onInterceptTouchEvent 与 onTouchEvent 分析与MotionEvent在ViewGroup与View中的分发

onInterceptTouchEvent 与 onTouchEvent 分析与MotionEvent在ViewGroup与View中的分发

       Notice：本文将紧接着

Android 触屏事件
OnTouch onClick onTouchEvent对于触屏事件的处理和分发

这一片博文来分析，假设您还没有读过这一片博文，强烈建议你先读一次上述博文

     OK，言归正传，我们開始吧

      近期，一直听到有人在争论关于dispatchTouchEvent这个函数 和 onInterceptTouchEvent这两个函数究竟是那一这真正的决定了MotionEvent的分发。

这里我还是统一给出答案吧。我们都知道假设我们不希望我们的ViewGroup阻拦我们的View获得MotionEvent。我们一般仅仅须要在onInterceptTouchEvent这个函数中return
false（而且该函数默认return false）。或者，在特定的情况下，假设我们希望某些时候交给我们的我们ViewGroup来处理有些情况下我们又希望我们的View来处理MotionEvent。这样的情况下，我们应该怎么来处理呢。我想大家都知道，我们须要在onInterceptTouchEvent做一些处理，由于大家都知道onInterceptTouchEvent是用来做MotionEvent事件的处理对象推断的。

    那么，答案非常明显了，处理MotionEvent仅仅有dispatchTouchEvent（事实上这一点从函数名称都能够看出来。更不要说onInterceptTouchEvent还仅仅是ViewGroup特有的方法）。

    好，我们先来整理我们眼下已知的。而且提出疑问然后再開始分析

 

    一、Android 中的MotionEvent事件分发处理是由dispatchTouchEvent来完毕的（底层直接调用该方法。我们这里不分析）

  

 
  二、通过上一篇博文我们知道dispatchTouchEvent > onTouch > onTouchEvent > onClick

 
          这里我还是简单啰嗦一下吧。事实上onTouchListener 和 onClickListener 仅仅是为了提供给上层便捷的处理接口，他们的存在仅仅是为了对于开发提供了便捷，可是在Android中我们通常採用onInterceptTouchEvent和onTouchEvent
联合来推断MotionEvent的处理对象

 
  三、我们在ViewGroup中採用onInterceptTouchEvent和onTouchEvent
联合来推断MotionEvent的处理对象

 
   OK。结论到此结束，我们应该提出问题。

 
    一、既然MotionEvent事件是由dispatchTouchEvent来分发的。那么他和onInterceptTouchEvent又是什么关系呢

        二、onInterceptTouchEvent和onTouchEvent
是如何来完毕事件分发的

    那么，到了如今，我们的主题仅仅有一个。解决上面的两个问题就可以。

     对于第一个问题，最好的解决方案，非常明显我们须要靠源代码来解决

     以下，我先给出ViewGroup重写之后的dispatchTouchEvent函数。这里我们须要注意的事他并没有调用super.dispatchTouchEvent。

而是实实在在的重写了全部逻辑（至于View源代码的dispatchTouchEvent，各位看官去我的上一篇博文里面去看看吧）

@Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
        }

        if (DBG_MOTION || DBG_TOUCH) {
            Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent 1: ev = " + ev + ",mFirstTouchTarget = "
                    + mFirstTouchTarget + ",this = " + this);
        }

        boolean handled = false;
        if (onFilterTouchEventForSecurity(ev)) {  //上一篇博文中提到的遮蔽推断事实上是错误的，这里指的应该是窗体被其它窗体遮挡。非常明显普通情况下是不会的
            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);//这里開始推断
                    /// M : add log to help debugging
                    if (intercepted == true) {
                        if (DBG_TOUCH) {
                            Xlog.d(TAG, "Touch event was intercepted event = " + ev + ",this = " + this);
                        }
                    }
                    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;
            }

