View的事件分发机制

一、点击事件的传递规则

1、事件分发过程

//伪代码
    fun dispatchTouchEvent(event: MotionEvent?) : Boolean{
        var consume = false
        if (onInterceptTouchEvent(event)) {
            consume = onTouchEvent(event)
        } else {
            consume = child.dispatchTouchEvent(event)
        }
        return consume
    }

dispatchTouchEvent
用来进行事件的分发，如果事件能传递给当前View，那么此方法一定会被调用，返回结果受当前View的onTouchEvent和下级View的dispatchEvent方法的影响，表示是否消耗当前事件。

onInterceptTouchEvent
用来判断是否拦截某个事件，如果当前View拦截了某个事件，那么在同一个时间序列当中，此方法不会被再次调用，返回结果表示是否拦截当前事件。

onTouchEvent
用来处理点击事件，返回结果表示是否消耗当前事件，如果不消耗，则在同一个事件序列中，当前View无法再次接受到事件。

2、View处理事件顺序

1、如果View设置了onTouchListener，先调用OnTouchListener.onTouch方法，如果为true则onTouchEvent 返回true。
2、如果当前设置了OnClickListener，在onTouch执行后会执行onClick，可见onClickListener的优先级最低，处于事件传递的尾端。
3、当一个点击事件产生后，它的传递过程遵循：Activity->Window->View，顶级View接收到事件后就会按照事件分发机制去分发事件。

3、总结

（1）同一个事件序列是指从手指触碰屏幕的那一刻起，到手指离开屏幕的那一刻结束，在这个过程中所产生的的一系列事件，以down事件开始，中间含有数量不定的move事件，最终以up事件结束。

（2）正常情况下一个事件序列只能被一个View拦截且消耗，因为一旦一个元素拦截了某此事件，那么同一个事件序列的所有事件都会直接交给它处理。

（3）某个View一旦决定拦截，那么这个事件序列都只能由它来处理，并且它的onInterceptTouchEvent不会再被调用。

（4）某个View一旦开始处理事件，如果它不消耗ACTION_DOWN事件，那么同一个事件序列中的其他事件都不会再交给它来处理，并且事件将重新交由它的父元素去处理，即父元素的onTouchEvent会被调用。

（5）如果View不消耗除ACTION_DOWN以外的其他事件，那么这个点击事件就会消失，此时父元素的onTouchEvent并不会被调用，并且当前View可以持续受到后续的事件，最终这些消失的点击事件会传递给Activity处理。

（6）ViewGroup默认不拦截任何事件。Android源码中ViewGroup的OnInterceptTouchEvent方法默认返回false。

（7）View没有onInterceptTouchEvent方法，一旦有点击事件传递给它，那么它的onTouchEvent方法就会调用。

（8）View的onTouchEvent默认都会消耗事件，除非它是不可点击的（clickable和longClickable都会false）。View的longClickable属性默认为false，clickable属性要分情况，如Button的clickable属性默认为true，而TextView的clickable属性默认为false。

（9）View的enable属性不影响onTouchEvent的默认返回值，哪怕一个View是disable状态的，只要它的clickable或者longClickable一个为true，那么它的onTouchEvent就返回true。

（10）onClick会发生的前提是当前View是可点击的，并且它收到了down和up的事件。

（11）事件传递是由外向内的，即事件总是先传递给父元素，然后由父元素分发给子View，通过requestDisallowInterceptTouchEvent方法可以在子元素中干预父元素的事件分发过程，ACTION_DOWN事件除外。

二、源码分析

1、Activity对点击事件的分发过程

touch事件最先传递给当前Activity，由Activity的dispatchTouchEvent来进行事件派发，具体工作由Activity内部的Window来完成，Window会将事件传递给decorView。

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

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

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

Window的唯一实现类是PhoneWindow。DecorView继承了FrameLayout，所以它的super.dispatchTouchEvent实际上就是调用了ViewGroup#dispatchTouchEvent。

2、顶级View对点击事件的分发过程

（1）事件拦截
先看ViewGroup.dispatchTouchEvent中部分代码，关于判断View是否拦截点击事件。

 @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;
            }

            ...
        }

        ...
    }

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

ViewGroup在以下两种情况下会判断是否拦截当前事件：事件类型为ACTION_DOWN或mFirstTouchTarget != null。mFirstTouchTarget指向成功处理了touchEvent的子元素，当ViewGroup不拦截事件并将事件交给子元素处理时mFirstTouchTarget != null 为true；一旦事件由当前ViewGroup拦截时，mFirstTouchTarget != null 为false。

FLAG_DISALLOW_INTERCEPT标志位，该标志位是通过requestDisallowInterceptTouchEvent设置的，一般用于子View中。该标志位一旦设置后，ViewGroup将无法拦截除了ACTION_DOWN以外的其他点击事件。因为在判断MotionEvent为ACTION_DOWN时会通过resetTouchState方法重置该标志位。

结论
当ViewGroup决定拦截事件后，那么后续的点击事件将会默认交给它处理并且不再调用它的onInterceptTouchEvent方法。FLAG_DISALLOW_INTERCEPT这个标志的作用是让ViewGroup不再拦截事件，前提是ViewGroup不拦截ACTION_DOWN事件。

（2）分发事件

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {

                    ...

