Android6.0 ViewGroup/View 事件分发机制详解

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之前自认为对于Android的事件分发机制还算比较了解，直到前一阵偶然跟人探讨该问题，才发现自己以前的理解有误，惭愧之余遂决定研习源码，彻底弄明白Android的事件分发机制，好了废话少说，直接开干。

首先，我们对Android中的touch事件做一下总结，主要分为以下几类：

1、Action_Down  用户手指触碰到屏幕的那一刻，会触发该事件；

2、Action_Move   在触碰到屏幕之后，手指开始在屏幕上滑动，会触发Action_Move事件；

3、Action_Up       在用户手指从屏幕上离开那一刻开始，会触发Action_Up事件；

4、Action_Cancel Cancel事件一般跟Up事件的处理是一样的，是由系统代码自己去触发，比如子view的事件被父view给拦截了，之前被分发的子view就会被发送cancel事件，或者用户手指在滑动过程中移出了边界。另外，在有多点触控事件时，还会陆续触发ACTION_POINTER_DOWN、ACTION_POINTER_UP等事件。

其次，我们知道Android中负责事件分发机制的方法主要有以下三个：

1、dispatchTouchEvent(MotionEvent event) --- 分发事件

      我们知道当用户触摸到手机屏幕时，最先接收到事件的并进行相应处理的应该是最外层的Activity，所以我们来看看Activity中是如何对事件进行分发的。

    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }
        if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
        }
        return onTouchEvent(ev);
    }

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

        从superDispatchTouchEvent()方法中我们可以看到，它又调用了mDecor的superDispatchTouchEvent()方法，再看看mDecor的superDispatchTouchEvent()方法，

    public boolean superDispatchTouchEvent(MotionEvent event) {
        return super.dispatchTouchEvent(event);
    }

       而mDecor是何许人也，其实就是PhoneWindow中的一个内部类DecorView的实例对象，是Activity的Window窗口中最根部的父容器，我们平时在Activity的onCreate()方法中，通过setContentView()给设置的布局容器，都属于mDecor的子View mContentView对象的子view，而DecorView又继承于FrameLayout，FrameLayout又继承于ViewGroup，由此我们就知道了，Activity是如何将事件分发到相应的View当中去的：

 Activity.dispatchTouchEvent(MotionEvent event) -> PhoneWindow.superDispatchTouchEvent(MotionEvent event) -> DecorView.superDispatchTouchEvent(MotionEvent event) ->  FrameLayout.dispatchTouchEvent(MotionEvent event) -> ViewGroup.dispatchTouchEvent(MotionEvent event) -> 再逐级分发到各个ViewGroup/View当中去

        另外从以上分析过程可以看出，有一点需注意，就是我们在继承ViewGroup或其子类复写dispatchTouchEvent时，在方法最后的返回值处，最好别直接写成return true或者return false，而应写成super.dispatchTouchEvent，否则无法对事件继续进行逐级分发，因为在ViewGroup类的dispatchTouchEvent(MotionEvent event)方法中，会对该布局容器内的所有子View进行遍历，然后再进行事件分发，详细分发过程稍后会给出。

2、onInterceptTouchEvent(MotionEvent event) --- 拦截事件

        onInterceptTouchEvent(MotionEvent event) 方法只存在于ViewGroup当中，是用来对布局容器内子View的事件进行拦截的，如果父容器View对事件进行了拦截，即return true，则子View不会收到任何事件分发。

3、onTouchEvent(MotionEvent event) --- 处理消费事件

        onTouchEvent(MotionEvent event)方法如果返回true，则表示该事件被当前View给消费掉了，它的父View的onTouchEvent()后续都不会得到调用，而是通过dispatchTouchEvent()逐级向上返回true到Activity；如果没人消费该事件，都返回false，则最终会交给Activity去进行处理。

在大致了解了dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent的作用之后，现在我们最需要理清的就是这三者之间的调用关系如何，为此我自己写了一个测试Demo，界面如下：

屏幕中有ViewGroupA、ViewGroupB、ViewC，依次进行嵌套

测试代码如下：

<com.android.hanyee.widgets.ViewGroupA xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/viewGroupA"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="@android:color/white">

