一步步探索学习Android Touch事件分发传递机制（三）

前言

该系列文章分三篇：

1. 一步步探索学习Android Touch事件分发传递机制（一）
   通过写demo打Log，以ACTION_DOWN事件为例，完整了解整个Android Touch事件分发传递机制。

2. 一步步探索学习Android Touch事件分发传递机制（二）
   探索了ACTION_MOVE和ACTION_UP事件的分发传递规律。

3. 一步步探索学习Android Touch事件分发传递机制（三）
   即本篇，将通过Android源码分析，从本质上认识Android Touch事件分发传递机制。

1. dispatchTouchEvent()方法源码分析

1.1 Activity的dispatchTouchEvent()方法

• 源码：

   /**     * Called to process touch screen events.  You can override this to     * intercept all touch screen events before they are dispatched to the     * window.  Be sure to call this implementation for touch screen events     * that should be handled normally.     *     * @param ev The touch screen event.     *     * @return boolean Return true if this event was consumed.     */    public boolean dispatchTouchEvent(MotionEvent ev) {        if (ev.getAction() == MotionEvent.ACTION_DOWN) {            //当判断是ACTION_DOWN事件，回调onUserInteraction()            onUserInteraction();        }        if (getWindow().superDispatchTouchEvent(ev)) {            //getWindow（）方法拿到的是PhoneWindow的实例            return true;        }        //如果没有找到消费该事件的子View，最终会交给Activity的onTouchEvent()处理        return onTouchEvent(ev);    }

• 分析：

  • 该方法首先判断传递进来的是不是一个ACTION_DOWN事件，if 是，就触发一个叫做 onUserInteraction()的回调方法。

  • onUserInteraction()方法在Activity.java中是个空实现。开发者可以在需要的时候重写它，比如用于判断是不是用户开始做与屏幕交互的事情了。

  • 然后，判断调用了getWindow().superDispatchTouchEvent(ev)。getWindow()拿到的是PhoneWindow的实例。

  • 这里需要简单说明一下Android窗口结构：
    

  • 继续看源码，看看PhoneWindow.java中的superDispatchTouchEvent(ev)方法。

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

  • 可以看到，这个方法内部又去调用了DecorView的superDispatchTouchEvent(ev)方法。

  • 一路这么调用，DecorView其实是FrameLayout的子类，它没重写这个方法，所以最后调用到了ViewGroup的dispatchTouchEvent()方法。

1.2 ViewGroup的dispatchTouchEvent()方法

• 源码：

    @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 = 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 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;    }

• 分析：

  • 这个方法的代码逻辑比较多，有两百多行。我们去繁就简，理出主要的逻辑就行，毕竟我们只关心Touch事件的流向和处理逻辑。

  • 首先来看一下，ViewGroup的dispatchTouchEvent()方法首先是要判断该事件自己要不要拦截下来自己处理。

         // Check for interception.            final boolean intercepted;            if (actionMasked == MotionEvent.ACTION_DOWN                    || mFirstTouchTarget != null) {                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;                if (!disallowIntercept) {                    intercepted = onInterceptTouchEvent(ev);                    ev.setAction(action); // restore action in case it was changed                } else {                    intercepted = false;                }            } else {                // There are no touch targets and this action is not an initial down                // so this view group continues to intercept touches.                intercepted = true;            }

  • 这里面我们看到一个很关键的boolean变量disallowIntercept，这个变量是控制是不是不允许父控件去拦截该事件的，取值是看mGroupFlags的取值。

  这里面涉及到一个方法：requestDisallowInterceptTouchEvent()方法。这个方法确定mGroupFlags的取值，控制请求父布局不拦截该事件，而是交给自己去做处理。这个方法在处理滑动冲突等场景时经常用到。但在这里为了整个源码分析的逻辑简洁清晰，不再具体分析该方法的代码。

  • 还有大家可以注意到一个判断条件： if (actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != null)，也就是说ViewGroup去判断这个事件该不该去拦截，首先是这个事件得是ACTION_DOWN事件或者该事件的mFirstTouchTarget（目标子View）是不为空的才会考虑要不要拦截。

