View事件分发机制

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本篇将介绍输入事件在View树中的分发机制。

事件产生

输入事件是通过InputManager的InputDispatcher分派给WMS的，在创建ViewRootImpl时会建立和WMS的关联，实际上是两个InputChannel，这两个InputChannel是一个socketpair分别负责读和写事件信息。这样事件就可以通过ViewRootImpl分发给view树。

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final class ViewPostImeInputStage extends InputStage {
    @Override
    protected int onProcess(QueuedInputEvent q) {
        if (q.mEvent instanceof KeyEvent) {
            return processKeyEvent(q);
        } else {
            ……
            final int source = q.mEvent.getSource();
            if ((source & InputDevice.SOURCE_CLASS_POINTER) != 0) {
                return processPointerEvent(q);
            } 
            ……
        }
    }
}

private int processPointerEvent(QueuedInputEvent q) {
    final MotionEvent event = (MotionEvent)q.mEvent;

    if (mView.dispatchPointerEvent(event)) {
        return FINISH_HANDLED;
    }
    return FORWARD;
}

public final boolean dispatchPointerEvent(MotionEvent event) {
    if (event.isTouchEvent()) {
        return dispatchTouchEvent(event);
    } else {
        return dispatchGenericMotionEvent(event);
    }
}

产生的事件最终是交给DecorView也就是view树的根节点进行分发的，对于Touch事件，它是通过dispatchTouchEvent进行分发的，DecorView实际上是一个ViewGroup，这里我们看它的dispatchTouchEvent方法

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@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
    if (mInputEventConsistencyVerifier != null) {
        mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
    }

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

        // Check for cancelation.
        final boolean canceled = resetCancelNextUpFlag(this)
                || actionMasked == MotionEvent.ACTION_CANCEL;

        // Update list of touch targets for pointer down, if needed.
        final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
        TouchTarget newTouchTarget = null;
        boolean alreadyDispatchedToNewTouchTarget = false;
        if (!canceled && !intercepted) {
            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 View[] children = mChildren;

                    final boolean customOrder = isChildrenDrawingOrderEnabled();
                    for (int i = childrenCount - 1; i >= 0; i--) {
                        final int childIndex = customOrder ?
                                getChildDrawingOrder(childrenCount, i) : i;
                        final View child = children[childIndex];
                        if (!canViewReceivePointerEvents(child)
                                || !isTransformedTouchPointInView(x, y, child, null)) {
                            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();
                            mLastTouchDownIndex = childIndex;
                            mLastTouchDownX = ev.getX();
                            mLastTouchDownY = ev.getY();
                            newTouchTarget = addTouchTarget(child, idBitsToAssign);
                            alreadyDispatchedToNewTouchTarget = true;
                            break;
                        }
                    }
                }

                if (newTouchTarget == null && mFirstTouchTarget != null) {
                    // Did not find a child to receive the event.
                    // Assign the pointer to the least recently added target.
                    newTouchTarget = mFirstTouchTarget;
                    while (newTouchTarget.next != null) {
                        newTouchTarget = newTouchTarget.next;
                    }
                    newTouchTarget.pointerIdBits |= idBitsToAssign;
                }
            }
        }

        // 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 {
            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;
            }
        }
        ……
    }

    return handled;
}

在ViewGroup的dispatchTouchEvent中，主要做了以下事情：

  1. 判断是否有允许事件拦截，如果允许拦截则通过onInterceptTouchEvent拦截事件，并将处理的结果保存。
  2. 如果未进行拦截处理，就需要为ACTION_DOWN寻找一个TargetView，实际上就是将事件分发给子view进行处理，如果子view对其进行处理即在dispatchTransformedTouchEvent返回true，则它就是TargetView,后续ACTION_MOVE,ACTION_UP事件会分发给它。
  3. 如果没有找到TargetView，则交由ViewGroup自身进行事件处理。
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public boolean dispatchTouchEvent(MotionEvent event) {
    if (mInputEventConsistencyVerifier != null) {
        mInputEventConsistencyVerifier.onTouchEvent(event, 0);
    }

    if (onFilterTouchEventForSecurity(event)) {
        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
                && li.mOnTouchListener.onTouch(this, event)) {
            return true;
        }

        if (onTouchEvent(event)) {
            return true;
        }
    }

    if (mInputEventConsistencyVerifier != null) {
        mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
    }
    return false;
}

子View对于事件先通过onTouch进行处理，如果onTouch消费了此次事件，则直接返回true表示子view处理了本次事件，否则通过onTouchEvent进行处理，同样的，返回true表示处理了本次事件，View的dispatchTouchEvent类似。

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private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
        View child, int desiredPointerIdBits) {
    final boolean handled;

    ……

    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;
}

在dispatchTransformedTouchEvent中当child为null时，即没有子view或者子view都不对事件进行处理，则通过ViewGroup自身的dispatchTouchEvent进行消化处理，否则是交给子view进行处理，处理的结果handled作为事件是否被消费的依据，如果handled为true表示事件被消费了，这样事件就不需要在view树中进行传递了。

view树的事件传递机制比较简答，读者可以分别阅读view和viewGroup的dispatchTouchEvent部分相关的代码即可。

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