Android View相关-事件分发机制详解-ViewGroup

上一篇文章中，我们从一个小例子和源码角度解析了View的事件分发过程，我们知道其执行流程是dispathcEvent -> onTouch -> onTouchEvent，在onTouchEvent中会经过判断处理OnClick事件和OnLongClick事件。那么本文我们来探讨下View的子类ViewGroup的事件分发流程。

举个栗子

这里使用一个自定义LinearLayout和一个自定义Button，自定义Button就是上一篇文章中的button，贴出代码：

@Override
public boolean dispatchTouchEvent(MotionEvent event) {
    switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            Log.e(TAG, "dispatchTouchEvent ACTION_DOWN");
            break;
        case MotionEvent.ACTION_MOVE:
            Log.e(TAG, "dispatchTouchEvent ACTION_MOVE");
            break;
        case MotionEvent.ACTION_UP:
            Log.e(TAG, "dispatchTouchEvent ACTION_UP");
            break;
        default:
            break;
    }
    return super.dispatchTouchEvent(event);
}

@Override
public boolean onTouchEvent(MotionEvent event) {
    switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            Log.e(TAG, "onTouchEvent ACTION_DOWN");
            break;
        case MotionEvent.ACTION_MOVE:
            Log.e(TAG, "onTouchEvent ACTION_MOVE");
            break;
        case MotionEvent.ACTION_UP:
            Log.e(TAG, "onTouchEvent ACTION_UP");
            break;
        default:
            break;
    }
    return super.onTouchEvent(event);
}

@Override
public boolean onInterceptTouchEvent(MotionEvent event) {
    switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            Log.e(TAG, "onInterceptTouchEvent ACTION_DOWN");
            break;
        case MotionEvent.ACTION_MOVE:
            Log.e(TAG, "onInterceptTouchEvent ACTION_MOVE");
            break;
        case MotionEvent.ACTION_UP:
            Log.e(TAG, "onInterceptTouchEvent ACTION_UP");
            break;
        default:
            break;
    }
    return super.onInterceptTouchEvent(event);
}

@Override
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {
    Log.e(TAG, "requestDisallowInterceptTouchEvent disallowIntercept"+disallowIntercept);
    super.requestDisallowInterceptTouchEvent(disallowIntercept);
}

同样是简单地打印了几个log，我们来看下在xml文件中的布局：

<com.xylitolz.androideventdispatchdemo.TestLinearLayout
        android:id="@+id/ll_test"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        android:gravity="center_horizontal">
        <com.xylitolz.androideventdispatchdemo.TestButton
            android:id="@+id/view_test_in_container"
            android:layout_width="wrap_content"
            android:layout_height="40dp"
            android:textAllCaps="false"
            android:text="testInContainer"/>
    </com.xylitolz.androideventdispatchdemo.TestLinearLayout>

就是使用自定义LinearLayout包裹自定义Button，那么我们直接来看看执行结果，首先是点击事件：

接下来是点击然后轻微移动，之后手指离开屏幕：

可以看到其执行流程是先从ViewGroup开始的，首先是ViewGroup dispatchTouchEvent —-> ViewGroup onInterceptTouchEvent —-> 子View dispatchTouchEvent —-> 子View onTouch —-> 子View onTouchEvent 若有move事件则是ACTION_DOWN —-> ACTION_MOVE —-> ACTION_UP的流程，接下来我们就从源码角度对这一执行顺序进行分析。

源码分析

dispatchTouchEvent

和前文一样，我们从dispatchTouchEvent开始分析，ViewGroup中的该方法代码较多

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

    // If the event targets the accessibility focused view and this is it, start
    // normal event dispatch. Maybe a descendant is what will handle the click.
    if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
        ev.setTargetAccessibilityFocus(false);
    }

    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();
        }

        // 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;
        }

        // If intercepted, start normal event dispatch. Also if there is already
        // a view that is handling the gesture, do normal event dispatch.
        if (intercepted || mFirstTouchTarget != null) {
            ev.setTargetAccessibilityFocus(false);
        }

        // 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 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 actionIndex = ev.getActionIndex(); // always 0 for down
                final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
                    : TouchTarget.ALL_POINTER_IDS;