                    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 (!child.canReceivePointerEvents()
                                    || !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;
                            }
       ...
      
    }

    protected boolean isTransformedTouchPointInView(float x, float y, View child,
            PointF outLocalPoint) {
        final float[] point = getTempPoint();
        point[0] = x;
        point[1] = y;
        transformPointToViewLocal(point, child);
        final boolean isInView = child.pointInView(point[0], point[1]);
        if (isInView && outLocalPoint != null) {
            outLocalPoint.set(point[0], point[1]);
        }
        return isInView;
    }

    /**
     * Adds a touch target for specified child to the beginning of the list.
     * Assumes the target child is not already present.
     */
    private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
        final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
        target.next = mFirstTouchTarget;
        mFirstTouchTarget = target;
        return target;
    }

分发事件的逻辑，首先遍历ViewGroup中的所有子元素，判断子元素是否能够接收到点击事件（!child.canReceivePointerEvents() || !isTransformedTouchPointInView(x, y, child, null)），根据子元素是否在执行动画与坐标是否在子元素内且可见。

在dispatchTransformedTouchEvent方法中调用了子元素的dispatchTouchEvent方法，由子元素继续向下分发事件。如果子元素消耗了事件，则dispatchTransformedTouchEvent返回true，同时通过addTouchTarget()更新mFirstTouchTarget，跳出for循环；如果子元素的dispatchTouchEvent返回false，则ViewGroup会把事件分发给下一个子元素。

addTouchTarget中完成了对mFirstTouchTarget的赋值，mFirstTouchTarget其实是一种单链表结构。mFirstTouchTarget是否被赋值，将直接影响到ViewGroup对事件的拦截策略，如果mFirstTouchTarget为null，那么ViewGroup就默认拦截接下来同一序列所有的点击事件。

（3）特殊情况
如果遍历所有的子元素后事件没有被合适地处理，有两种可能：
1、ViewGroup没有子元素
2、子元素处理了点击事件，但是在dispatchTouchEvent中返回了false，这一般是因为子元素在onTouchEvent中返回了false。

    public boolean dispatchTouchEvent(MotionEvent ev) {

        ...

            // 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 {

        ...
        
    }

    private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
            View child, int desiredPointerIdBits) {
        final boolean 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;
    }

在这种情况下，ViewGoup会自己处理点击事件，这时就回到了View的dispatchTouchEvent方法处理。

一开始有个疑问就是，为什么很多情况 如FrameLayout嵌套RecyclerView，当MOVE事件产生后，FrameLayout.onInterceptTouchEvent方法不调用，原因只能是RecyclerView将父容器的FLAG_DISALLOW_INTERCEPT标志值设为true

View对点击事件的处理过程

（1）dispatchTouchEvent 分发事件

    /**
     * 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) {

        ...

        boolean result = false;

        ...

        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);
        }

        ...

        return result;
    }

1、首先判断有没有设置OnTouchListener，如果onTouchListener.onTouch()返回true，则onTouchEvent方法不会调用。
2、接着判断onTouchEvent方法，如果返回true则表示消耗掉此次事件。

（2）onTouchEvent

    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;
            }
        }

        ...
        
    }

1、当View处于不可用状态下，如果此时View是可点击的，onTouchEvent仍然会消耗点击事件但不会触发点击回调。
2、在处理View本身的touch事件前，还会对View的代理mTouchDelegate做一层检测，如果为true则返回。

关于点击回调

    public boolean onTouchEvent(MotionEvent event) {
       
       ...

        if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
            switch (action) {
                case MotionEvent.ACTION_UP:
                   
                    ...

                    boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
                    if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
                        
                        ...

                        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)) {
                                    performClickInternal();
                                }
                            }
                        }

                        ...

                    break;

                    ...
                    
            }

            return true;
        }

        return false;
    }

    public boolean performClick() {
        // We still need to call this method to handle the cases where performClick() was called
        // externally, instead of through performClickInternal()
        notifyAutofillManagerOnClick();

        final boolean result;
        final ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnClickListener != null) {
            playSoundEffect(SoundEffectConstants.CLICK);
            li.mOnClickListener.onClick(this);
            result = true;
        } else {
            result = false;
        }

        sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);

        notifyEnterOrExitForAutoFillIfNeeded(true);

        return result;
    }

3、只要View的CLICKABLE和LONG_CLICKABLE有一个为true，那么它就会消耗这个事件，即onTouchEvent返回true，不管它是不是disable状态。

4、当ACTION_UP发生时，会触发performClick方法，在performClick中回调点击监听。

5、View的LONG_CLICKABLE属性默认为false，而CLICKABLE的值与具体的View有关，设置了相应的事件后对应的标志值也会被置为true。

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

    public void setOnLongClickListener(@Nullable OnLongClickListener l) {
        if (!isLongClickable()) {
            setLongClickable(true);
        }
        getListenerInfo().mOnLongClickListener = l;
    }

三、ACTION_CANCEL

ACTION_CANCEL事件是收到前驱事件后，后续事件被父控件拦截的情况下产生，onTouchEvent的事件回传到父控件只会发生在ACTION_DOWN事件中

结尾

摘抄自《Android开发艺术探索》