ViewGroup事件分发

ViewGroup继承自View从写了父类的dispatchTouchEvent而且多了一个onInterceptTouchEvent方法此方法代表是否拦截此事件的分发

 // Check for interception.            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;            }

当我们按下的时候回来到这里，首先会执行onInterceptTouchEvent判断是否拦截，而disallowIntercept代表不允许拦截默认为false我们也可以通过方法设置disallowIntercept的值来达到拦截目的，onInterceptTouchEvent默认返回false，我们可以通过重写这个方法达到拦截目的。

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;                            }

我们直接掉到上面那段代码，此段代码也在dispatchTouchEvent方法中，我们主要关心的是dispatchTransformedTouchEvent方法而在此方法中发现了下面两行代码

if (child == null) {                handled = super.dispatchTouchEvent(event);            } else {                handled = child.dispatchTouchEvent(event);            }

当我们child为空时，也就是把我们上网viewgroup当成view处理，调用suoer.dispatchTouchEvent当child不为空时也就调用子类的dispatchTouchEvent，可以从源码看出disTransformedTouchEvent的调用在一个for循环中，因为viewgroup并不一定一个孩子当dispatchtransformTouchevent返回为true时，会调用newTouchTarget=addTouchTarget(...)

private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {        final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);        target.next = mFirstTouchTarget;        mFirstTouchTarget = target;        return target;    }

在addTouchTarget方法中，会给mFirstTouchTarget赋值，也就是child赋值给它。而当我们的move或up方法进来时当mFirstTouchTarget不为空时，会判断完是否拦截然后直接走下面的代码，而当mfirstTouchTargrt为空时直接走下面的代码

 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;                        }

而当执行up时mFirstTouchTarget清空resetTouchState()--->clearTouchTargets()然年就被清空了

由上总结，当我们拦截down事件时，那么mFirstTouchTarget就为空了，子view就一直不会响应触摸事件，而要想给mFirstTouchTarget赋值就必须在Down中并且子view 的dispatchtouchevent返回true

在子类中重写dispatchTouchEvent中设置getParent().requestDisallowInterceptTouchEvent(boolean disallowIntercept)阻止父类拦截触摸事件intercept=false，这句话要在super.dispatchTouchEvent()之前