Android事件分发机制解析

2022-03-08 本文已影响0人 Horps

概述

手指触摸事件是由InputManagerService服务来监听并发送到对应窗口的对应Activity的，大体来说就是该服务会监听设备的各种输入事件，然后会有一个InputEventReceiver来接收事件变化，然后发送给Activity或Dialog，这部分是C/C++部分完成的，我们这里先只分析用户层的分发机制。
Activity

根据上面的信息，我们以Activity的dispatchTouchEvent方法作为事件入口。

在开始之前，我们要先有一个概念：dispatchTouchEvent方法的返回值为 true表示尝试自己消费（只是尝试）而不往下分发，返回false表示继续往下分发，Activity的这个逻辑同样在更上一级的Framework层。
```
public boolean dispatchTouchEvent(MotionEvent ev) {
    if (ev.getAction() == MotionEvent.ACTION_DOWN) {
        onUserInteraction();
    }
    if (getWindow().superDispatchTouchEvent(ev)) {
        return true;
    }
    return onTouchEvent(ev);
}
```
onUserInteraction没有默认实现，暂不考虑。可以看到，这里会调用getWindow().superDispatchTouchEvent(ev)方法判断是返回true还是返回onTouchEvent方法的返回值。getWindow方法获取的就是PhoneWindow：
```
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
    return mDecor.superDispatchTouchEvent(event);
}
```
PhoneWindow的mDecor就是DecorView：
```
public boolean superDispatchTouchEvent(MotionEvent event) {
    return super.dispatchTouchEvent(event);
}
```
DecoView继承自ViewGroup，所以调用的就是ViewGroup的dispatchTouchEvent方法，也就是分发给子View们优先尝试捕获，下面会说到。

如果子View的询问中有捕获的则会返回true，这一点在ViewGroup的dispatchTouchEvent分析中会看到，则这里也会返回true通知上一级已经消费事件；如果子View中没有捕获该事件，则会调用Activity本身的onTouchEvent方法：
```
public boolean onTouchEvent(MotionEvent event) {
    if (mWindow.shouldCloseOnTouch(this, event)) {
        finish();
        return true;
    }

    return false;
}
```
这里会返回false，默认不消费。

而Activity中是没有onInterceptTouchEvent方法的。

ViewGroup

下面贴出dispatchTouchEvent的部分代码：

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
    // 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;
      //onFilterTouchEventForSecurity判断是否view已显示
    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.
          //注意这里，当事件是第一次按下或者之前有消费者消费了事件时才会重新调用onInterceptTouchEvent判断是否拦截，否则一律拦截，这就表示如果ACTION_DOWN有拦截了那么后面的ACTION_MOVE和ACTION_UP等事件就不会再判断了，直接拦截
        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 isMouseEvent = ev.getSource() == InputDevice.SOURCE_MOUSE;
        final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0
                && !isMouseEvent;
        TouchTarget newTouchTarget = null;
        boolean alreadyDispatchedToNewTouchTarget = false;
        if (!canceled && !intercepted) {
            // If the event is targeting accessibility focus we give it to the
            // view that has accessibility focus and if it does not handle it
            // we clear the flag and dispatch the event to all children as usual.
            // We are looking up the accessibility focused host to avoid keeping
            // state since these events are very rare.
            View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
                    ? findChildWithAccessibilityFocus() : null;

            if (actionMasked == MotionEvent.ACTION_DOWN
                    || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                    || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                final int actionIndex = ev.getActionIndex(); // always 0 for 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 =
                            isMouseEvent ? ev.getXCursorPosition() : ev.getX(actionIndex);
                    final float y =
                            isMouseEvent ? ev.getYCursorPosition() : ev.getY(actionIndex);
                    // Find a child that can receive the event.
                    // Scan children from front to back.
                    final ArrayList<View> preorderedList = buildTouchDispatchChildList();
                    final boolean customOrder = preorderedList == null
                            && isChildrenDrawingOrderEnabled();
                    final View[] children = mChildren;
                    for (int i = childrenCount - 1; i >= 0; i--) {
                        final int childIndex = getAndVerifyPreorderedIndex(
                                childrenCount, i, customOrder);
                        final View child = getAndVerifyPreorderedView(
                                preorderedList, children, childIndex);
                        if (!child.canReceivePointerEvents()
                                || !isTransformedTouchPointInView(x, y, child, null)) {
                            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;
}