    <com.android.hanyee.widgets.ViewGroupB
        android:id="@+id/viewGroupB"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_margin="60dp"
        android:orientation="vertical"
        android:background="@android:color/holo_blue_dark">

        <com.android.hanyee.widgets.ViewC
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_margin="60dp"
            android:background="@android:color/holo_green_dark" />
    </com.android.hanyee.widgets.ViewGroupB>
</com.android.hanyee.widgets.ViewGroupA>

public class ViewGroupA extends LinearLayout {
    public ViewGroupA(Context context) {
        super(context);
    }

    public ViewGroupA(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public ViewGroupA(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        Log.d("hanyee", this.getClass().getSimpleName() + " onInterceptTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.onInterceptTouchEvent(ev);
        Log.d("hanyee", this.getClass().getSimpleName() + " onInterceptTouchEvent return super.onInterceptTouchEvent(ev)=" + result);
        return result;
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.d("hanyee", this.getClass().getSimpleName() + " dispatchTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.dispatchTouchEvent(ev);
        Log.d("hanyee", this.getClass().getSimpleName() + " dispatchTouchEvent return super.dispatchTouchEvent(ev)= " + result);
        return result;
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        Log.d("hanyee", this.getClass().getSimpleName() + " onTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.onTouchEvent(ev);
        Log.d("hanyee", this.getClass().getSimpleName() + " onTouchEvent return super.onTouchEvent(ev)=" + result);
        return result;
    }

}

public class ViewGroupB extends LinearLayout {
    public ViewGroupB(Context context) {
        super(context);
    }

    public ViewGroupB(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public ViewGroupB(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        Log.d("hanyee", this.getClass().getSimpleName() + " onInterceptTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.onInterceptTouchEvent(ev);
        Log.d("hanyee", this.getClass().getSimpleName() + " onInterceptTouchEvent return super.onInterceptTouchEvent(ev)=" + result);
        return result;
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.d("hanyee", this.getClass().getSimpleName() + " dispatchTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.dispatchTouchEvent(ev);
        Log.d("hanyee", this.getClass().getSimpleName() + " dispatchTouchEvent return super.dispatchTouchEvent(ev)= " + result);
        return result;
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        Log.d("hanyee", this.getClass().getSimpleName() + " onTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.onTouchEvent(ev);
        Log.d("hanyee", this.getClass().getSimpleName() + " onTouchEvent return super.onTouchEvent(ev)=" + result);
        return result;
    }
}

public class ViewC extends View {
    public ViewC(Context context) {
        super(context);
    }

    public ViewC(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public ViewC(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.d("hanyee", this.getClass().getSimpleName() + " dispatchTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.dispatchTouchEvent(ev);
        Log.d("hanyee", this.getClass().getSimpleName() + " dispatchTouchEvent return super.dispatchTouchEvent(ev)= " + result);
        return result;
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        Log.d("hanyee", this.getClass().getSimpleName() + " onTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.onTouchEvent(ev);
        Log.d("hanyee", this.getClass().getSimpleName() + " onTouchEvent return super.onTouchEvent(ev)=" + result);
        return result;
    }
}

测试情景1：ViewGroupA、ViewGroupB、ViewC都没有消费事件

测试结果如下图：

由图中log可以看出，如果没有任何view消费事件的话，事件的传递顺序如下：

ViewGroupA.dispatchTouchEvent -> ViewGroupA.onInterceptTouchEvent(return false, 没有进行拦截) -> ViewGroupB.dispatchTouchEvent -> ViewGroupB.onInterceptTouchEvent(return false, 没有进行拦截) -> ViewC.dispatchTouchEvent -> ViewC.onTouchEvent(return false, 没有消费)  -> ViewC.dispatchTouchEvent(return false, 将onTouchEvent的处理结果回传给ViewGroupB) -> ViewGroupB.onTouchEvent(return false, 也没有消费)  -> ViewB.dispatchTouchEvent(return false, 将onTouchEvent的处理结果回传给ViewGroupA) -> ViewGroupA.onTouchEvent(return false, 也没有消费)  -> ViewA.dispatchTouchEvent(return false, 最终将onTouchEvent的处理结果回传给Activity) -> Activity对事件进行最终处理

