记录一下Android事件分发机制

前言

现在网上已经很多事件分发机制解析的文章了，很多文章讲的很详细。但是发现每次看完相关的文章后，当时看的挺明白，过后就忘，算了还是自己写篇文章记录一下，加深印象。加深理解。

储备知识

看本篇文章就默认的认为你已经掌握了以下几点：
1、事件分发相关的三个方法dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent，其中默认情况下：ViewGroup中没有onTouchEvent方法；View中没有onInterceptTouchEvent方法；dispatchTouchEvent同时存在于ViewGroup和View中。
2、触摸事件包括按下(Down)、移动(Move)、抬起(Up)、取消(Cancel)等。
3、触摸事件第一次被处理是在Activity的dispatchTouchEvent方法
4、Window类的唯一实现类是PhoneWindow；DecorView继承自FrameLayout
下面从Activity开始分析。

源码

Activity#dispatchTouchEvent(MotionEvent event)

/**
 * 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();//默认是个空方法，由用户自己实现，开发屏保相关的功能可以使用该方法
    }
    if (getWindow().superDispatchTouchEvent(ev)) {//如果为true，则不会再调用Activity的OnTouchEvent(ev)方法
        return true;
    }
    return onTouchEvent(ev);
}

由储备知识点可以知道getWindow()获取的是PhoneWindow对象，所以看PhoneWindow的superDispatchTouchEvent(MotionEvent ev)方法：

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

mDecor是DecorView类型对象，跟进去发现就是单纯的调用父类的方法dispatchTouchEvent，具体代码就不贴了、贴一下ViewGroup的dispatchTouchEvent方法：

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
    //代码省略

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

        //省略代码：down事件来的时候，就重置之前保存的各种状态
        
        // Check for interception.
        final boolean intercepted;
        if (actionMasked == MotionEvent.ACTION_DOWN
                || mFirstTouchTarget != null) {
             //判断mGroupFlags是否设置了FLAG_DISALLOW_INTERCEPT，若设置了。则不拦截，默认不设置
            final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
            if (!disallowIntercept) {
            	//注释1===调用onInterceptTouchEvent方法
                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;
        }
		//省略部分带码
		
        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.

            if (actionMasked == MotionEvent.ACTION_DOWN
                    || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                    || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {


                final int childrenCount = mChildrenCount;
                if (newTouchTarget == null && childrenCount != 0) {
                    //...
                    //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);
						//省略
						//判断触摸点是否在当前子view中、若不在继续遍历下一个子view
                        if (!canViewReceivePointerEvents(child)
                                || !isTransformedTouchPointInView(x, y, child, null)) {
                            ev.setTargetAccessibilityFocus(false);
                            continue;
                        }
                        //省略
                        //注释2===判断当前子view是否消费点击事件 若消费掉、就调出循环不在遍历剩余子view
                        if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                            // ...
                            
                            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();
                }
                //略。。
            }
        }

        // Dispatch to touch targets.
        //没有子view接受触摸事件、交给自己处理，注意第三个参数为null
        //注释3
        if (mFirstTouchTarget == null) {
            // No touch targets so treat this as an ordinary view.
            handled = dispatchTransformedTouchEvent(ev, canceled, null,
                    TouchTarget.ALL_POINTER_IDS);
        } else {//已有接收触摸Down事件的子view剩余事件均交给它
            // 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;
                //已经传递过的事件、不再传递
                if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
                    handled = true;
                } else {
                	//分发剩余事件
                    if (dispatchTransformedTouchEvent(ev, cancelChild,
                            target.child, target.pointerIdBits)) {
                        handled = true;
                    }
                    //略
                }
                predecessor = target;
                target = next;
            }
        }
    }
    return handled;
}

以上代码大致逻辑是Down事件进来后，先判断FLAG_DISALLOW_INTERCEPT是否禁止分发，一般为false，然后调用注释1处的onInterceptTouchEvent方法

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;
}

默认返回false（不拦截）。然后回遍历子View、判断触摸点落在哪个子view内部，如果找到那个子View后调用注释2处的dispatchTransformedTouchEvent方法：

/**
 * Transforms a motion event into the coordinate space of a particular child view,
 * filters out irrelevant pointer ids, and overrides its action if necessary.
 * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
 */
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
        View child, int desiredPointerIdBits) {
    final boolean handled;

    //略
    // Perform any necessary transformations and dispatch.
    //根据child是否为null 调用父类View或子View的dispatchTouchEvent方法
    if (child == null) {
        handled = super.dispatchTouchEvent(transformedEvent);
    } else {
    	//略
        handled = child.dispatchTouchEvent(transformedEvent);
    }
    return handled;
}

假如这里找到这里找到子View。那事件就会被分发到子View中。若果子View还是一个ViewGroup、那调用逻辑跟以上分析的一致。若子View不是ViewGroup。就会调用View的dispatchTouchEvent：

