View事件分发、滑动冲突--《Android开发艺术探索》阅读笔记——第三章part2

目录

一、事件分发

1、事件传递规则

2、源码分析

1）Activity的分发过程：

2）顶级View（ViewGroup）的分发过程：分发规则的伪代码来源的详细说明

3)view的处理过程：主要是onTouchListener、OnTouchEvent、onClick的关系

二、滑动冲突

1、冲突场景

2、解决原理

3、举个例子

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View的事件分发，指的是 在多层级ViewGroup/View中对触摸事件的分发。触摸事件即MotionEvent，系统要把它传递给具体的view取处理。

事件序列，从Down开始，到多个Move，再到Up结束，就是一个事件序列。

一、事件分发

1、事件传递规则

事件分发主要3个方法：

public boolean dispatchTouchEvent(MotionEvent ev)，负责事件的传递分发，事件到达view时一定回调此方法。返回值表示是否消费此事件，受onTouchEvent和子view的dispatchTouchEvent返回值影响。

public boolean onInterceptTouchEvent(MotionEvent ev)，用于判断是否拦截事件，也是返回值的含义。在dispatchTouchEvent内部调用。view拦截了某个事件，那后续这一事件序列都会默认拦截，不再调用此方法。

public boolean onTouchEvent(MotionEvent event)，用于处理事件，返回值表示是否消费此事件。

在dispatchTouchEvent内部调用。如果不消耗某一事件，那当前view不再接受同一事件序列的事件。 注意，如果设置了onTouchListener且onTouch返回值是true，那么不会调用onTouchEvent(会给父view处理)。onTouchEvent中发现设置了onClickListener那onCLick会被调用。

三者关系：事件到达view时，会调用dispatchTouchEvent，然后内部调用onInterceptTouchEvent判断是否拦截，如果不拦截就调用子view的dispatchTouchEvent。用伪代码表示：

public boolean dispatchTouchEvent(MotionEvent event){
        boolean consumed;
        if (onIntercepterTouchEvent(event)){
            consumed = onTouchEvent(event);
        }else {
            consumed = child.dispatchTouchEvent(event);
        }

        return consumed;
    }

事件发生时，传递顺序是：Activity-Window-View。如果所有activity的所有子View都不处理，那么最终Activity的onTouchEvent会被调用。

另外几点规则：

1）ViewGroup默认不拦截任何事件。即onInterceptTouchEvent方法返回false

2)  View没有onInterceptTouchEvent（）

3）View的onTouchEvent默认会消耗事件（返回True）,除非它是不可点击的——-clickable 和 longClickable同时为false.注意：View的longClickable默认都为false,clickable要根据控件属性判断。

4）View的enable属性不影响onTouchEvent的默认返回值。（哪怕是disable）

5)  事件总是先传递给父元素，然后由父元素分发给子View.通过requestDisallowIntercept()分发可以在子元素中干预父元素的事件分发过程，但是ACTION_DOWN事件除外。

2、源码分析

1）Activity的分发过程：

public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }
        //调用window的superDispatchTouchEvent来分发。（实际window只有一个实现PhoneWindow）
        if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
        }
        //如果window内所有view没有消费，就会走acticity的onTouchEvent
        return onTouchEvent(ev);
    }

看PhoneWindow：

// This is the top-level view of the window, containing the window decor.
    private DecorView mDecor;

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

看DecorView：

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

继续跟就到了ViewGroup的dispatchTouchEvent。DecorView内部的子View实际就是StatusBar、ActionBar、ContentView这些。

2）顶级View（ViewGroup）的分发过程：分发规则的伪代码来源的详细说明

 @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        ...

        boolean handled = false;
        if (onFilterTouchEventForSecurity(ev)) {
            final int action = ev.getAction();
            final int actionMasked = action & MotionEvent.ACTION_MASK;

            // Handle an initial down.
            // 事件DOWN时会清除 子view的 "不允许父view拦截"的flag。因为Down事件是事件序列的开始，一定会询问本身是否拦截。
            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;
            //是Down 或者 MOVE、UP但没有拦截DOWN让子View处理了（mFirstTouchTarget != null）
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                if (!disallowIntercept) {
                    //如果子View没有不允许拦截，就正常询问自己是否拦截
                    intercepted = onInterceptTouchEvent(ev);
                    ev.setAction(action); // restore action in case it was changed
                } else {
                    //如果子View设置了不允许拦截，就不拦截
                    intercepted = false;
                }
            } else {
                // There are no touch targets and this action is not an initial down
                // so this view group continues to intercept touches.
                //不是Down且子view没有处理，就是说本身拦截了DOWN，那后续的MOVE、UP就不再调用onInterceptTouchEvent询问，直接拦截。
                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;
            
            //如果没有拦截
            if (!canceled && !intercepted) {

                ...

                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 preorderedList = buildTouchDispatchChildList();
                        final boolean customOrder = preorderedList == null
                                && isChildrenDrawingOrderEnabled();
                        final View[] children = mChildren;
                        
                        //就遍历子view
                        for (int i = childrenCount - 1; i >= 0; i--) {
                            final int childIndex = getAndVerifyPreorderedIndex(
                                    childrenCount, i, customOrder);
                            final View child = getAndVerifyPreorderedView(
                                    preorderedList, children, childIndex);

                            ...