ACTION_DOWN事件时会调用resetTouchState方法：

private void resetTouchState() {
    clearTouchTargets();
    resetCancelNextUpFlag(this);
    mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
    mNestedScrollAxes = SCROLL_AXIS_NONE;
}

可以看到，这里的mGroupFlags会移除FLAG_DISALLOW_INTERCEPT标签，所以disallowIntercept是false，所以intercepted等于onInterceptTouchEvent的返回值：

public boolean onInterceptTouchEvent(MotionEvent ev) {
    if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
            && ev.getAction() == MotionEvent.ACTION_DOWN
            && ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
            && isOnScrollbarThumb(ev.getX(), ev.getY())) {
        return true;
    }
    return false;
}

这里的默认实现中只要是touch事件都会返回false，表示不会拦截。

接下来会判断事件是否取消，如果既没有取消，intercepted又为false，则会进入if (!canceled && !intercepted) 代码块中。这个代码块中做了什么呢？

通过buildTouchDispatchChildList方法找出所有的子View：

ArrayList<View> buildOrderedChildList() {
    final int childrenCount = mChildrenCount;
    if (childrenCount <= 1 || !hasChildWithZ()) return null;

    if (mPreSortedChildren == null) {
        mPreSortedChildren = new ArrayList<>(childrenCount);
    } else {
        // callers should clear, so clear shouldn't be necessary, but for safety...
        mPreSortedChildren.clear();
        mPreSortedChildren.ensureCapacity(childrenCount);
    }

    final boolean customOrder = isChildrenDrawingOrderEnabled();
    for (int i = 0; i < childrenCount; i++) {
        // add next child (in child order) to end of list
        final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
        final View nextChild = mChildren[childIndex];
        final float currentZ = nextChild.getZ();

        // insert ahead of any Views with greater Z
        int insertIndex = i;
        while (insertIndex > 0 && mPreSortedChildren.get(insertIndex - 1).getZ() > currentZ) {
            insertIndex--;
        }
        mPreSortedChildren.add(insertIndex, nextChild);
    }
    return mPreSortedChildren;
}

可以看到，这里会按照子View的z坐标进行排序，z坐标越大的会放在集合靠后的位置，即在View层级中越往上的View放在集合中较后的位置。

然后会执行for 循环：for (int i = childrenCount - 1; i >= 0; i--)。可见这里会先询问层级靠上的View。

child.canReceivePointerEvents方法验证View是否可见或者应用了动画，这里也可以解释位移View动画中原先的位置为何能响应点击事件；isTransformedTouchPointInView表示View是否从原先的位置位移到了此时点击触摸点的位置。这里的意义就是排除不可见、无动画且非移动到触摸点的View，缩小事件分发范围。

然后，getTouchTarget(child)方法会获取消费事件的View：

private TouchTarget getTouchTarget(@NonNull View child) {
    for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
        if (target.child == child) {
            return target;
        }
    }
    return null;
}

这里我们是ACTION_DOWN事件，所以此时取到的这个newTouchTarget是null。

接着会调用dispatchTransformedTouchEvent方法：

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) {
          //如果ViewGroup中判断此时是事件取消了，则event会保存这个取消状态
        event.setAction(MotionEvent.ACTION_CANCEL);
          //因为此时是取消状态，child如果为空则交给这里的父容器本身尝试去处理，因为super调用的就是View的dispatchTouchEvent方法，那个方法里只会分发和View本身相关的
        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;
    }

    final MotionEvent transformedEvent;
    if (newPointerIdBits == oldPointerIdBits) {
        if (child == null || child.hasIdentityMatrix()) {
            if (child == null) {
                handled = super.dispatchTouchEvent(event);
            } else {
                  //hasIdentityMatrix会判断是否产生过偏移
                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;
        }
          //如果两次事件触摸点前后没发生变化则使用之前的event（主要是getPointerIdBits获取的值还能继续用）
        transformedEvent = MotionEvent.obtain(event);
    } else {
          //如果两次事件触摸点前后发生变化了就替换成新的触摸点id
        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);
          //有需要的话进行转换
        if (! child.hasIdentityMatrix()) {
            transformedEvent.transform(child.getInverseMatrix());
        }

        handled = child.dispatchTouchEvent(transformedEvent);
    }

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

可以看到，这个方法里会尝试在子View中找到消费事件的View，如果返回true表示找到了，那么此时会记录mLastTouchDownIndex、mLastTouchDownX和mLastTouchDownY，alreadyDispatchedToNewTouchTarget在这里置为true，然后会break跳出循环。而newTouchTarget会通过addTouchTarget方法生成：

