Android事件分发机制源码分析之ViewGroup篇
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上一篇分析了Android事件分发机制源码分析之View篇 ，按照计划我们这篇分析一下关于ViewGroup的事件分发。
那么我们首先要清楚理解一下View和ViewGroup之间的关系，我选取LinearLayout为例子，我们看一下LinearLayout的继承关系：
从图中看出的继承关系是，LinearLayout是继承ViewGroup,而ViewGroup是继承View，View则是继承我们的所以类的基类Object。那就是说，我们平时使用的LinearLayout、RelativeLayout等等常用Layout都是继承ViewGroup的，Button、TextView是继承View的，但是ViewGroup实际也是一个View。那么我们现在结合一下Demo来分析一下ViewGroup事件分发源码。
示例代码
我们接着用回上一篇的CustomButton.class，本篇添加一个CustomLayout.class.
public class CustomLayout extends LinearLayout { private static final String TAG = "CustomLayout"; public CustomLayout(Context context, AttributeSet attrs) { super(context, attrs); } public CustomLayout(Context context) { super(context); } @Override public boolean dispatchTouchEvent(MotionEvent ev) { int action = ev.getAction(); switch (action) { case MotionEvent.ACTION_DOWN: Log.i(TAG, "dispatchTouchEvent ACTION_DOWN"); break; case MotionEvent.ACTION_MOVE: Log.i(TAG, "dispatchTouchEvent ACTION_MOVE"); break; case MotionEvent.ACTION_UP: Log.i(TAG, "dispatchTouchEvent ACTION_UP"); break; default: break; } boolean b = super.dispatchTouchEvent(ev); Log.i(TAG, "dispatchTouchEvent return " + b); return b; } @Override public boolean onTouchEvent(MotionEvent event) { int action = event.getAction(); switch (action) { case MotionEvent.ACTION_DOWN: Log.i(TAG, "onTouchEvent ACTION_DOWN"); break; case MotionEvent.ACTION_MOVE: Log.i(TAG, "onTouchEvent ACTION_MOVE"); break; case MotionEvent.ACTION_UP: Log.i(TAG, "onTouchEvent ACTION_UP"); break; default: break; } boolean b = super.onTouchEvent(event); Log.i(TAG, "onTouchEvent return " + b); return b; } @Override public boolean onInterceptTouchEvent(MotionEvent event) { int action = event.getAction(); switch (action) { case MotionEvent.ACTION_DOWN: Log.i(TAG, "onInterceptTouchEvent ACTION_DOWN"); break; case MotionEvent.ACTION_MOVE: Log.i(TAG, "onInterceptTouchEvent ACTION_MOVE"); break; case MotionEvent.ACTION_UP: Log.i(TAG, "onInterceptTouchEvent ACTION_UP"); break; default: break; } boolean b = super.onInterceptTouchEvent(event); Log.i(TAG, "onInterceptTouchEvent return " + b); return b; }}我是继承常用的LinearLayout，然后重写和打印部分事件分发代码。
然后看一下布局文件：
<?xml version="1.0" encoding="utf-8"?><RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="horizontal"> <com.example.pc.myapplication.CustomLayout android:layout_width="match_parent" android:layout_height="match_parent"> <com.example.pc.myapplication.CustomButton android:id="@+id/button" android:layout_width="100dip" android:layout_height="100dip" android:text="button" /> </com.example.pc.myapplication.CustomLayout></RelativeLayout>一个充满屏幕的CustomLayout里面包含着一个固定宽高的CustomButton。直接编译运行我们的程序，看看打印台打印情况：
点击非按钮范围的Layout：
点击CustomButton按钮：
从上一篇的 Android事件分发机制源码分析之View篇 中我们说过在ViewGroup的分析中会涉及到onInterceptHoverEvent()，我可以从打印台中看出ViewGroup的事件分发的不同。现在我们从源码中，探究一下原因。
源码分析。依据Android API 23 Platform
我们按照打印台的打印顺序来先分析一个ViewGroup的dispatchTouchEvent()。由于源码过长，我们分段分析。
最开始第一步只是一些对View是否可以获得焦点的判断、设置标志位以及初始化一些布尔值，并且在ACTION_DOWN事件产生的时候清楚以外的状态并且准备开始新一轮的手势操作，不是我们的重要；我们看下面的代码：
··· // 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; } ···这里第二步设置一个intercepted的布尔值去判断当前ViewGroup是否要进行拦截事件，在后面的if条件中，第一个是判断是否按下事件，第二个是mFirstTouchTarget！=null，然而mFirstTouchTarget是什么呢？我们找一下涉及这个变量的代码。