  • 这说明mFirstTouchTarget为空的情况下，ACTION_MOVE和ACTION_UP事件是不会经过这个拦截判断的，而是直接intercepted = true表示事件被直接拦截掉。这一点刚好印证了我在一步步探索学习Android Touch事件分发传递机制（二）中提到的ACTION_MOVE和ACTION_UP事件的分发规律。

  • 那么mFirstTouchTarget（目标子View）这个变量到底是什么呢？什么时候会为空呢？

  • 可以看到ViewGroup的dispatchTouchEvent()方法的后续代码，是一个for循环：

    ... for (int i = childrenCount - 1; i >= 0; i--) {                                    final int childIndex = getAndVerifyPreorderedIndex(                                            childrenCount, i, customOrder);                                    final View child = getAndVerifyPreorderedView(                                            preorderedList, children, 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.                                }                                ...

    • 这个for循环一个一个的遍历子View，寻找看事件的发生坐标在哪个View的范围中，如果找到了，就设置mFirstTouchTarget为child,并且把alreadyDispatchedToNewTouchTarget设置为true。

    • 那么事件最终交给自己处理还是目标子View（mFirstTouchTarget）处理？

    • 很简单，经过上面的分析可以知道，如果遍历完之后mFirstTouchTarget不为null，就传给mFirstTouchTarget（目标子View）处理；如果为null，就自己消费掉。

    • 那其实不管是给目标子View处理还是自己处理，都会跑到View的dispatchTouchEvent()方法。看源码可以知道，当mFirstTouchTarget为null的时候，ViewGoup会调用super.dispatchTouchEvent(event),毕竟ViewGroup本质上是View的子类，所以其实ViewGroup调用的还是View的dispatchTouchEvent()方法。那么我们下面分析View的dispatchTouchEvent()方法。

  1.3 View的dispatchTouchEvent()方法

  • 源码：

      /**     * Pass the touch screen motion event down to the target view, or this     * view if it is the target.     *     * @param event The motion event to be dispatched.     * @return True if the event was handled by the view, false otherwise.     */    public boolean dispatchTouchEvent(MotionEvent event) {        // If the event should be handled by accessibility focus first.        if (event.isTargetAccessibilityFocus()) {            // We don't have focus or no virtual descendant has it, do not handle the event.            if (!isAccessibilityFocusedViewOrHost()) {                return false;            }            // We have focus and got the event, then use normal event dispatch.            event.setTargetAccessibilityFocus(false);        }        boolean result = false;        if (mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onTouchEvent(event, 0);        }        final int actionMasked = event.getActionMasked();        if (actionMasked == MotionEvent.ACTION_DOWN) {            // Defensive cleanup for new gesture            stopNestedScroll();        }        if (onFilterTouchEventForSecurity(event)) {            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;    }

• 分析：

  • 代码量相对于ViewGroup的dispatchTouchEvnet()方法来说少很多。

  • 主要是首先判断这个View本身有没有设置OnTouchListener监听，如果有，就直接跑去调用该接口下的onTouch()方法。该方法如果return true,这事件就是被消费掉了。return false,事件还是会传回给onTouchEvent()方法。

  注意：值得注意的是，ViewGroup本身并没有重写View的onTouchEvnet()方法，所以这里如果回传，也是调用的父类View.java的onTouchEvent()方法。

2. onTouchEvent()方法源码分析

2.1 Activity的onTouchEvent()方法

• 源码：

 /**     * Called when a touch screen event was not handled by any of the views     * under it.  This is most useful to process touch events that happen     * outside of your window bounds, where there is no view to receive it.     *     * @param event The touch screen event being processed.     *     * @return Return true if you have consumed the event, false if you haven't.     * The default implementation always returns false.     */    public boolean onTouchEvent(MotionEvent event) {        if (mWindow.shouldCloseOnTouch(this, event)) {            finish();            return true;        }        return false;    }

• 分析：

  • 这个方法代码很少，默认Activity的onTouchEvent()方法是返回false的，也就是说默认不处理触摸事件。

  • 只有在PhoneWindow的shouldCloseOnTouch（）方法返回true才会处理触摸事件，直接finish整个Activity.

