的，所以我们从activity入手。和事件相关的三个方法：dispatchTouchEvent，onInterceptTouchEvent，onTouchEvent。
我们先看activity的dispatchTouchEvent：
/** * Called to process touch screen events. You can override this to * intercept all touch screen events before they are dispatched to the * window. Be sure to call this implementation for touch screen events * that should be handled normally. * * @param ev The touch screen event. * * @return boolean Return true if this event was consumed. */ public boolean dispatchTouchEvent(MotionEvent ev) { if (ev.getAction() == MotionEvent.ACTION_DOWN) { onUserInteraction(); } if (getWindow().superDispatchTouchEvent(ev)) { return true; } return onTouchEvent(ev); }先看一下一下api说明：
调用这个方法来处理屏幕的触摸事件，你可以重写这个方法在所有触摸事件交由window处理之前将它拦截。（这样看来activity的事件是要交给window去处理的）
这个方法先是判断是不是down事件，如果是，执行用户自定义的交互事件，这个onUserInteraction是个空方法，需要自己实现。然后交由当前activity所在window处理。
window类是一个虚类，看一下api的说明：
/**
* Abstract base class for a top-level window look and behavior policy. An
* instance of this class should be used as the top-level view added to the
* window manager. It provides standard UI policies such as a background, title
* area, default key processing, etc.
*
*The only existing implementation of this abstract class is
* android.view.PhoneWindow, which you should instantiate when needing a
* Window.
*/
window类是一个顶级窗口类，包含了显示和行为的处理策略。这个类的实例应该被用来作为添加到window manager的顶级view。它提供了标准UI策略例如背景，标题区域、默认key进程等。这个类提供的唯一实现类是android.view.PhoneWindow。所以我们找到PhoneWindow的superDispatchTouchEvent：
public boolean superDispatchTouchEvent(MotionEvent event) { return super.dispatchTouchEvent(event); }跟一下发现，调到了ViewGroup的dispatchTouchEvent。至此，触摸事件由activity－－》window－－》ViewGroup。
ViewGroup的分发处理：
public boolean dispatchTouchEvent(MotionEvent ev) { …. // 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(); }对于down事件，总是会清空FLAG_DISALLOW_INTERCEPT； // 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; } 这段代码是对是否已经拦截的判断。可以看出，只要不是down事件，inercepeted都会是true。如果是down事件或者mFirstTouchTarget不是null，去检查FLAG_DISALLOW_INTERCEPT标识位，如果允许拦截，就会去调用onInterceptTouchEvent，intercepted标识位取onInterceptTouchEvent，如果不允许拦截，intercepted为false，并存储action以防发生改变。intercepted不论从字面意思还是代码上都可以看出，是用来标识是否由当前view group拦截了的。mFirstTouchTarget是TouchTarget的一个实例，First touch target in the linked list of touch targets. 是touch target 单列表的第一个实例。TouchTarget是用来描述捕获到的可触摸view以及触摸点的id的类。 // 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); } 如果由当前viewGroup拦截，那么开启正常的事件分发；同样的如果已经有处理这个手势的view（有别的view处理了）也正常分发事件。 // 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 accessiiblity 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; 前面如果由当前viewGroup处理 ev.setTargetAccessibilityFocus(false);这里childWithAccessibilityFocus就会是null，否则就是找到具有处理能力的子view。 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); 移除早先的TouchTargets以防它们已经不是同步的了。 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 = buildOrderedChildList(); final boolean customOrder = preorderedList == null && isChildrenDrawingOrderEnabled(); final View[] children = mChildren; for (int i = childrenCount - 1; i >= 0; i--) { final int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i; final View child = (preorderedList == null) ? children[childIndex] : preorderedList.get(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; } 按一定的顺序（从前到后）遍历所有子 view，如果这个子 view 可以接收事件，就把childWithAccessibilityFocus置为null。其实就是过滤找出来这个能接收事件的view。 if (!canViewReceivePointerEvents(child) || !isTransformedTouchPointInView(x, y, child, null)) { ev.setTargetAccessibilityFocus(false); continue; } 又检查了一遍之前找到的child是否能接受手势事件(跟进去看，是检查child是否可见和child是否有动画存在可见或有动画都返回true)和是不是手指位置在这个子 view child中。不是的话就重新去找，并ev.setTargetAccessibilityFocus(false); 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; } 检查child是否正在接受手势，如果是，跳出遍历。 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(); }dispatchTransformedTouchEvent这个方法跟进去，发现一大堆东西，其实就是说了如果child为空，调用super.dispatchTouchEvent(event)，否则调用child.dispatchTouchEvent(event)。明显这里传的child不是null，所以调用的是child.dispatchTouchEvent(event)。如果child.dispatchTouchEvent(event)返回了true，就是说child处理了手势。那么就会alreadyDispatchedToNewTouchTarget设为true；并addTouchTarget，看看里面怎么写的：