private void cancelAndClearTouchTargets(MotionEvent event) { if (mFirstTouchTarget != null) { boolean syntheticEvent = false; if (event == null) { final long now = SystemClock.uptimeMillis(); event = MotionEvent.obtain(now, now, MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0); event.setSource(InputDevice.SOURCE_TOUCHSCREEN); syntheticEvent = true; } for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) { resetCancelNextUpFlag(target.child); dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits); } clearTouchTargets(); if (syntheticEvent) { event.recycle(); } } }······ /** * Clears all touch targets. */ private void clearTouchTargets() { TouchTarget target = mFirstTouchTarget; if (target != null) { do { TouchTarget next = target.next; target.recycle(); target = next; } while (target != null); mFirstTouchTarget = null; } }从上面的源码可以看到，cancelAndClearTouchTargets()这方法是对mFirstTouchTarget的清空，这个方法也是在dispatchTouchEvent()的开头且是//Check for interception的判断之前的部分代码，这里我们知道了ViewGroup的dispatchTouchEvent()一开始是对mFirstTouchTarget的这个变量作一个置空处理的。那么mFirstTouchTarget的赋值呢？我们继续找：
/** * 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; }好了，我们找到了mFirstTouchTarget的赋值部分，从它的封装的方法了解到是处理获得子View并赋值的。而为什么要有if(actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null)这个判断的呢？这里解释一下，比如说，子View消耗了ACTION_DOWN事件，然后这里可以由ViewGroup继续判断是否要拦截接下来的ACTION_MOVE事件之类的；又比如说，如果第一次DOWN事件最终不是由子View消耗掉的，那么显然mFirstTouchTarget将为null，所以也就不用判断了，直接把intercept设置为true，此后的事件都是由这个ViewGroup处理。
我们回到//Check for interception的代码中的 final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;第二个值是常量，第一个值在一开始的resetTouchState()有设置，同样我们可以对mGroupFlags变量的搜索可以找到public的设置方法requestDisallowInterceptTouchEvent()可以对mGroupFlags变量进行修改。看看它的源码:
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) { if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) { // We're already in this state, assume our ancestors are too return; } if (disallowIntercept) { mGroupFlags |= FLAG_DISALLOW_INTERCEPT; } else { mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT; } // Pass it up to our parent if (mParent != null) { mParent.requestDisallowInterceptTouchEvent(disallowIntercept); } }从方法名中我们看出它是请求不允许拦截触摸事件。在示例代码中我们也有重写这个方法，说明这个方法是可以设置我们的ViewGroup是否要拦截这个触摸事件的。如果没有重写去设置requestDisallowInterceptTouchEvent方法，那么程序就会继续往下走到调用onInterceptTouchEvent(ev)方法，我们找到这个方法：
/** * Implement this method to intercept all touch screen motion events. This * allows you to watch events as they are dispatched to your children, and * take ownership of the current gesture at any point. * * <p>Using this function takes some care, as it has a fairly complicated * interaction with {@link View#onTouchEvent(MotionEvent) * View.onTouchEvent(MotionEvent)}, and using it requires implementing * that method as well as this one in the correct way. Events will be * received in the following order: * * <ol> * <li> You will receive the down event here. * <li> The down event will be handled either by a child of this view * group, or given to your own onTouchEvent() method to handle; this means * you should implement onTouchEvent() to return