            // 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;
            if (DBG_MOTION) {
                Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent 2: actionMasked = " + actionMasked
                        + ",intercepted = " + intercepted + ",canceled = " + canceled + ",split = "
                        + split + ",mChildrenCount = " + mChildrenCount + ",mFirstTouchTarget = "
                        + mFirstTouchTarget + ",this = " + this);
            }

            TouchTarget newTouchTarget = null;
            boolean alreadyDispatchedToNewTouchTarget = false;
if (!canceled && !intercepted) {     //根据interceptd 分析转折点
                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 View[] children = mChildren;

                        final boolean customOrder = isChildrenDrawingOrderEnabled();
                        for (int i = childrenCount - 1; i >= 0; i--) {
                            final int childIndex = customOrder ?
                                    getChildDrawingOrder(childrenCount, i) : i;
                            final View child = children[childIndex];
                            if (!canViewReceivePointerEvents(child)
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                if (DBG_MOTION) {
                                    Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent continue 6: i = "
                                            + i + ",count = " + childrenCount + ",child = " + child
                                            + ",this = " + this);
                                }
                                continue;
                            }

                            newTouchTarget = getTouchTarget(child);
                            if (DBG_MOTION) {
                                Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent to child 3: child = "
                                        + child + ",childrenCount = " + childrenCount + ",i = " + i
                                        + ",newTouchTarget = " + newTouchTarget + ",idBitsToAssign = "
                                        + idBitsToAssign + ",mFirstTouchTarget = " + mFirstTouchTarget
                                        + ",this = " + this);
                            }
                            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();
                                mLastTouchDownIndex = childIndex;
                                mLastTouchDownX = ev.getX();
                                mLastTouchDownY = ev.getY();
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                alreadyDispatchedToNewTouchTarget = true;
                                break;
                            }
                        }
                    }

                    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) {
                if (DBG_MOTION) {
                    Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent mFirstTouchTarget = null, canceled = "
                            + canceled + ",this = " + this);
                }
                // 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 (DBG_MOTION) {
                            Xlog.d(TAG, "dispatchTouchEvent middle 5: cancelChild = " + cancelChild
                                    + ",mFirstTouchTarget = " + mFirstTouchTarget + ",target = "
                                    + target + ",predecessor = " + predecessor + ",next = " + next
                                    + ",this = " + this);
                        }
                        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 (DBG_MOTION) {
            Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent end 4: handled = " + handled + ",mFirstTouchTarget = "
                    + mFirstTouchTarget + ",this = " + this);
        }

        if (!handled && mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
        }
        return handled;
    }

      通过上面我标红色的代码(做标记地方。显示有问题)。我们非常明显的看得出来通过onInterceptTouchEvent终于交给了dispatchTransformedTouchEvent来处理。那么dispatchTransformedTouchEvent又做了什么呢

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 (DBG_MOTION) {
            Xlog.d(TAG, "dispatchTransformedTouchEvent 1: event = " + event + ",cancel = "
                    + cancel + ",oldAction = " + oldAction + ",desiredPointerIdBits = "
                    + desiredPointerIdBits + ",mFirstTouchTarget = " + mFirstTouchTarget
                    + ",child = " + child + ",this = " + this);
        }

        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);
            if (DBG_MOTION) {
                Xlog.d(TAG, "Dispatch cancel action end: handled = " + handled + ",oldAction = "
                        + oldAction + ",child = " + child + ",this = " + this);
            }
            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) {
            Xlog.i(TAG, "Dispatch transformed touch event without pointers in " + this);
            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);
                }
                if (DBG_MOTION) {
                    Xlog.d(TAG, "dispatchTransformedTouchEvent 2 to child " + child
                            + ",handled = " + handled + ",mScrollX = " + mScrollX + ",mScrollY = "
                            + mScrollY + ",mFirstTouchTarget = " + mFirstTouchTarget + ",event = "
                            + event + ",this = " + this);
                }
                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);
        }

        if (DBG_MOTION) {
            Xlog.d(TAG, "dispatchTransformedTouchEvent 3 to child " + child + ",handled = "
                    + handled + ",mScrollX = " + mScrollX + ",mScrollY = " + mScrollY
                    + ",mFirstTouchTarget = " + mFirstTouchTarget + ",transformedEvent = "
                    + transformedEvent + ",this = " + this);
        }