看到这里大伙可能会有些疑问，怎么就只有Down事件，而没有后续的Move、Up等事件，这是因为没有任何子View消费Down事件，Down事件最终被最外层的Activity给处理掉了，所以后续的所有Move、Up等事件都不会再分发给子View了，这里在后面的源码分析时会提到。

测试情景2：ViewC消费了事件

测试结果如下图：

由图中的log可以看出，一旦ViewC消费了Down事件，它的父容器ViewGroupB，祖父容器ViewGroupA的onTouchEvent都不会被调用了，而是直接通过dispatchTouchEvent将Down以及后续的Move、Up事件的处理结果返回至Activity。

测试情景3：仅点击ViewGroupB，让ViewGroupB消费事件

测试结果如下图：

从图中log可以看出，如果点击ViewGroupB，事件根本就不会传递到ViewC，ViewGroupB在消费了Down事件之后，再直接由父容器ViewGroupA的dispatchTouchEvent将ViewGroupB的onTouchEvent处理结果true回传给Activity，接下来后续的Move、Up事件都只会传递至ViewGroupB，而不会分发给ViewC。

测试情景4：让ViewGroupB对事件进行拦截

测试结果如图：

从图中log可以看出，如果ViewGroupB的onInterceptTouchEvent 返回true，对子view的事件进行拦截，则ViewC不会收到任何的点击事件，事件流变成了ViewGroupA -->ViewGroupB --> ViewGroupA，而没有经过ViewC

通过上述几种情景，我们可以大致了解，ViewGroupA的dispatchTouchEvent最先被调用，主要负责事件分发，然后会调用其onInterceptTouchEvent，如果返回true，则后续的ViewGroupB、ViewC都不会收到任何的点击事件，相反如果返回false，就放弃拦截事件，接着会遍历调用子View的dispatchTouchEvent方法将事件分发给ViewGroupB，如果ViewGroupB也没有拦截事件，则又会遍历调用子View的dispatchTouchEvent方法将事件分发给ViewC，如果ViewC在onTouchEvent中消费了事件返回true，则会将true通过dispatchTouchEvent方法逐级返回给其父容器直至Activity中，而且不会调用各个父容器对应的onTouchEvent方法，如果子View在onTouchEvent中没消费事件返回false，则通过dispatchTouchEvent方法将false返回给ViewGroupB，ViewGroupB就知道子View没有消费事件，就会调用自己的onTouchEvent来处理该事件，然后同理递归着ViewC在onTouchEvent中对于事件的处理逻辑，直到ViewGroupA将事件处理完反馈给Activity。

前面列了这么多现象，并归纳总结出以上dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent之间的调用关系，相信大家对于Android事件的分发机制已经有了较为清晰的认识，但作为一名程序员，知其然，还得知其所以然，下面就带领大家一起研读下源码，看看到底为啥是这样的调用关系。从上面的情景log中大家应该可以看出，事件分发机制的最初始的入口就是ViewGroup的dispatchTouchEvent，下面就看看其代码：

    @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 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;

                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 = 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;
                            }

                            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;
    }

这方法看似比较长，但我们只挑比较重要的点来看，在第32行会根据disallowIntercept来判断是否对子view来进行事件拦截，子view可以通过调用requestDisallowInterceptTouchEvent()方法来改变其值，如果可以进行拦截，则会调用onInterceptTouchEvent()方法，根据其返回值来判断需不需要对子View进行拦截，默认情况下onInterceptTouchEvent()方法返回的是false，所以如果我们在自定义View时如果想拦截的话，可以重写这个方法返回true就行了。

然后在第58行的if条件中，会根据是否取消canceled以及之前的是否拦截的标志intercepted来判断是否走进下面的逻辑代码块，这里我们只看intercepted，如果没有拦截，则会进入if后面的逻辑代码块，直到第89行的for循环，我们会看到ViewGroup在对所有子View进行遍历，以方便接下来的事件分发，再看到107、108行的判断，canViewReceivePointerEvents()用来判断是否该View能够接受处理事件，

    private static boolean canViewReceivePointerEvents(View child) {
        return (child.mViewFlags & VISIBILITY_MASK) == VISIBLE
                || child.getAnimation() != null;
    }