true, so you will * continue to see the rest of the gesture (instead of looking for * a parent view to handle it). Also, by returning true from * onTouchEvent(), you will not receive any following * events in onInterceptTouchEvent() and all touch processing must * happen in onTouchEvent() like normal. * <li> For as long as you return false from this function, each following * event (up to and including the final up) will be delivered first here * and then to the target's onTouchEvent(). * <li> If you return true from here, you will not receive any * following events: the target view will receive the same event but * with the action {@link MotionEvent#ACTION_CANCEL}, and all further * events will be delivered to your onTouchEvent() method and no longer * appear here. * </ol> * * @param ev The motion event being dispatched down the hierarchy. * @return Return true to steal motion events from the children and have * them dispatched to this ViewGroup through onTouchEvent(). * The current target will receive an ACTION_CANCEL event, and no further * messages will be delivered here. */ public boolean onInterceptTouchEvent(MotionEvent ev) { return false; }这里注释很长，但是代码就很短，就一个返回值，这里返回False，说明ViewGrup默认不拦截点击事件，事件可以继续往下传递。另外说明了在ViewGroup中，调用dispatchTouchEvent()–>onInterceptTouchEvent()的顺序。
接下来的第三步源码，是检测cancel,通过标记和action检查cancel，然后将结果赋值给局部boolean变量canceled。不是我们的重点。
继续走：
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;首先可以看见获取一个boolean变量标记split来标记，默认是true，作用是是否把事件分发给多个子View，这个同样在ViewGroup中提供了public的方法setMotionEventSplittingEnabled()设置，如下：
public void setMotionEventSplittingEnabled(boolean split) { // TODO Applications really shouldn't change this setting mid-touch event, // but perhaps this should handle that case and send ACTION_CANCELs to any child views // with gestures in progress when this is changed. if (split) { mGroupFlags |= FLAG_SPLIT_MOTION_EVENTS; } else { mGroupFlags &= ~FLAG_SPLIT_MOTION_EVENTS; } }第四步事件分发。
//如果没有取消当前动作并且ViewGroup未拦截事件，那么事件就传递到接收了该点击事件的View 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; //是否ACTION_DOWN的事件 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; //判断了childrenCount个数是否不为0 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. //拿到了子View的list集合preorderedList final ArrayList<View> preorderedList = buildOrderedChildList(); final boolean customOrder = preorderedList == null && isChildrenDrawingOrderEnabled(); final View[] children = mChildren; //倒序遍历所有的子view 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. //寻找能够获得焦点的Children(View) if (childWithAccessibilityFocus != null) { if (childWithAccessibilityFocus != child) { continue; } childWithAccessibilityFocus = null; i = childrenCount - 1; } if (!canViewReceivePointerEvents(child) || !isTransformedTouchPointInView(x, y, child, null)) { ev.setTargetAccessibilityFocus(false); continue; } //查找当前子View是否在mFirstTouchTarget.next这条target链中的某一个targe中，如果在则返回这个target，否则返回null。 //在这段代码的if判断通过说明找到了接收Touch事件的子View（消耗了这个Touch事件），即newTouchTarget，那么，既然已经找到了，所以执行break跳出for循环。 