事件分发(ViewGroup)

第二篇：事件分发(ViewGroup)

ViewGroup是继承自View，包含很多View和ViewGroup。上一篇文章说过只要touch一个View，就会执行dispatchTouchEvent方法，ViewGroup也是View，同样会首先执行dispatchTouchEvent方法。

事件的传递是从外层向内层传递的，查看dispatchTouchEvent源码，分析ViewGroup的dispatchTouchEvent：

 // 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;            }

这段代码可以看出，ViewGroup会在MotionEvent.ACTION_DOWN和mFirstTouchTarget != null的两种情况下判断是否拦截。由后面的代码分析可知当ViewGroup的子元素成功处理事件就不为null。这里的标记位FLAG_DISALLOW_INTERCEPT，是通过requestDisallowInterceptTouchEvent()方法设置的，一般在子View中设置。

MotionEvent.ACTION_DOWN事件到来时，会做重置状态的操作，如下：

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();            }

分情况分析：

当ViewGroup决定自己拦截事件的时候 ,mFirstTouchTarget为null,会执行如下代码：

// 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 {                ...                }            }

执行dispatchTransformedTouchEven()方法，如下，child（第三个参数）的为null,调用super.dispatchTouchEvent(event);其实就是View的dispatchTouchEvent()，交由View来处理，事件处理在上一篇View事件分发。

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,            View child, int desiredPointerIdBits) {        final boolean handled;        // Canceling motions is a special case.  We don't need to perform any transformations        // or filtering.  The important part is the action, not the contents.        final int oldAction = event.getAction();        if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {            event.setAction(MotionEvent.ACTION_CANCEL);            if (child == null) {                handled = super.dispatchTouchEvent(event);            } else {                handled = child.dispatchTouchEvent(event);            }            event.setAction(oldAction);            return handled;        }        // Calculate the number of pointers to deliver.        final int oldPointerIdBits = event.getPointerIdBits();        final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;        // If for some reason we ended up in an inconsistent state where it looks like we        // might produce a motion event with no pointers in it, then drop the event.        if (newPointerIdBits == 0) {            return false;        }        // If the number of pointers is the same and we don't need to perform any fancy        // irreversible transformations, then we can reuse the motion event for this        // dispatch as long as we are careful to revert any changes we make.        // Otherwise we need to make a copy.        final MotionEvent transformedEvent;        if (newPointerIdBits == oldPointerIdBits) {            if (child == null || child.hasIdentityMatrix()) {                if (child == null) {                    handled = super.dispatchTouchEvent(event);                } else {                    final float offsetX = mScrollX - child.mLeft;                    final float offsetY = mScrollY - child.mTop;                    event.offsetLocation(offsetX, offsetY);                    handled = child.dispatchTouchEvent(event);                    event.offsetLocation(-offsetX, -offsetY);                }                return handled;            }            transformedEvent = MotionEvent.obtain(event);        } else {            transformedEvent = event.split(newPointerIdBits);        }        // Perform any necessary transformations and dispatch.        if (child == null) {            handled = super.dispatchTouchEvent(transformedEvent);        } else {            final float offsetX = mScrollX - child.mLeft;            final float offsetY = mScrollY - child.mTop;            transformedEvent.offsetLocation(offsetX, offsetY);            if (! child.hasIdentityMatrix()) {                transformedEvent.transform(child.getInverseMatrix());            }            handled = child.dispatchTouchEvent(transformedEvent);        }        // Done.        transformedEvent.recycle();        return handled;    }

ViewGroup不拦截事件的时候，事件会分发给他的子View：

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);                            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);                        }

其中：dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign))实际调用的是子View的dispatchTouchEvent方法，如果子元素的dispatchTouchEvent方法返回true,mFirstTouchTarget会被赋值跳出循环。如下所示：

 newTouchTarget = addTouchTarget(child, idBitsToAssign); alreadyDispatchedToNewTouchTarget = true;

赋值最终在addTouchTarget方法：TouchTarget其实是单链表结构

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

这几行代码完成了对mFirstTouchTarget的赋值并终止了对子元素的遍历。如果子元素的dispatchTouchEvent返回false，ViewGroup就会把事件分给下一个元素（如果还有子元素的话）。

如果子View的dispatchTouchEvent返回false或者没有子元素，ViewGroup会自己处理点击事件（和拦截自己处理一样，第三个参数child为null,调用View的dispatchTouchEvent方法，交由View来处理，事件处理上一篇View事件分发）。

// 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 {...}