/** * Adds a touch target for specified child to the beginning of the list. * Assumes the target child is not already present. */private TouchTarget addTouchTarget(View child, int pointerIdBits) { TouchTarget target = TouchTarget.obtain(child, pointerIdBits); target.next = mFirstTouchTarget; mFirstTouchTarget = target; return target;}可以看到，这里mFirstTouchTarget会被赋值，而且TouchTarget是个单链表结构，mFirstTouchTarget指向了之前找到的child。
继续跟viewGroup的dispatchTouchEvent方法：
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); 如果遍历了全部子元素还是没有找到可以处理的，就把自己视作普通view、 并且 dispatchTransformedTouchEvent 传了null， 里面就是调用super.dispatchTouchEvent(event)也就是view的dispatchTouchEvent。 } 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; 如果找到了处理的child，并且处理完了，直接handle设为true作为处理结果。 } 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; } } 上面这段是遍历touchTarget 单链表，对每个接收事件的child 进行分发（dispatchTransformedTouchEvent） // 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; } 总结一下viewGroup的分发：
1. down事件都会重置FLAG_DISALLOW_INTERCEPT标识，而FLAG_DISALLOW_INTERCEPT子view可以通过requestDisallowInterceptTouchEvent 设置，来控制viewGroup父类是否执行onInterceptTouchEvent（）；所以requestDisallowInterceptTouchEvent方法对down事件不起作用。如果requestDisallowInterceptTouchEvent设置为true，那么除了down事件其他事件都不会拦截。所以requestDisallowInterceptTouchEvent应该写在子类的down事件中（此时父类down已经处理了），在move和up到来时起作用。
2. viewGroup父类在down事件一定会去调用onInterceptTouchEvent判断是不是需要拦截。
3. viewGroup父类重写onInterceptTouchEvent，down事件不能返回true，就是说down事件不能被拦截。因为从代码上看，需要在down事件去遍历子元素，找到分发给哪个child处理，如果intercepted为true，都不会走这段代码，导致一直都找不到可以接收事件的子元素。这也是为什么除了down事件其他都直接返回intercepted 为true，因为在move和up事件，mFirstTouchTarget不会为null。并且cancelChild一直都是true，move和up事件通过dispatchTransformedTouchEvent下发。
接下来就是view的分发，因为无论是super.dispatchTouchEvent还是child.dispatchTouchEvent都是调用的view：
/** * Pass the touch screen motion event down to the target view, or this * view if it is the target. * * @param event The motion event to be dispatched. * @return True if the event was handled by the view, false otherwise. */ public boolean dispatchTouchEvent(MotionEvent event) { // If the event should be handled by accessibility focus first. if (event.isTargetAccessibilityFocus()) { // We don't have focus or no virtual descendant has it, do not handle the event. if (!isAccessibilityFocusedViewOrHost()) { return false; } // We have focus and got the event, then use normal event dispatch. event.setTargetAccessibilityFocus(false); } 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(); }如果是down事件，停止折叠滚动 if (onFilterTouchEventForSecurity(event)) { //noinspection SimplifiableIfStatement ListenerInfo li = mListenerInfo; if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED && li.mOnTouchListener.onTouch(this, event)) { result = true; }如果view可以点击且有onTouchListener且onTouch方法返回true；result设为true； if (!result && onTouchEvent(event)) { result = true; } }如果onTouchEvent返回true；result设为true。 if (!result && mInputEventConsistencyVerifier != null) { mInputEventConsistencyVerifier.onUnhandledEvent(event, 0); } // Clean up after nested scrolls if this is the end of a gesture; // also cancel it if we tried an ACTION_DOWN but we didn't want the rest // of the gesture. if (actionMasked == MotionEvent.ACTION_UP || actionMasked == MotionEvent.ACTION_CANCEL || (actionMasked == MotionEvent.ACTION_DOWN && !result)) { stopNestedScroll(); }up、取消或者result为false情况下的down事件都会停止nestScroll。 return result; }相比viewGroup，view的分发简单一些。可以看到，响应的顺序是onTouchListener－－》onTouchEvent。如果在onTouchListener的onTouch方法中返回true，那么这个view的onTouchEvent方法不会调用。
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