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); //正常的如果找不到newTouchTarget，即是这个Touch事件还没有被子View消耗，有相应记录的TouchTarget，即调用dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)传递给子View，然后根据返回值去判断子View是否消耗的这个Touch事件 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(); //将接收这个事件的Child(View)赋值给mFirstTouchTarget，并将原来的mFirestTouchTarget给TouchTarget列表的下一个，并且跳出这个循环，因为事件被Child(View)消耗了，所以newTouchTarget依然也有值 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(); } //该种情况是，如果事件不是ACTION_DOWN,mFisrtTarget就不会被重置为null,但是如果要newTouchTarget有值，必须是事件在Child(View)里面被消耗了，简单概括事件被分下去了，，却没被消耗，于是就把事件添加进最近的TouchTarget里面去了 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; } } }普通的代码注释我直接写在上面，我们重点开一下dispatchTransformedTouchEvent()。其实dispatchTransformedTouchEvent()将Touch事件传递给特定的子View，它实际上在内部是调用了子元素的disPatchTouchEvent方法，找一下它的源码：
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, View child, int desiredPointerIdBits) { final boolean handled; // Canceling motions is a special case. We don't need to perform any transformations // or filtering. The important part is the action, not the contents. final int oldAction = event.getAction(); //传进来的cancel值为false且不是取消的动作，所以不执行这里的代码 if (cancel || oldAction == MotionEvent.ACTION_CANCEL) { event.setAction(MotionEvent.ACTION_CANCEL); if (child == null) { handled = super.dispatchTouchEvent(event); } else { handled = child.dispatchTouchEvent(event); } event.setAction(oldAction); return handled; } // Calculate the number of pointers to deliver. final int oldPointerIdBits = event.getPointerIdBits(); final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits; // If for some reason we ended up in an inconsistent state where it looks like we // might produce a motion event with no pointers in it, then drop the event. if (newPointerIdBits == 0) { return false; } // If the number of pointers is the same and we don't need to perform any fancy // irreversible transformations, then we can reuse the motion event for this // dispatch as long as we are careful to revert any changes we make. // Otherwise we need to make a copy. final MotionEvent transformedEvent; 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); } else { transformedEvent = event.split(newPointerIdBits); } // Perform any necessary transformations and dispatch. //如果子View为空就会调用View的dispatchTouchEvent方法去处理 if (child == null) { // 1 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()); } //子View是ViewGroup就调用ViewGroup,如果子View是View就调用View的 handled = child.dispatchTouchEvent(transformedEvent); } // Done. transformedEvent.recycle(); return handled; }看我标记了1的部分代码，如果传递的child为null就调用父类的dispatchTouchEvent否则就调用子类的dispatchTouchEvent，而上面的代码中child不为null，所以执行子元素的dispatchTouchEvent，这里完成了ViewGroup到子View的事件传递。验证了super.dispatchTouchEvent()–>onInterceptTouchEvent()–>child.dispatchTouchEvent()的顺序。
如果子元素的dispatchTouchEvent返回的是true，那么调用dispatchTransformedTouchEvent这个方法内部的for循环就不会继续下去,直接break跳出了循环，因为已经找到了处理事件的子View，那就无需继续遍历了。而代码将会执行如下：
mLastTouchDownX = ev.getX(); mLastTouchDownY = ev.getY(); newTouchTarget = addTouchTarget(child, idBitsToAssign); alreadyDispatchedToNewTouchTarget = true; break;这里的addTouchTarget()就是我们上面提及过的方法，因为这里一个子View能够消耗事件，那么mFirstTouchTarget会指向子View，
但是如果子元素的dispatchTouchEvent返回的是false，那么当前ViewGroup会把点击事件传递给下一个子元素进行处理，执行for循环查找下一个子元素。如果子View都不能消耗事件，那么mFirstTouchTarget将为null。
接着dispatchTouchEvent()的代码，这部分是非ACTION_DOWN的事件将从以下开始处理：