2.2 View的onTouchEvent()方法

前面说了，ViewGroup没有重写View的onTouchEvent()方法，所以继承VIewGroup时，调用的还是View的onTouchEvent()。

• 源码：

/**     * Implement this method to handle touch screen motion events.     * <p>     * If this method is used to detect click actions, it is recommended that     * the actions be performed by implementing and calling     * {@link #performClick()}. This will ensure consistent system behavior,     * including:     * <ul>     * <li>obeying click sound preferences     * <li>dispatching OnClickListener calls     * <li>handling {@link AccessibilityNodeInfo#ACTION_CLICK ACTION_CLICK} when     * accessibility features are enabled     * </ul>     *     * @param event The motion event.     * @return True if the event was handled, false otherwise.     */    public boolean onTouchEvent(MotionEvent event) {        final float x = event.getX();        final float y = event.getY();        final int viewFlags = mViewFlags;        final int action = event.getAction();        final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE                || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)                || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;        if ((viewFlags & ENABLED_MASK) == DISABLED) {            if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {                setPressed(false);            }            mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;            // A disabled view that is clickable still consumes the touch            // events, it just doesn't respond to them.            return clickable;        }        if (mTouchDelegate != null) {            if (mTouchDelegate.onTouchEvent(event)) {                return true;            }        }        if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {            switch (action) {                case MotionEvent.ACTION_UP:                    mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;                    if ((viewFlags & TOOLTIP) == TOOLTIP) {                        handleTooltipUp();                    }                    if (!clickable) {                        removeTapCallback();                        removeLongPressCallback();                        mInContextButtonPress = false;                        mHasPerformedLongPress = false;                        mIgnoreNextUpEvent = false;                        break;                    }                    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:                    if (event.getSource() == InputDevice.SOURCE_TOUCHSCREEN) {                        mPrivateFlags3 |= PFLAG3_FINGER_DOWN;                    }                    mHasPerformedLongPress = false;                    if (!clickable) {                        checkForLongClick(0, x, y);                        break;                    }                    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, x, y);                    }                    break;                case MotionEvent.ACTION_CANCEL:                    if (clickable) {                        setPressed(false);                    }                    removeTapCallback();                    removeLongPressCallback();                    mInContextButtonPress = false;                    mHasPerformedLongPress = false;                    mIgnoreNextUpEvent = false;                    mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;                    break;                case MotionEvent.ACTION_MOVE:                    if (clickable) {                        drawableHotspotChanged(x, y);                    }                    // Be lenient about moving outside of buttons                    if (!pointInView(x, y, mTouchSlop)) {                        // Outside button                        // Remove any future long press/tap checks                        removeTapCallback();                        removeLongPressCallback();                        if ((mPrivateFlags & PFLAG_PRESSED) != 0) {                            setPressed(false);                        }                        mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;                    }                    break;            }            return true;        }        return false;    }

• 分析：

  • 代码同样很长，那么我们只去理清主要的逻辑。

  • 当事件传递到该方法处，首先会判断这个View是不是enabled状态，是不是clickable状态。

  • 然后会根据Touch事件的类型做出不同的响应。比如View接收到Down事件和up事件等时候的表现效果。

  注意：注意到一段比较重要的代码
  if (!post(mPerformClick)) {
performClick();
}
  这段代码在判断是up事件之后调用了performClick()方法，这个方法回去回调onClickListener接口里面的onClick()方法。
  这结合前面dispatchTouchEvent()方法中ACTION_DOWN事件会去调用onTouch，可见onTouch比onClick优先。

3. onInterceptTouchEvent()方法源码分析

只有ViewGroup有onInterceptTouchEvent()方法

• 源码：

  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;    }

• 分析：

  • 这个方法默认是返回false，表示不拦截触摸事件的。

  • 只有在
    ev.isFromSource(InputDevice.SOURCE_MOUSE)
    && ev.getAction() == MotionEvent.ACTION_DOWN
    && ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
    && isOnScrollbarThumb(ev.getX(), ev.getY())
    这么多条件同时成立时才会拦截。

• 有需要的话，比如处理滑动冲突的时候，可以重写该方法，retrun true，拦截触摸事件。