                // Clean up earlier touch targets for this pointer id in case they
                // have become out of sync.
                removePointersFromTouchTargets(idBitsToAssign);

                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 = buildOrderedChildList();
                    final boolean customOrder = preorderedList == null
                        && isChildrenDrawingOrderEnabled();
                    final View[] children = mChildren;
                    for (int i = childrenCount - 1; i >= 0; i--) {
                        final int childIndex = customOrder
                            ? getChildDrawingOrder(childrenCount, i) : i;
                        final View child = (preorderedList == null)
                            ? children[childIndex] : preorderedList.get(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);
                    }
                    if (preorderedList != null) preorderedList.clear();
                }

                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 {
            // 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;
            }
        }

        // Update list of touch targets for pointer up or cancel, if needed.
        if (canceled
            || actionMasked == MotionEvent.ACTION_UP
            || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
            resetTouchState();
        } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
            final int actionIndex = ev.getActionIndex();
            final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
            removePointersFromTouchTargets(idBitsToRemove);
        }
    }

    if (!handled && mInputEventConsistencyVerifier != null) {
        mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
    }
    return handled;
}

这里将整个步骤分解为四部，首先看第一部分：

• step1

  这一部分主要是当TouchEvent为ACTION_DOWN时进行一些初始化操作，例如重设触摸状态、开始新的手势等，若有兴趣可以查看cancelAndClearTouchTargets和resetTouchState源码

• step2

  检查是否需要拦截Touch事件，核心方法onInterceptTouchEvent，这里生成了一个局部变量intercepted后续也会用它来判断是否拦截，这个值会被onInterceptTouchEvent的返回值所改变。

• step3

  该事件Action为ACTION_DOWN并且mFirstTouchTarget值不为null，则继续if语句中的代码，那么mFirstTouchTarget是干嘛的。mFirstTouchTarget是TouchTarget类的对象，而TouchTarget是ViewGroup中的一个内部类，它封装了被触摸的View及这次触摸所对应的ID，该类主要用于多点触控。比如：三个指头依次按到了同一个Button上。 我们需要重点关注mFirstTouchTarget，因为它也贯穿了整个dispatchTouchEvent方法中。若mFirstTouchTarget不为空，表示ViewGroup没有拦截Touch事件并且子View消费了Touch；若mFirstTouchTarget为空，则表示ViewGroup拦截了Touch事件或者虽然ViewGroup没有拦截Touch事件但是子View也没有消费Touch。此时需要ViewGroup自身处理Touch事件。

  若(mFirstTouchTarget!=null)，当处理后续到来的ACTION_MOVE和ACTION_UP时仍会调用该代码判断是否需要拦截Touch事件。

  接下来开始遍历所有的子View

  判断当前的x,y坐标是否落在子View身上，如果在，执行child.dispatchTouchEvent(ev)，就进入了View的dispatchTouchEvent代码中了，当child.dispatchTouchEvent(ev)返回true，则为mMotionTarget设为child，然后return true;

  ViewGroup实现捕获到DOWN事件，如果代码中不做TOUCH事件拦截，则开始查找当前x,y是否在某个子View的区域内，如果在，则把事件分发下去。

• step4

  ACTION_MOVE中，ViewGroup捕获到事件，然后判断是否拦截，如果没有拦截，则直接调用mMotionTarget.dispatchTouchEvent(ev)

• step5

  ACTION_UP中，ViewGroup捕获到事件，然后判断是否拦截，如果没有拦截，则直接调用mMotionTarget.dispatchTouchEvent(ev)

onInterceptTouchEvent

拦截事件

复写onInterceptTouchEvent并在需要拦截的事件中返回true，则代表事件被当前ViewGroup拦截，则不再下发给子View。

不拦截事件

若父控件复写了onInterceptTouchEvent方法并在需要拦截的操作里返回了true，那么若子View不希望父控件处理该操作，可以在子View中复写requestDisallowInterceptTouchEvent，返回true则子View仍能获取到相关操作

小结

本篇流程总结有些混乱，回头会重新梳理再发出来，暂时先这样吧