/**
 * 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) {
    //默认返回结果false 不消费事件
    boolean result = false;
    
    if (onFilterTouchEventForSecurity(event)) {
        
        //noinspection SimplifiableIfStatement
        //判断当前view是否设置了onTouchListener回调、若设置并返回true。则不再调用onTouchEvent方法
        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnTouchListener != null
                && (mViewFlags & ENABLED_MASK) == ENABLED
                && li.mOnTouchListener.onTouch(this, event)) {
            result = true;
        }
		//若设置并返回true。则不再调用onTouchEvent方法
        if (!result && onTouchEvent(event)) {
            result = true;
        }
    }
	//略
    return result;
}

在子view中onTouchEvent方法的调用受制于onTouchListener的设置及返回值、默认不设置、则会调用onTouchEvent

/**
 * 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) {
    if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
        switch (action) {
            case MotionEvent.ACTION_UP:
                
                boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
                if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
                    //在down中检查是否有长点击事件，若没有执行以下逻辑
                    if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
                        // This is a tap, so remove the longpress check
                        removeLongPressCallback();
                            // 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();
                            }
                            //使用一个runnable来处理点击事件，这让其他视图view在点击事件开始之前更新状态，如果post失败，再直接调用；performClick就是处理onClickListener的方法
                            if (!post(mPerformClick)) {
                                performClick();
                            }

                    }

                   
                }

                break;

            case MotionEvent.ACTION_DOWN:
                mHasPerformedLongPress = false;

                if (!clickable) {
                	//检查长点击事件
                    checkForLongClick(0, x, y);
                    break;
                }
                break;

            case MotionEvent.ACTION_CANCEL:
                break;

            case MotionEvent.ACTION_MOVE:
                
                break;
        }

        return true;
    }

    return false;
}

onTouchEvent对各个事件的处理有一个条件、就是clickable=true、若成立，则返回true、并处理其中的点击事件，若不成立则返回false、回到上一个方法。若是一个clickable==false的子view、则view的dispatchTouchEvent返回的为false表示没有分发此事件、继续往回返、返到dispatchTransformedTouchEvent方法也返回false、继续往回返回到viewgroup中对子view遍历调用都为false、没有找到消费该down事件的子view、该方法继续往下走、调用到注释3、调用dispatchTransformedTouchEvent但是此时传的child参数为null、所以调用super.dispatchTouchEvent，即view的dispatchTouchEvent方法、逻辑再走一遍、会走view的onTouchEvent方法、此viewgroup的clickable默认也为false、即返回false、然后一层层往上传回来、注释3处的handle为false。此事件就处理完毕、没有一个view消费此事件。
回过头来看、在viewgroup的dispatchTouchEvent方法的遍历子view调用dispatchTransformedTouchEvent那处代码、若有子view是clickable的，比如Button控件、则onTouchEvent返回true，所以dispatchTransformedTouchEvent返回true、说明找到消费此事件的子View了、然后在注释3出、就不会走第一个分支逻辑、会走第二个、把剩余的事件均交给此子View处理。讲到这里、只ViewGroup的dispatchTouchEvent方法中调用的onInterceptTouchEvent方法返回默认值false的情况。
如果onInterceptTouchEvent返回true，则下面的注释2中if判断那一堆逻辑就不会走了、不会遍历子View、而是直接进入注释3、调用dispatchTransformedTouchEvent，参数child为null、则直接调用view的dispatchTouchEvent方法。则会调用到自己的onTouchEvent方法

总结

1. 事件分发从Activity的dispatchTouchEvent开始、经历PhoneWindow----->DecorView----->ViewGroup#dispatchTouchEvent方法
2. ViewGroup的dispatchTouchEvent----------调用------>ViewGroup的onInterceptTouchEvent。
3. onInterceptTouchEvent：若返回false，则遍历子view，找到处理触摸事件的子view，并调用child.dispatchTouchEvent方法把触摸事件分发下去；若返回true，则不遍历子view，直接调用super.dispatchTouchEvent方法，自己处理触摸事件；默认返回false。

3.1.onInterceptTouchEvent返回false:
      child.dispatchTouchEvent此方法中会判断onTouchListener是否不为null，且onTouch方法是否返回true。若不成立则调用自己的onTouchEvent方法；若成立，则不会执行自己的onTouchEvent方法

3.1.1.onTouchEvent返回true:
          触摸事件被消费，接下来的move和up等事件均交由此view处理
3.1.2.onTouchEvent返回false:
          最终会返回到第3点，遍历子view并判断是否有子view消费事件的那个逻辑，没有子view消费此事件

3.2.onInterceptTouchEvent返回true:
      自己拦截触摸事件

4. 第3.1.2和第3.2之后的逻辑都是需要自己询问自己是否拦截此触摸事件的逻辑，会调用super.dispatchTouchEvent方法
   5.在super.dispatchTouchEvent方法中(同3.1中child.dispatchTouchEvent)，判断是否设置onTouchListener且
   onTouch方法是否返回true，若不成立则调用自己的onTouchEvent方法；

最后来张流程图吧

写的匆忙，如有不当的地方请指正