android view事件分发机制

android 事件分发机制

1.android 点击事件

当用户触摸屏幕时(View 或 ViewGroup派生的控件)，将产生点击事件(Touch事件)
一共包含四种事件类型
事件类型具体动作
MotionEvent.ACTION_DOWN 按下View（所有事件的开始）

MotionEvent.ACTION_UP 抬起View（与DOWN对应）

MotionEvent.ACTION_MOVE 滑动View

MotionEvent.ACTION_CANCEL 结束事件（非人为原因）

一般情况下，事件列都是以DOWN事件开始、UP事件结束，中间有无数的MOVE事件，当一个点击事件（MotionEvent ）产生后，
系统需把这个事件传递给一个具体的 View 去处理

2.事件的传递

事件传递顺序: activity ——>ViewGroup——>View
事件分发过程由哪些方法协作完成？
dispatchTouchEvent() 、onInterceptTouchEvent()和onTouchEvent()

dispatchTouchEvent():分发(传递)事件,当点击事件可以传递时，这个方法就会被调用
onTouchEvent():处理点击事件,在dispatchTouchEvent()内部调用
onInterceptTouchEvent():判断是否拦截某个事件,在ViewGroup的dispatchTouchEvent()内部调用
下面我们逐个来分析各事件的传递

2.1.Activity的事件传递

当一个点击事件发生时，事件最先传到Activity的dispatchTouchEvent()进行事件分发,源码入下:

/**     * 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) {            onUserInteraction();        }        if (getWindow().superDispatchTouchEvent(ev)) {            return true;        }        return onTouchEvent(ev);    }

分析2：getWindow().superDispatchTouchEvent(ev)
说明：
a. getWindow() = 获取Window类的对象
b. Window类是抽象类，其唯一实现类 = PhoneWindow类；即此处的Window类对象 = PhoneWindow类对象
c. Window类的superDispatchTouchEvent() = 1个抽象方法，由子类PhoneWindow类实现
DecorView类是PhoneWindow类的一个内部类
b. DecorView继承自FrameLayout，是所有界面的父类，FrameLayout是ViewGroup的子类，故DecorView的间接父类 = ViewGroup

这就实现了事件从activity 传递到viewGroup,我们看看源码：(截取部分代码)

2.2.ViewGroup事件分发

我们看源码中核心分发方法dispatchTouchEvent

@Override    public boolean dispatchTouchEvent(MotionEvent ev) {                    ...            // Check for interception.            final boolean intercepted;            if (actionMasked == MotionEvent.ACTION_DOWN                    || mFirstTouchTarget != null) {                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;                if (!disallowIntercept) {                            // 重点分析1：ViewGroup每次事件分发时，都需调用onInterceptTouchEvent()询问是否拦截事件            // 判断值1：disallowIntercept = 是否禁用事件拦截的功能(默认是false)，可通过调用requestDisallowInterceptTouchEvent（）修改            // 判断值2： onInterceptTouchEvent(ev)                        // a. 若在onInterceptTouchEvent()中返回false（即不拦截事件），intercepted值为true，从而进入到条件判断的内部                        // b. 若在onInterceptTouchEvent()中返回true（即拦截事件），intercepted值为false，从而跳出了这个条件判断                        // c. 关于onInterceptTouchEvent() ->>分析1                                                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;                        //重点分析2：只有当canceled 、intercepted都为false 时，事件传递才能进行下去，接下来遍历所有子view            找到被触摸或者点击的子view            if (!canceled && !intercepted) {                ...                    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;                        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);                            //重点分析3：将事件继续传递到子view 中，                            // 若该控件可点击，那么点击时，dispatchTouchEvent的返回值必定是true，因此会导致条件判断成立                            alreadyDispatchedToNewTouchTarget 赋值为true,并且直接跳出遍历                            // 于是给ViewGroup的dispatchTouchEvent（）直接返回了true，即直接跳出                            // 即把ViewGroup的点击事件拦截掉                               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.                //没有找到目标view,则viewGroup自己处理该事件，调用onTouch——>onTouchEvent()                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;                }            }            ...        return handled;    }

2.3.View的事件分发过程

接下来我们看view的分发

public boolean dispatchTouchEvent(MotionEvent event) {        ...        if (onFilterTouchEventForSecurity(event)) {            if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {                result = true;            }            //noinspection SimplifiableIfStatement            ListenerInfo li = mListenerInfo;            /**重点分析1：*/            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;    }