    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();
        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.
        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());
            }

            handled = child.dispatchTouchEvent(transformedEvent);
        }

        // Done.
        transformedEvent.recycle();
        return handled;
    }

我们看到在方法的末尾第55行，如果child为Null，则会调用ViewGroup的父类View的dispatchTouchEvent，否则就会调用child自身的dispatchTouchEvent方法进行事件分发处理。如果child是ViewGroup，则会又递归调用ViewGroup的dispatchTouchEvent方法逻辑进行事件分发，如果是View，则跟child为Null情况一样，都是会调到View的dispatchTouchEvent方法，接下来我们看看View的dispatchTouchEvent方法，

    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();
        }

        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;
            }

            if (!result && onTouchEvent(event)) {
                result = 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();
        }

        return result;
    }

    public boolean onTouchEvent(MotionEvent event) {
        final float x = event.getX();
        final float y = event.getY();
        final int viewFlags = mViewFlags;
        final int action = event.getAction();

        if ((viewFlags & ENABLED_MASK) == DISABLED) {
            if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
                setPressed(false);
            }
            // A disabled view that is clickable still consumes the touch
            // events, it just doesn't respond to them.
            return (((viewFlags & CLICKABLE) == CLICKABLE
                    || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
                    || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);
        }

        if (mTouchDelegate != null) {
            if (mTouchDelegate.onTouchEvent(event)) {
                return true;
            }
        }

        if (((viewFlags & CLICKABLE) == CLICKABLE ||
                (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
                (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
            switch (action) {
                case MotionEvent.ACTION_UP:
                    boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
                    if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
                        // take focus if we don't have it already and we should in
                        // touch mode.
                        boolean focusTaken = false;
                        if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
                            focusTaken = requestFocus();
                        }

                        if (prepressed) {
                            // The button is being released before we actually
                            // showed it as pressed.  Make it show the pressed
                            // state now (before scheduling the click) to ensure
                            // the user sees it.
                            setPressed(true, x, y);
                       }

                        if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
                            // This is a tap, so remove the longpress check
                            removeLongPressCallback();

                            // Only perform take click actions if we were in the pressed state
                            if (!focusTaken) {
                                // Use a Runnable and post this rather than calling
                                // performClick directly. This lets other visual state
                                // of the view update before click actions start.
                                if (mPerformClick == null) {
                                    mPerformClick = new PerformClick();
                                }
                                if (!post(mPerformClick)) {
                                    performClick();
                                }
                            }
                        }

                        if (mUnsetPressedState == null) {
                            mUnsetPressedState = new UnsetPressedState();
                        }

                        if (prepressed) {
                            postDelayed(mUnsetPressedState,
                                    ViewConfiguration.getPressedStateDuration());
                        } else if (!post(mUnsetPressedState)) {
                            // If the post failed, unpress right now
                            mUnsetPressedState.run();
                        }

                        removeTapCallback();
                    }
                    mIgnoreNextUpEvent = false;
                    break;

                case MotionEvent.ACTION_DOWN:
                    mHasPerformedLongPress = false;

                    if (performButtonActionOnTouchDown(event)) {
                        break;
                    }

                    // Walk up the hierarchy to determine if we're inside a scrolling container.
                    boolean isInScrollingContainer = isInScrollingContainer();

                    // For views inside a scrolling container, delay the pressed feedback for
                    // a short period in case this is a scroll.
                    if (isInScrollingContainer) {
                        mPrivateFlags |= PFLAG_PREPRESSED;
                        if (mPendingCheckForTap == null) {
                            mPendingCheckForTap = new CheckForTap();
                        }
                        mPendingCheckForTap.x = event.getX();
                        mPendingCheckForTap.y = event.getY();
                        postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
                    } else {
                        // Not inside a scrolling container, so show the feedback right away
                        setPressed(true, x, y);
                        checkForLongClick(0);
                    }
                    break;

                case MotionEvent.ACTION_CANCEL:
                    setPressed(false);
                    removeTapCallback();
                    removeLongPressCallback();
                    mInContextButtonPress = false;
                    mHasPerformedLongPress = false;
                    mIgnoreNextUpEvent = false;
                    break;

                case MotionEvent.ACTION_MOVE:
                    drawableHotspotChanged(x, y);