// 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);// 1 } 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)) {// 2 handled = true; } if (cancelChild) { if (predecessor == null) { mFirstTouchTarget = next; } else { predecessor.next = next; } target.recycle(); target = next; continue; } } predecessor = target; target = next; } }首先是一个if判断语句，判断mFirstTouchTarget是否为Null，如果为null，那么调用1处的代码：dispatchTransformedTouchEvent(ev,canceled,null,TouchTarget.ALL_POINTER_IDS)，这个方法上面出现过了，这里第三个参数为null，那么我们看方法里面，会执行super.dispatchTouchEvent(event);这里意思是说，如果找不到子View来处理事件，那么最后会交由ViewGroup来处理事件。这里验证了一开始我们的实例代码，在点击非CustomButton的CustomLayout部分，只打印了CustomLayout的重写部分代码。接着，如果在上面已经找到一个子View来消耗事件了，那么这里的mFirstTouchTarget不为空，接着会往下执行。
接着有一个if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget)判断，这里就是区分了ACTION_DOWN事件和别的事件，因为在上面dispatchTransformedTouchEvent()返回true的情况分析我们知道，如果子View消耗了ACTION_DOWN事件，那么alreadyDispatchedToNewTouchTarget和newTouchTarget已经有值了，所以就直接置handled为true并返回；那么如果alreadyDispatchedToNewTouchTarget和newTouchTarget值为null，那么就不是ACTION_DOWN事件，即是ACTION_MOVE、ACTION_UP等别的事件的话，就会调用2的部分代码，把这些事件分发给子View。
上面就基本分析了ViewGroup的事件分发的基本流程走向，下面我们用一张图来理顺一下上面我们分析的源码。
图2
图形剖析
可能上面的源码分析，有部分同学还是不能清晰理解ViewGroup的事件分发，那么我们直接通过示例代码来理解。
- CustomLayout不拦截、CustomButton不消费；
我们的CustomButton不消费就是onTouchEvent返回false，看看打印情况。
11-01 14:18:29.275 29088-29088/com.example.pc.myapplication I/CustomLayout: dispatchTouchEvent ACTION_DOWN11-01 14:18:29.275 29088-29088/com.example.pc.myapplication I/CustomLayout: onInterceptTouchEvent ACTION_DOWN11-01 14:18:29.275 29088-29088/com.example.pc.myapplication I/CustomLayout: onInterceptTouchEvent return false11-01 14:18:29.275 29088-29088/com.example.pc.myapplication I/CustomButton: dispatchTouchEvent ACTION_DOWN11-01 14:18:29.275 29088-29088/com.example.pc.myapplication I/MainActivity: onTouch ACTION_DOWN11-01 14:18:29.275 29088-29088/com.example.pc.myapplication I/MainActivity: onTouch return false11-01 14:18:29.275 29088-29088/com.example.pc.myapplication I/CustomButton: onTouchEvent ACTION_DOWN11-01 14:18:29.275 29088-29088/com.example.pc.myapplication I/CustomButton: onTouchEvent return false11-01 14:18:29.275 29088-29088/com.example.pc.myapplication I/CustomButton: dispatchTouchEvent return false11-01 14:18:29.275 29088-29088/com.example.pc.myapplication I/CustomLayout: onTouchEvent ACTION_DOWN11-01 14:18:29.275 29088-29088/com.example.pc.myapplication I/CustomLayout: onTouchEvent return false11-01 14:18:29.275 29088-29088/com.example.pc.myapplication I/CustomLayout: dispatchTouchEvent return false- CustomLayout不拦截，CustomButton消费；
我们一开始的实例代码就是这个情况，我们可以再看看打印数据
11-01 14:15:47.361 16958-16958/com.example.pc.myapplication I/CustomLayout: dispatchTouchEvent ACTION_DOWN11-01 14:15:47.361 16958-16958/com.example.pc.myapplication I/CustomLayout: onInterceptTouchEvent ACTION_DOWN11-01 14:15:47.361 16958-16958/com.example.pc.myapplication I/CustomLayout: onInterceptTouchEvent return false11-01 14:15:47.361 16958-16958/com.example.pc.myapplication I/CustomButton: dispatchTouchEvent ACTION_DOWN11-01 14:15:47.361 16958-16958/com.example.pc.myapplication I/MainActivity: onTouch ACTION_DOWN11-01 14:15:47.362 