重点分析1：
条件1：mOnTouchListener != null，说明：mOnTouchListener变量在View.setOnTouchListener（）方法里赋值，也就是说
如果有设置view 的触摸事件回调，那么，view 会优先执行onTouch方法
条件2：(mViewFlags & ENABLED_MASK) == ENABLED，该条件是判断当前点击的控件是否enable，由于很多View默认enable，
故该条件恒定为true

条件3：mOnTouchListener.onTouch(this, event),说明：即回调控件注册Touch事件时的onTouch（）；需手动复写设置,
以button 为例，

button.setOnTouchListener(new OnTouchListener() {          @Override          public boolean onTouch(View v, MotionEvent event) {                   return false;          }      });

若在onTouch()返回true，就会让上述三个条件全部成立，从而使得View.dispatchTouchEvent（）直接返回true，事件分发结束
若在onTouch()返回false，就会使得上述三个条件不全部成立，从而使得View.dispatchTouchEvent（）中跳出If，执行onTouchEvent(event)

继续来看onTouchEvent()事件处理

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;            }        }        //若该控件可点击，则进入switch判断中        if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {            switch (action) {             //若当前的事件是抬起View（主要分析）                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;            }            // 若该控件可点击，就一定返回true            return true;        }            // 若该控件不可点击，就一定返回false        return false;    }

当view 可点击切且手势为抬手，进过一些判断，会进入执行performClick()方法，这里会判断是否有设置点击事件setOnclickListener()，
如果有，执行onClick(),并返回true

分析到这里，我们可以发现，view的 onTouch()的执行先于 onClick()

总结：
1.dispatchTouchEvent() 逐上至下调用 Activity——>viewGroup——>View
2.onTouchEvent() 逐下至上级返回 view——>ViewGroup——>Activity

3.View的滑动冲突

滑动冲突有三种解决方式

外部拦截法
内部拦截法

3.1 外部拦截法

点击事件都先进过父容器的拦截处理，如果父容器需要此事件，就拦截，不需要则不拦截，外部拦截方法需要重写父容器的onInterceptTouchEvent方法，在内部做相应的拦截即可，类似这样的伪代码：

    public boolean onInterceptTouchEvent(MotionEvent event){        boolean intercepted = false;    int x = (int)event.getX();     int y = (int)event.getY();    switch(event.getAction()){        case MotionEvent.ACTION_DOWN:{            intercepted = false;            break;          }        case MotionEvent.ACTION_MOVE:{            if(父容器需要当前点击事件)            {            intercepted = true;              }else {                intercepted = false;            }            break;          }        case MotionEvent.ACTION_UP:{              intercepted = false;              break;          }            default:        break;        }    mLastXIntercept = x;    mLastYIntercept = y;    return intercepted;}

3.2 内部拦截法

内部拦截法指父容器不拦截任何事件，所有事件都传给子元素，如果子元素需要则消耗掉，不需要则交还给父容器处理，，需配合requestDisallowInterceptTouchEvent()方法才能正常工作，我们需要重写子元素的
dispatchTouchEvent()方法

public boolean dispatchTouchEvent(MotionEvent event){        int x = (int)event.getX();     int y = (int)event.getY();    switch(event.getAction()){        case MotionEvent.ACTION_DOWN:{              parent.requestDisallowInterceptTouchEvent(true);              break;          }        case MotionEvent.ACTION_MOVE:{              int deltaX = x-mLastX;              int deltaY = y-mLastY;              if(父容器需要当前点击事件)              {                parent.requestDisallowInterceptTouchEvent(false);              }              break;          }        case MotionEvent.ACTION_UP:{              break;          }        default:        break;        }      mLastX = x;      mLastY = y;    return super.dispatchTouchEvent(event);}

除了子元素需要处理外，父元素也需要默认拦截除了ACTION_DOWN以外的其他事件，这样当子元素调用
parent.requestDisallowInterceptTouchEvent(false)方法时，父元素才能继续拦截所需的事件。
父元素修改如下：

    public Boolean onInterceptTouchEvent(MotionEvent event){            int action =  event.getAction();        if(action == MotionEvent.ACTION_DOWN){                renturn false;        }else {                return true;        }        }

文章有部分参考自：https://www.jianshu.com/p/38015afcdb58