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

从上面的代码来推断我们如果我们不重载

public boolean onInterceptTouchEvent(MotionEvent ev) {
        return false;
    }

那么非常明显了，我们的ViewGroup不会吃掉本次MotionEvent

从上一博文中的MyImageView被点击来推断

当我们点击我们ImageView时

我们继续接着我们的Log来分析

MotionEvent事件首先传递给了我们的MyFrameLayout，这时，我们要注意尽管我们设置了onTouchListener和OnClickListener。可是由于我们没有重载onInterceptTouchEvent默认的onInterceptTouchEvent直接return
false;所以我们的MyFrameLayout没有资格消费本次的MotionEvent事件。终于经过推断dispatchTransformedTouchEvent把事件交给了我们的MyImageView

handled = child.dispatchTouchEvent(transformedEvent);

当然
由于我们的MyImageView 也设置了onTouchListener和onClickListener，所以他顺理成章的消费了本次MotionEvent

当我们再点击我们FrameLayout

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这时尽管我们的onInterceptTouchEvent还是return  false；可是我们的touchTarget是空的，所以

if (child == null) {
            handled = super.dispatchTouchEvent(transformedEvent);
        } else 

我们的MyFrameLayout 也还是终于享受到了MotionEvent

  所以说  不一定我们的onInterceptTouchEvent
返回了false 我们的ViewGroup就不能接收到OnTouchEvent了

这里，我们再次考虑
 当我们的onInterceptTouchEvent 被我们重载  并返回了true 那么走的路线就不一样了

if (!canceled && !intercepted) {     //根据interceptd 分析转折点</span>

if (mFirstTouchTarget == null) {
                if (DBG_MOTION) {
                    Xlog.d(TAG, "(ViewGroup)dispatchTouchEvent mFirstTouchTarget = null, canceled = "
                            + canceled + ",this = " + this);
                }
                // No touch targets so treat this as an ordinary view.
                handled = dispatchTransformedTouchEvent(ev, canceled, null,
                        TouchTarget.ALL_POINTER_IDS);

这里注意，由于我们的onInterceptTouchEvent return true;我们直接跳过了Child的推断并终于导致我们的child == null;那么非常明星我们的MotionEvent仅仅能被我们的ViewGroup消费了。

 
      那么，到了这里我们開始大胆的做出如果性结论：

 
      一、仅仅要我们的onInterceptTouchEvent return
true 那么我们的MotionEvent 与ChildView 无缘

 
       二、假设我们的onInterceptTouchEvent  return false。那么我们的ChildView  会优先获得MotionEvent
。可是当我们的ChildView  并不在TouchTarget上，我们的ViewGroup依旧有机会得到本次MotionEvent 。

 
   那么接下来，我们全部做的就仅仅能验证我们的结论了。

 
  这里我们先验证第一条// 这里的验证我们将根据ACTION的不同来分析。

 
  这里我们对我们的MyFrameLayout  做出改变（重载onInterceptTouchEvent ）

@Override
    public boolean onInterceptTouchEvent(MotionEvent event) {
        switch(event.getAction()){
        case MotionEvent.ACTION_DOWN:{
            Log.d(TAG,"MyFrameLayout onInterceptTouchEvent ACTION_DOWN return true");
            return true;
        }
        case MotionEvent.ACTION_MOVE:{
            Log.d(TAG,"MyFrameLayout onInterceptTouchEvent ACTION_MOVE return true");
            return true;
        }
        case MotionEvent.ACTION_UP:{
            Log.d(TAG,"MyFrameLayout onInterceptTouchEvent ACTION_UP return true");
            return true;
        }
        }
        return true;
    }