16958-16958/com.example.pc.myapplication I/MainActivity: onTouch return false11-01 14:15:47.362 16958-16958/com.example.pc.myapplication I/CustomButton: onTouchEvent ACTION_DOWN11-01 14:15:47.364 16958-16958/com.example.pc.myapplication I/CustomButton: onTouchEvent return true11-01 14:15:47.364 16958-16958/com.example.pc.myapplication I/CustomButton: dispatchTouchEvent return true11-01 14:15:47.364 16958-16958/com.example.pc.myapplication I/CustomLayout: dispatchTouchEvent return true11-01 14:15:47.378 16958-16958/com.example.pc.myapplication I/CustomLayout: dispatchTouchEvent ACTION_UP11-01 14:15:47.378 16958-16958/com.example.pc.myapplication I/CustomLayout: onInterceptTouchEvent ACTION_UP11-01 14:15:47.378 16958-16958/com.example.pc.myapplication I/CustomLayout: onInterceptTouchEvent return false11-01 14:15:47.378 16958-16958/com.example.pc.myapplication I/CustomButton: dispatchTouchEvent ACTION_UP11-01 14:15:47.378 16958-16958/com.example.pc.myapplication I/MainActivity: onTouch ACTION_UP11-01 14:15:47.378 16958-16958/com.example.pc.myapplication I/MainActivity: onTouch return false11-01 14:15:47.378 16958-16958/com.example.pc.myapplication I/CustomButton: onTouchEvent ACTION_UP11-01 14:15:47.378 16958-16958/com.example.pc.myapplication I/CustomButton: onTouchEvent return true11-01 14:15:47.378 16958-16958/com.example.pc.myapplication I/CustomButton: dispatchTouchEvent return true11-01 14:15:47.378 16958-16958/com.example.pc.myapplication I/CustomLayout: dispatchTouchEvent return true11-01 14:15:47.384 16958-16958/com.example.pc.myapplication I/MainActivity: onClick Event- CustomLayout拦截；
因为我们在CustomLayout的onInterceptTouchEvent的返回值设置为true，进行拦截，于是程序会走到CustomLayout的onTouchEvent()，而且不会去到CustomButton的onInterceptTouchEvent()。看打印数据：
11-01 14:19:42.615 30336-30336/com.example.pc.myapplication I/CustomLayout: dispatchTouchEvent ACTION_DOWN11-01 14:19:42.615 30336-30336/com.example.pc.myapplication I/CustomLayout: onInterceptTouchEvent ACTION_DOWN11-01 14:19:42.615 30336-30336/com.example.pc.myapplication I/CustomLayout: onInterceptTouchEvent return true11-01 14:19:42.615 30336-30336/com.example.pc.myapplication I/CustomLayout: onTouchEvent ACTION_DOWN11-01 14:19:42.615 30336-30336/com.example.pc.myapplication I/CustomLayout: onTouchEvent return false11-01 14:19:42.615 30336-30336/com.example.pc.myapplication I/CustomLayout: dispatchTouchEvent return false总结：
- 在是否拦截当前事件，返回true表示拦截，如果拦截了事件，那么将不会分发给子View。比如说：ViewGroup拦截了这个事件，那么所有事件都由该ViewGroup处理，它内部的子View将不会获得事件的传递。（但是ViewGroup是默认不拦截事件的）注意：View是没有这个方法的，也即是说，继承自View的一个子View不能重写该方法，也无需拦截事件，因为它下面没有View了，它要么处理事件要么不处理事件，所以最底层的子View不能拦截事件；
- 事件传递过程是由外向内的，即事件总是先传给父元素，然后再由父元素分发给子VIew,子View可以通过requestDisallowInterceptTouchEvent来干预父元素的分发过程，但是影响不到ACTION_DOWN事件；
- 如果ViewGroup找到了能够处理该事件的View，则直接交给子View处理，自己的onTouchEvent不会被触发；
- 子View可以通过调用getParent().requestDisallowInterceptTouchEvent(true); 阻止ViewGroup对其MOVE或者UP事件进行拦截；
下一篇我们来继续分析Android事件分发机制源码分析之Activity篇
参考文章：
- http://blog.csdn.net/yanbober/article/details/45912661
- http://blog.csdn.net/a553181867/article/details/51287844
- http://blog.csdn.net/leesidong/article/details/6973261
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