Android中View的事件分发和拦截机制

1.点击事件，就是MotionEvent,所谓点击事件的事件分发，其实就是对MotionEvent事件的分发过程。点击事件的分发过程由三个很重要的方法来共同完成：dispatchTouchEvent、 onInterceptTouchEvent和onTouchEvent.

下列的伪代码(Pseudo code)形象生动地展示了这三个方法之间的逻辑关系:

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

2.传递规则: 


对于一个根ViewGroup来说，点击事件产生后，首先会传递给他，这时它的dispatchTouchEvent就会被调用，如果这个ViewGroup的onInterceptTouchEvent方法返回true,就表示它要拦截当前事件，接着这个ViewGroup就会处理这个事件，即onTouchEvent方法被调用，事件得到消耗；如果这个ViewGroup的onInterceptTouchEvent方法返回false就表示它不拦截当前事件，这时当前事件就会继续传递给他的子元素，接着子元素的dispatchTouchEvent方法就会被调用，如此反复直到事件被最终处理。


当一个点击事件产生后，它的传递过程遵循如下顺序Activity->Window->View,如果最终所有元素都没能处理这个时间，那么该事件最终将会被传递给Activity处理，即Activity的onTouchEvent方法被调用


注意:
1) ViewGroup默认不拦截任何事件， ViewGroup的onInterceptTouchEvent方法默认返回false
2) View没有onInterceptTouchEvent方法，一旦有点击事件传递给它，那么它的onTouchEvent方法就会被调用
3) View的onTouchEvent默认会消耗事件(返回true), 除非它是不可点击的
4) View的enable属性不影响onTouchEvent的默认返回值，哪怕一个View是disable状态，主要它的clickable或者longClickable有一个为true,那么它的onTouchEvent就返回true
5)事件传递过程是由外向内的，即事件总是先传递给父元素，然后再由父元素分发给子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，但是ACTION_DOWN事件除外

6)当某个View已经决定对事件进行拦截，那么同一事件序列内的所有事件都将交给它来处理，而且它的onInterceptTouchEvent方法将不再被调用

简而概之，事件的分发路径就是：activity -> window -> decor view -> view

3.源码分析

ViewGroup源码分析

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.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();  //ACTION_DOWN事件将重置触控状态，所以FLAG_DISALLOW_INTERCEPT标记不会影响对该事件的拦截}
        // Check for interception.        final boolean intercepted;

// First touch target in the linked list of touch targets. 触控对象列表的首个对象

//mFirstTouchTarget 当ViewGroup的子元素成功处理事件时，该对象将被赋值并指向该子元素        if (actionMasked == MotionEvent.ACTION_DOWN                || mFirstTouchTarget != null) {            final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;            if (!disallowIntercept) {  // FLAG_DISALLOW_INTERCEPT标记一旦被设置，则ViewGroup将无法拦截除了ACTION_DOWN事件外的所有触控事件                intercepted = onInterceptTouchEvent(ev);                ev.setAction(action); // restore action in case it was changed            } else {                intercepted = false;            }        } else { //当触控事件未发生时或事件由ViewGroup本身拦截时，则ViewGroup的onInterceptTouchEvent方法不会再被调用            // There are no touch targets and this action is not an initial down            // so this view group continues to intercept touches.            intercepted = true;        }        ...        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.            View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()                    ? findChildWithAccessibilityFocus() : null;            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) {                    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();

//当ViewGroup不拦截事件时，事件将下发给它的子View去处理                    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 (preorderedList != null) preorderedList.clear();                }                ...            }        }        // Dispatch to touch targets.        if (mFirstTouchTarget == null) {  //ViewGroup没有子元素或者处理事件的子元素在OnTouch方法中返回了false,此时ViewGroup自行处理事件            // No touch targets so treat this as an ordinary view.            handled = dispatchTransformedTouchEvent(ev, canceled, null,                    TouchTarget.ALL_POINTER_IDS);        } ...    }    ...    return handled;}

/** * 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) { //对mFirstTouchTarget进行赋值    final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);    target.next = mFirstTouchTarget;    mFirstTouchTarget = target;    return target;}

View的源码分析：

//view是单独的一个元素，它没有子元素，所以不能向下继续分发事件，只能自己处理事件

public boolean dispatchTouchEvent(MotionEvent event) {    ...    boolean result = false;

    ...    if (onFilterTouchEventForSecurity(event)) {        ...        ListenerInfo li = mListenerInfo;        if (li != null && li.mOnTouchListener != null                && (mViewFlags & ENABLED_MASK) == ENABLED                && li.mOnTouchListener.onTouch(this, event)) { //这里先对OnTouchListener是否被设置进行判断            result = true;        }        if (!result && onTouchEvent(event)) { //这里表明OnTouchListener优先级高于onTouchEvent,设置了listener,就不调用onTouchEvent方法            result = true;        }    }   ...    return result;}

public boolean onTouchEvent(MotionEvent event) {    ...    final int action = event.getAction();    if ((viewFlags & ENABLED_MASK) == DISABLED) {        if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {            setPressed(false);        }        // A disabled view that is clickable still consumes the touch                  // events, it just doesn't respond to them.
//这里表明即使处于disabled状态的view依然可以消耗触碰事件,只要Clickable或long_clickable其中之一为true        return (((viewFlags & CLICKABLE) == CLICKABLE                || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)                || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);    }    ...    return false;}