简而言之。就是任何情况下，我们都要我们的onInterceptTouchEvent  return true；

并分别改写我们的FrameLayout
和ImageView的OnTouchEvent函数

 @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.d(TAG,"MyFrameLayout onTouchEvent"+event.getAction());
        switch(event.getAction()){
        case MotionEvent.ACTION_DOWN:{
            Log.d(TAG,"MyFrameLayout onTouchEvent ACTION_DOWN return true");
            break;
        }
        case MotionEvent.ACTION_MOVE:{
            Log.d(TAG,"MyFrameLayout onTouchEvent ACTION_MOVE return true");
            break;
        }
        case MotionEvent.ACTION_UP:{
            Log.d(TAG,"MyFrameLayout onTouchEvent ACTION_UP return true");
            break;
        }
        }
        return super.onTouchEvent(event);
    }

 @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.d(TAG,"MyImageView onTouchEvent"+event.getAction());
        switch(event.getAction()){
        case MotionEvent.ACTION_DOWN:{
            Log.d(TAG,"MyImageView onTouchEvent ACTION_DOWN return true");
            break;
        }
        case MotionEvent.ACTION_MOVE:{
            Log.d(TAG,"MyImageView onTouchEvent ACTION_MOVE return true");
            break;
        }
        case MotionEvent.ACTION_UP:{
            Log.d(TAG,"MyImageView onTouchEvent ACTION_UP return true");
            break;
        }
        }
        return super.onTouchEvent(event);
    }

首先我们点击我们的MyImageView,我们来看一下Log

非常明显。一旦我们的onInterceptTouchEvent
 return true,我们的子View就与MotionEvent 无缘了

接下来。我们继续点击我们的FrameLayout

Log 图，我就不贴了，和上面一模一样，为什么呢，非常显然我们的子View就与MotionEvent 无缘了

接下来。我们继续做出改变

我们在FrameLayout中将onInterceptTouchEvent
 中的MotionEvent.ACTION_DOWN returnfalse。

依旧点击我们的点击我们的MyImageView

这里我们来分析一下Log   首先我们须要知道3 == MotionEvent.ACTION_CANCEL

那么，首先由于我们的onInterceptTouchEvent  在MotionEvent.ACTION_DOWN
returnfalse；所以我们的FrameLayout没办法继续往下走了（不会走onTouch 和 onTouchEvent）转而MotionEvent事件转交给我们的ImageView

所以我们的ImageView总算是得到了MotionEvent.ACTION_DOWN

可是在经过MotionEvent.ACTION_MOVE时。由于我们的onInterceptTouchEvent
 返回了true。所以非常明显我们的FrameLayout告诉了系统 这次的MotionEvent事件我要了。所以我们的ImageView的到了MotionEvent.ACTION_CANCEL
 反而我们的FrameLayout 能够继续往下走了。

所以说我们的ImageView猜中了开头缺有猜中结局。尽管得到了ACTION_DOWN，但却什么都做不了仅仅能以ACTION_CANCEL草草收场，所以也不会触发onClickListener

同理我们的FrameLayout尽管得到了ACTION_UP
可是没有ACTION_DOWN 也是徒劳。

相同不会OnClickListener

到了这里，我们相同也是能够类推的
假设我们的FrameLayout在onInterceptTouchEvent  在ACTION_MOVE中返回false，我们的ImageView能够接受到ACTION_MOVE
可是终于接受的还是ACTION_CANCEL依旧接受不到ACTION_UP ，反而我们的FrameLayout
仅仅能得到ACTION_UP 。简而言之就是说根据onInterceptTouchEvent  的返回值不同各MotionEvent事件终于要么被ViewGroup
要么被ChildView获取

从上面的Log，我们依旧能够看到的是 onInterceptTouchEvent
并不会在每一次 MotionEvent事件（ACTION_DOWN、ACTION_MOVE、ACTION_UP
等）调用，例假设在ACTION_DOWN return true 交给了ViewGroup 而View没有得到的话。ACTION_MOVE时就不会调用，可是假设return
false，ChildView 得到了ACTION_MOVE时就会再次调用