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

然后在接下来的判断中，mFirstTouchTarget不等于null，alreadyDispatchedToNewTouchTarget等于true，mFirstTouchTarget就是我们在上面询问中捕获到的消费者，这些条件都成立了，所以handled赋值为true，最后返回handled。

ACTION_DOWN事件走完之后我们看接下来的MOVE和UP事件会不会有什么变化。

DOWN事件流程中我们如果捕获了一个消费者，那么getTouchTarget方法获取的newTouchTarget就不会为null了：

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;
}

所以这里会直接break，并不会再次执行dispatchTransformedTouchEvent方法尝试捕获，那我们的MOVE和UP事件在哪里分发的呢？上面我们说到根据alreadyDispatchedToNewTouchTarget为true来直接给handled赋值为true，alreadyDispatchedToNewTouchTarget只在一个地方赋值为true的，那就是for循环捕获消费者的时候，dispatchTransformedTouchEvent方法返回true时，那这里我们没有去重新捕获消费者(因为我们之前捕获到了)，所以alreadyDispatchedToNewTouchTarget是false，那么在最后的while代码中的alreadyDispatchedToNewTouchTarget判断中会走else分支，在这里会再次调用dispatchTransformedTouchEvent方法分发事件，如果返回true表示分发成功，那么handled赋值为true返回。

所以dispatchTouchEvent方法中总共有两处调用dispatchTransformedTouchEvent方法的地方，第一次调用除了分发事件，分发事件成功（即返回true）时还会保存消费者对象，同时会保存一个标志位alreadyDispatchedToNewTouchTarget为true，表示已经分发过了，这样在最后的while代码块中会跳过dispatchTransformedTouchEvent方法的第二次调用，所以alreadyDispatchedToNewTouchTarget保证了不会重复分发两次，即这两处调用是互斥的；第二次调用只是单纯地分发事件，和它关联的逻辑只有修改handled为true。

在最后会判断如果事件取消了或者手指抬起了则会调用resetTouchState方法，这样mFirstTouchTarget就又置为空了，所以下次按下又会重新走之前的逻辑。

综上所知，在触摸事件第一次触发的时候会分发事件并保存消费者（如果分发成功的话），在本次事件结束之前（取消或者手指抬起），DOWN事件之后的类型分发并不会再次去找新的消费者，都会直接分发到之前已经消费了DOWN事件的child，也就是说，我们从手指按下到抬起，这其中的的DOWN、MOVE和UP等类型的分发都会被同一个View消费。

ViewGroup中则没有重写onTouchEvent方法，至于为什么你可以好好理解一下，因为如果ViewGroup作为一个消费者的时候（上面的super.dispatchTouchEvent方法返回true）它其实就是一个View的身份，因为消费者消费的肯定是和自身相关的逻辑，这和子View无关，只有当它要分发事件的时候它才作为一个ViewGroup的身份去处理的。

View

那接下来，我们来看一下View的分发逻辑。先看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();
      //首先手指按下时会先停止父容器滚动
    if (actionMasked == MotionEvent.ACTION_DOWN) {
        // Defensive cleanup for new gesture
        stopNestedScroll();
    }

    if (onFilterTouchEventForSecurity(event)) {
          //如果是有滚动条且拖动过的话认为是消费了事件
        if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
            result = true;
        }
        //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;
}

onFilterTouchEventForSecurity方法我们前面说过了，如果View没有隐藏则返回true，所以成立。

然后会判断是否通过setOnTouchListener方法设置过OnTouchListener，如果设置过，则调用他的onTouch方法，如果onTouch方法返回true则表示已消费。