                    // Be lenient about moving outside of buttons
                    if (!pointInView(x, y, mTouchSlop)) {
                        // Outside button
                        removeTapCallback();
                        if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
                            // Remove any future long press/tap checks
                            removeLongPressCallback();

                            setPressed(false);
                        }
                    }
                    break;
            }

            return true;
        }

        return false;
    }

从第7~16行可以看出，当View为disable状态，而又clickable时，是会消费掉事件的，只不过在界面上没有任何的响应。第18~22行，关于TouchDelegate，根据对官方文档的理解就是说有两个View, ViewB在ViewA中，ViewA比较大，如果我们想点击ViewA的时候，让ViewB去响应点击事件，这时候就需要使用到TouchDelegate， 简单的理解就是如果该View有自己的事件委托处理人，就交给委托人处理。从第24~26行可以看出，只有当View是可点击状态时，才会进入对应各种事件的详细处理逻辑，否则会直接返回false，表明该事件没有被消费。在第59行，可以看到在Action_Up事件被触发时，会执行performClick()，也就是View的点击事件，由此可知，view的onClick()回调是在Action_Up事件中被触发的。第134行直接返回了true，可以看出只要View处于可点击状态，并且进入了switch的判断逻辑，就会被返回true，表明该事件被消费掉了，也就是说只要View是可点击的，事件传到了其OnTouchEvent，都会被消费掉。而平时我们在调用setOnClickListener方法给View设置点击事件监听时，都会将其点击状态修改为可点击状态。

    public void setOnClickListener(@Nullable OnClickListener l) {
        if (!isClickable()) {
            setClickable(true);
        }
        getListenerInfo().mOnClickListener = l;
    }

追溯完View的事件分发流程，我们再返回到ViewGroup的dispatchTouchEvent方法的122行，如果对应得child消费了点击事件，就会通过对应的dispatchTouchEvent方法返回true并最终在122行使得条件成立，然后会进入到138行，调用addTouchTarget对newTouchTarget进行赋值，并且mFirstTouchTarget跟newTouchTarget的值都一样，然后将alreadyDispatchedToNewTouchTarget置为true

    private TouchTarget addTouchTarget(View child, int pointerIdBits) {
        TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
        target.next = mFirstTouchTarget;
        mFirstTouchTarget = target;
        return target;
    }

然后来到了163行，由于mFirstTouchTarget和newTouchTarget在addTouchTarget中都被赋值了，所以会直接进入172行的while循环，由于之前在138、139行对mFirstTouchTarget、newTouchTarget、alreadyDispatchedToNewTouchTarget都赋值了，使得174行条件成立，所以就直接返回true了，至此，ViewGroup就完成了对子View的遍历及事件分发，由于事件被消费掉了，所以ViewGroup对应的所有外围容器都会递归回调dispatchTouchEvent将true传递给Activity，到这也就解释了测试情景2的产生原理。在Down相关事件被消费掉之后，后续的Move、Up事件在dispatchTouchEvent方法的68~70行不符合判断条件，直接会来到179行的dispatchTransformedTouchEvent方法继续进行分发，待子View进行消费。

如果在ViewGroup的dispatchTouchEvent方法第58行被拦截了(对应测试情景4)，或者107~108行不成立(对应测试情景3)，或者122行返回false(即子View没有消费事件，对应测试情景1)，则会直接进入到第163行，这时mFirstTouchTarget肯定为空，所以会又调用dispatchTransformedTouchEvent方法，而且传进去的child为空，最终就会直接走到dispatchTransformedTouchEvent方法的55行，然后调用super.dispatchTouchEvent，之后的处理逻辑跟前面调View的dispatchTouchEvent逻辑一样。

终上所述，整个Android的事件分发机制可以大致概括成如下的流程图

PS：以上相关的系统代码均为Android6.0的系统源码，整个Android事件分发机制还算有点复杂，完全给整明白写下这篇文章还费了些时间，中间查阅了一些资料，可能有些地方还存在些理解偏差，还请大家指出相互学习进步

Android6.0 ViewGroup/View 事件分发机制详解

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原文地址：http://blog.csdn.net/meiliwanghan/article/details/51567408