Android触摸事件传递机制—源码解读

时间：12-14来源：作者：点击数：3

在做需求开发中，我使用ViewPager嵌套Fragment，而Fragment又嵌套了Recyclerview组件。这样就导致RV滑动与Viewpager滑动事件产生手势冲突的问题。在深入理解Android触摸事件的传递机制后，我解决了这个问题。遂决定写下这篇博客，来深入分析Android触摸事件传递机制涉及到的相关源码。

一、事件类型

MotionEvent.ACTION_DOWN：手指按下屏幕触发。
MotionEvent.ACTION_MOVE：手指在屏幕上移动触发。
MotionEvent.ACTION_UP：手指抬起时触发。
MotionEvent.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。

二、负责事件分发机制的方法

2.1 分发事件`(dispatchTouchEvent)`

我们知道当用户触摸到手机屏幕时，最先接收到事件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);
}

从以上代码中我们可以看到调用getWindow().superDispatchTouchEvent()，而这里的getWindow()返回的是Window抽象类，其实就是PhoneWindow类，继承于Window抽象类，然后调用PhoneWindow的superDispatchTouchEvent()

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

从superDispatchTouchEvent()方法中可以看到，它又调用了mDecor的superDispatchTouchEvent()方法，再看mDecor的superDispatchTouchEvent()方法

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

而mDecor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，由此可知，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

所以，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。

2.2 拦截事件`(onInterceptTouchEvent)`

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。

2.3 处理消费事件`(onTouchEvent)`

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。

三、Demo

界面如下：

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

源代码如下：

<com.android.phc.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.phc.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.phc.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.phc.widgets.ViewGroupB>
</com.android.phc.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("phc", this.getClass().getSimpleName() + " onInterceptTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.onInterceptTouchEvent(ev);
        Log.d("phc", this.getClass().getSimpleName() + " onInterceptTouchEvent return super.onInterceptTouchEvent(ev)=" + result);
        return result;
    }
 
    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.d("phc", this.getClass().getSimpleName() + " dispatchTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.dispatchTouchEvent(ev);
        Log.d("phc", this.getClass().getSimpleName() + " dispatchTouchEvent return super.dispatchTouchEvent(ev)= " + result);
        return result;
    }
 
    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        Log.d("phc", this.getClass().getSimpleName() + " onTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.onTouchEvent(ev);
        Log.d("phc", 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("phc", this.getClass().getSimpleName() + " onInterceptTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.onInterceptTouchEvent(ev);
        Log.d("phc", this.getClass().getSimpleName() + " onInterceptTouchEvent return super.onInterceptTouchEvent(ev)=" + result);
        return result;
    }
 
    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.d("phc", this.getClass().getSimpleName() + " dispatchTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.dispatchTouchEvent(ev);
        Log.d("phc", this.getClass().getSimpleName() + " dispatchTouchEvent return super.dispatchTouchEvent(ev)= " + result);
        return result;
    }
 
    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        Log.d("phc", this.getClass().getSimpleName() + " onTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.onTouchEvent(ev);
        Log.d("phc", 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("phc", this.getClass().getSimpleName() + " dispatchTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.dispatchTouchEvent(ev);
        Log.d("phc", this.getClass().getSimpleName() + " dispatchTouchEvent return super.dispatchTouchEvent(ev)= " + result);
        return result;
    }
 
    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        Log.d("phc", this.getClass().getSimpleName() + " onTouchEvent -> " + ViewUtils.actionToString(ev.getAction()));
        boolean result = super.onTouchEvent(ev);
        Log.d("phc", this.getClass().getSimpleName() + " onTouchEvent return super.onTouchEvent(ev)=" + result);
        return result;
    }
}

3.1 ViewGroupA、ViewGroupB、ViewC都没有消费事件

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。

3.2 ViewC消费了事件

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。

3.3 仅点击ViewGroupB，让ViewGroupB消费事件

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。

3.4 让ViewGroupB对事件进行拦截

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。

四、源码解析

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;
}

可以看到只有当view处于可见状态且没有做动画时才能接收处理事件，再看isTransformedTouchPointInView()是用来判断当前事件是否触发在该view的范围之内，这里我们可以回想前面的测试情景3，当我们点击ViewGroupB时，ViewC完全没有收到任何事件，就是因为点击事件不在ViewC的范围之类，在isTransformedTouchPointInView()进行判断时就给过滤掉了，所以ViewC不会收到任何分发的事件。再看看第122行，会调用dispatchTransformedTouchEvent()来将事件分发给对应的view进行处理，让我们进入其方法体看看

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;
}

同样我们捡重点的看，第23行用来做过滤，看是否有窗口覆盖在上面，第27~29行三个判断条件说明了，当View的touch事件监听器不为空，View是enable状态，且touch事件监听回调方法onTouch方法返回true三个条件同时满足时，则会最终返回true，而且第33行的onTouchEvent方法都不会得到执行，这说明View的OnTouchListener监听回调的优先级要高于onTouchEvent，如果我们给View设置了OnTouchListener监听，并且在回调方法onTouch()中返回true，View的onTouchEvent就得不到执行，其dispatchTouchEvent方法就会直接返回true给父容器，相反如果返回false，或者没有设置OnTouchListener监听，才会执行onTouchEvent()方法对分发来的事件进行处理。接着再去看看onTouchEvent()中如何对事件进行处理的。

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：