ViewGroup源码-Touch事件

在Android-27中查看源码：

在看完View源码的触摸事件(参考文章：View源码-Touch事件)后，我们接着来看看容器类View的触摸事件。因为容器类的View都继承自ViewGroup，所以我们在ViewGroup的源码中来看看是如何处理触摸事件的。

同样的先来看ViewGroup源码中的dispatchTouchEvent方法：

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;
}

首先，会判断是否不允许拦截，可以通过requestDisallowInterceptTouchEvent进行设置，默认允许拦截。如果允许拦截，会调用onInterceptTouchEvent，看看该View是否拦截了触摸事件，默认不拦截即可以将触摸事件向子View传递。

接下来，如果触摸事件没有取消并且没有拦截，在手指按下的时候：

final ArrayList preorderedList = buildTouchDispatchChildList();
...
for (int i = childrenCount - 1; i >= 0; i--){
    ...
    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;
    }
...
}

1. 按照绘制子View的顺序，找到该ViewGoup的所有子view，并保存到ArrayList中。
2. 根据视图顺序依次遍历子View。判断当前子view是否是TouchTarget，若是则跳出循环。否则调用dispatchTransformedTouchEvent方法，如果当前子View的dispatchTouchEvent返回为true,找到该子View在ViewGroup中的index,并将该子View作为新的TouchTarget。
3. 清楚保存了所有子View的ArrayList。

然后在TouchTarget形成的链式结构中，处理触摸事件：

// 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 {
    // 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;
    }
}

1. 如果子View没有处理触摸事件，则由当前的ViewGroup处理，然后返回处理结果。因为ViewGroup是View的子类，所以还是由View的dispatchTouchEvent处理。
2. 如果子View中处理了触摸事件，根据TouchTarget生成的链式结构，不断循环，分别调用里面View的dispatchTouchEvent方法。

从上面的分析可以看出整个ViewGroup的Touch事件的整个传递过程如下：

1. 是否调用了requestDisallowInterceptTouchEvent方法设置不允许拦截，默认允许拦截。若允许拦截，则再调用onInterceptTouchEvent,看是否拦截。
2. 如果触摸事件被拦截，则调用ViewGroup的dispatchTransformedTouchEvent方法，其实是调用View的dispatchTouchEvent方法。否则继续向下。
3. 当触摸事件为MotionEvent.ACTION_DOWN时，首先获取根据绘制顺序保存了所有子View的ArrayLsit，然后再根据视图顺序去遍历子View。
   i. 如果从TouchTarget形成的链表中发现该View,则跳出循环，即找到了TouchTarget。否则继续向下。
   ii. 在子View中寻找，当子View的dispatchTouchEvent方法返回为true时,则找到了新的TouchTarget，并将其添加到TouchTarget形成的链表中。
4. 遍历TouchTarget形成的链表，然后对链表里面的子View分别调用dispatchTouchEvent，最后将处理结果返回。