                            //判断子view能否接受事件：是否在播动画、是否点击在区域内
                            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);
                            
                            //child不为空就会调child.dispatchTouchEvent(event)
                            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();
                                 //mFirstTouchTarget赋值，就是说child处理了事件
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                alreadyDispatchedToNewTouchTarget = true;
                                break;
                            }

                            ...
                        }
                        if (preorderedList != null) preorderedList.clear();
                    }

                    if (newTouchTarget == null && mFirstTouchTarget != null) {
                        ...
                    }
                }
            }

            // Dispatch to touch targets.
            //所有子View都没处理事件
            if (mFirstTouchTarget == null) {
                // No touch targets so treat this as an ordinary view.
                //那么这里穿的child是null，就是调用super.dispatchTouchEvent(event)，就是View.dispatchTouchEvent(event)
                handled = dispatchTransformedTouchEvent(ev, canceled, null,
                        TouchTarget.ALL_POINTER_IDS);
            } else {
                ...
            }

            ...
        }

        ...
        return handled;
    }

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

        // Canceling motions is a special case.  We don't need to perform any transformations
        // or filtering.  The important part is the action, not the contents.
        final int oldAction = event.getAction();
        if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
            event.setAction(MotionEvent.ACTION_CANCEL);
            if (child == null) {
                //如果所有子view都没有处理，就调用View本身的分发
                handled = super.dispatchTouchEvent(event);
            } else {
                //分发到子view，同样开始遍历
                handled = child.dispatchTouchEvent(event);
            }
            event.setAction(oldAction);
            return handled;
        }
...

3)view的处理过程：主要是onTouchListener、OnTouchEvent、onClick的关系

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)) {
                //设置了OnTouchListener且onTouch返回 true，则看下面一行
                result = true;
            }
            //设置了OnTouchListener且onTouch返回 true就不会走onTouchEvent了
            if (!result && onTouchEvent(event)) {
                result = true;
            }
        }

        ...

        return result;
    }

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.
            //即使disabled，也是可点击也会消费事件
            return clickable;
        }
        if (mTouchDelegate != null) {
            if (mTouchDelegate.onTouchEvent(event)) {
                return true;
            }
        }

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

                            // 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)) {
                                    //走onClick了
                                    performClickInternal();
                                }
                            }
                        }

                        ...
                    break;

                case MotionEvent.ACTION_DOWN:
                    ...
                    break;
                case MotionEvent.ACTION_MOVE:
                    ...
                    break;
            }

            return true;
        }

        return false;
    }

二、滑动冲突

1、冲突场景

ViewGroup的滑动和子view的滑动有冲突。

外部横向，里面竖向，那就是看外部啥时候拦截。

外部里面都是相同方向，那就要看具体的业务规则了。

2、解决原理

因为ViewGroup的onInterceptTouchEvent默认是不拦截的，所以：

解决滑动冲突的要点：重写onInterceptTouchEvent，根据冲突场景的规则来 判断是否拦截，即isIntercept的赋值规则。其他代码不动即可。

（还有个内部拦截法，用子view调用parent.requestDisallowInterceptTouchEvent(true)，就不让parent拦截）

@Override
    public boolean onInterceptTouchEvent(MotionEvent motionEvent) {
        //解决滑动冲突
        boolean isIntercept = false;

        int x = (int) motionEvent.getX();
        int y = (int) motionEvent.getY();

        switch (motionEvent.getAction()) {
            case MotionEvent.ACTION_DOWN:
                //down不能拦截，因为一拦截就都给你了。
                mLastXIntercept = x;
                mLastYIntercept = y;
                break;
            case MotionEvent.ACTION_MOVE:

                //是横向的滑动
                boolean isHorizontal = Math.abs(x - mLastXIntercept) > Math.abs(y - mLastYIntercept);
                  
                //解决滑动冲突的要点：根据冲突场景的规则来 判断是否拦截，即isHorizontal如果取值
                if (isHorizontal) {
                    //此处一旦拦截，后面的ACTION就不会在调用onInterceptTouchEvent
                    isIntercept = true;
                }
                break;
            case MotionEvent.ACTION_UP:
                //up不能拦截，因为拦截了没啥意义。因为如果前面拦截了Down，那自然Up也会给到。
                //另外，如果只拦截了UP，那子view即使处理了其他事件，那也不会走onClick。
                break;
        }

        Log.i(TAG, "onInterceptTouchEvent: isIntercept：" + isIntercept);

        //此处要记录下 拦截时的 开始的 触摸点（仅用于下面onTouchEvent处理滑动使用）
        mLastX = mLastXIntercept;
        mLastY = mLastYIntercept;

        mLastXIntercept = x;
        mLastYIntercept = y;

        return isIntercept;
    }

3、举个例子

之前写的横向滑动View：《Android进阶之光》自定义ViewGroup实例：横向滑动HorizontalView