接着判断，如果之前onTouch方法返回了true的话，这里的result是true，则不会执行到本身的onTouchEvent方法，这里可以得知，OnTouchListener会优先尝试执行，如果OnTouchListener的onTouch方法返回了true则不会再去调用onTouchEvent方法；若是OnTouchListener的onTouch方法返回了false，则会根据onTouchEvent方法的返回值去决定是否消费。

来看一下onTouchEvent方法：

public boolean onTouchEvent(MotionEvent event) {
    final float x = event.getX();
    final float y = event.getY();
    final int viewFlags = mViewFlags;
    final int action = event.getAction();
      //判断是否可以点击
    final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
            || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
            || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;
      //TouchDelegate用来设置比View本身更大区域的事件捕获，作用是把溢出View边界的event的location修改到View内
    if (mTouchDelegate != null) {
        if (mTouchDelegate.onTouchEvent(event)) {
            return true;
        }
    }
      //如果clickable为false，则View不会执行下面的逻辑
    if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
          ...
        ...
        return true;
    }

    return false;
}

这个方法比较冗长，只贴了部分关键代码.

可以看到，这里只要是clickable为true，onTouchEvent方法都会返回true，否则会返回false。

总结

现在我们来总结一下Android的事件分发机制：
1. 从Activity的dispatchTouchEvent方法开始，调用PhoneWindow的superDispatchTouchEvent方法尝试往下分发，如果这个方法返回true，说明子View中有组件消费了，那么这里会返回true给Framework层表示已消费。如果子View中没有组件消费该事件，则会调用Activity本身的onTouchEvent方法，默认会返回false；
2. PhoneWindow的superDispatchTouchEvent方法调用了DecorView的superDispatchTouchEvent方法，这里又调用了super的dispatchTouchEvent方法，DecorView继承自ViewGroup，所以就走到了ViewGroup的dispatchTouchEvent方法；
3. ViewGroup的dispatchTouchEvent方法中会把所有的View按照z坐标排序，z坐标越大的放在集合后面，然后从末端开始循环这个集合，这表示会先尝试分发给最内层的View，因为View的添加顺序都是最后添加的View在最上层，所以它的z坐标是最大的，也就是说最上层的View就是最内层的View，如果找到了消费事件的View则保存这个消费对象为mFirstTouchTarget，然后跳出循环；
4. ACTION_DOWN类型事件处理之后，后面跟随的MOVE或UP等事件会自动交给之前保存的mFirstTouchTarget指向的View来消费，直到新一轮的触摸事件开始；
5. 第3步中是调用的dispatchTransformedTouchEvent方法来查找可消费View的，第3条只是说明了大体的流程，细节就是，在这个方法内部会调用每一个合法child的dispatchTouchEvent方法，如果child还是ViewGroup的话就会走该child的分发工作，原理和上述一样，相当于递归，不过是更内部一层ViewGroup的事件分发，如果child是View的话就会走View的dispatchTouchEvent逻辑；
6. ViewGroup的dispatchTouchEvent方法的工作是向子View分发事件的，而View的dispatchTouchEvent方法则是用来决定它内部是要通过什么途径来消费事件，首先会先尝试调用OnTouchListener的onTouch方法（如果OnTouchListener不为空的话），如果onTouch方法返回了true，则View的onTouchEvent方法不会执行，否则会调用onTouchEvent方法，这两处处理有一个返回true则dispatchTouchEvent就返回true；
7. View的onTouchEvent方法中就是处理ACTION_DOWN、ACTION_UP等具体的事件了，值得注意的是我们添加的OnClickListener是在这里尝试判断执行的。
总的来说，一次触摸事件从ACTION_DOWN开始，事件会从最上面的Activity开始尝试分发，如果中间某个ViewGroup拦截了的话就在该ViewGroup内部消费，如果没被拦截则会一直走到最内层的子View，如果最内层的子View没有消费该事件则会一层一层地往父容器上面分发，最终如果都没有捕获该事件则交给Activity的onTouchEvent方法，这个方法默认返回false，所以最终没消费的话会再往上交给最初事件分发者。如果在以上分发过程中有View消费了该事件，则会记住该View，ACTION_DOWN之后的ACTION_MOVE、ACTION_UP等事件会直接分发给该View处理，直到新一次的ACTION_DOWN开始会重新走分发逻辑。

Android事件分发机制解析

概述

Activity

ViewGroup

View

总结

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