Android-事件分发机制

本文传达的内容：

通过一个实例结合源码分析MotionEvent对象的传递过程。

实验：

自定义两个ViewGroup重写它们的dispatchTouchEvent(),onInterceptTouchEvent(),onTouchEvent()方法,和一个View重写它的dispatchTouchEvent(),onTouchEvent()方法。

MyGroupViewA:

public class MyGroupViewA extends LinearLayout {

    public MyGroupViewA(Context context) {
        super(context);
    }

    public MyGroupViewA(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public MyGroupViewA(Context context, AttributeSet attrs,
                        int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.d("dhn", "ViewGroupA dispatchTouchEvent" + ev.getAction());
        return super.dispatchTouchEvent(ev);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        Log.d("dhn", "ViewGroupA onInterceptTouchEvent" + ev.getAction());
        return super.onInterceptTouchEvent(ev);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.d("dhn", "ViewGroupA onTouchEvent" + event.getAction());
        return super.onTouchEvent(event);
    }
}

MyGroupViewB:

public class MyGroupViewB extends LinearLayout {

    public MyGroupViewB(Context context) {
        super(context);
    }

    public MyGroupViewB(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public MyGroupViewB(Context context, AttributeSet attrs,
                        int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.d("dhn", "ViewGroupB dispatchTouchEvent" + ev.getAction());
        return super.dispatchTouchEvent(ev);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        Log.d("dhn", "ViewGroupB onInterceptTouchEvent" + ev.getAction());
        return super.onInterceptTouchEvent(ev);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.d("dhn", "ViewGroupB onTouchEvent" + event.getAction());
        return true;
    }
}

MyView:

public class MyView extends View {
    public MyView(Context context) {
        super(context);
    }

    public MyView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public MyView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.d("dhn", "View onTouchEnent" + event.getAction());
        return super.onTouchEvent(event);
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent event) {
        Log.d("dhn", "View dispatchTouchEvent" + event.getAction());
        return super.dispatchTouchEvent(event);
    }
}

布局文件：activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <com.dhn.touchevnettest.MyGroupViewA
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@android:color/holo_blue_bright">

        <com.dhn.touchevnettest.MyGroupViewB
            android:layout_width="300dp"
            android:layout_height="300dp"
            android:background="@android:color/holo_green_dark">

            <com.dhn.touchevnettest.MyView
                android:id="@+id/myView"
                android:layout_width="100dp"
                android:layout_height="100dp"
                android:background="@android:color/darker_gray"/>


        </com.dhn.touchevnettest.MyGroupViewB>

    </com.dhn.touchevnettest.MyGroupViewA>

</RelativeLayout>

效果图：

 

实验一：点击最小的方块

结果：

/com.dhn.touchevnettest D/dhn﹕ ViewGroupA dispatchTouchEvent0
/com.dhn.touchevnettest D/dhn﹕ ViewGroupA onInterceptTouchEvent0
/com.dhn.touchevnettest D/dhn﹕ ViewGroupB dispatchTouchEvent0
/com.dhn.touchevnettest D/dhn﹕ ViewGroupB onInterceptTouchEvent0
/com.dhn.touchevnettest D/dhn﹕ View dispatchTouchEvent0
/com.dhn.touchevnettest D/dhn﹕ View onTouchEnent0
/com.dhn.touchevnettest D/dhn﹕ ViewGroupB onTouchEvent0
/com.dhn.touchevnettest D/dhn﹕ ViewGroupA onTouchEvent0

分析：

首先事件从上层传递到MyGroupViewA对象，其dispatchTouchEvent()被调用，打印出ViewGroupA dispatchTouchEvent0。然后调用super.dispatchTouchEvent()即ViewGroup.dispatchTouchEvent(),该方法部分如下:

code1:

 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;
            }

如果是ACTION_DOWN事件，或mFirstTouchTarget != null()则会调用onInterceptionTouchEvent()，这里显然会进入该方法，在该方法中，我们打印ViewGroupA onInterceptTouchEvent0，然后调用surper.onInterceptTouchEvent()，即ViewGroup.onInterceptTouchEvent(),我们来看下它的源码：

code2:

    public boolean onInterceptTouchEvent(MotionEvent ev) {
        return false;
    }

该方法返回false，所以intercepted被赋值为false(code1第6行)，接着往下执行ViewGroup.dispatchTouchEvent()：

code3:

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;
                    }
                }
            }

遍历每个子元素，65行调用dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign))，我们看下这个方法:

code4:

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

里面有这么一段，所以会调用MyGroupViewB.dispatchTouchEvent()方法，成功将事件传递给子View。MyGroupViewB按同样的步骤将事件传递给MyView(期间打印了ViewGroupB dispatchTouchEvent0, ViewGroupB onInterceptTouchEvent0)。MyView的dispatchTouchEvent()方法被调用（打印View dispatchTouchEvent0），然后调用super.dispatchTouchEvent()方法即View.dispatchEvent()方法,我们看下该方法片段：

code5：

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

            if (!result && onTouchEvent(event)) {
                result = true;
            }

如果设置了OnTouchListeener则会优先调用onTouch()方法，该方法的返回这也会影响是否会调用onTouchEvent()。这里我们没有设置OnTouchListener则会调用第8行的onTouchEvent()方法（打印View onTouchEnent0），接着调用super.onTouchEvent方法，即View.onTouchEvent()方法，第8行看到该方法的返回值会影响result的值，若返回true则result为true从而导致dispatchTouchEvent()方法返回true,不改写的情况下返回false。该方法返回后回到MyGroupViewB.dispathTouchEvent()方法的65行，因为返回false,所以不进入for循环，跳出循环，进入如下代码片段：

code6:

            if (mFirstTouchTarget == null) {
                // No touch targets so treat this as an ordinary view.
                handled = dispatchTransformedTouchEvent(ev, canceled, null,
                        TouchTarget.ALL_POINTER_IDS);

mFirstTouchTarget==null成立，再次进入dispatchTransformedTouchEvent(ev, canceled, null,TouchTarget.ALL_POINTER_IDS)方法，因为传入的第三个参数是null，根据code4所以这次会调用super.dispatchTouchEvent()方法，即View.dispatchTouchEvent()方法，这里会打印：ViewGroupB onTouchEvent0。然后从MyGroupViewB.dispathTouchEvent()方法返回，进入MyGroupViewA.dispathTouchEvent()方法的65行往下执行（和MyGroupViewB类似）期间打印：ViewGroupA onTouchEvent0。这样就从MyGroupViewA.dispathTouchEvent()返回了，事件分发在这三个控件的部分也就结束了。

实验二：

将MyGroupViewA的onInterceptedTouchEvent()方法直接返回true。

结果：

01-19 18:00:02.768 23804-23804/com.dhn.touchevnettest D/dhn﹕ ViewGroupA dispatchTouchEvent0
01-19 18:00:02.828 23804-23804/com.dhn.touchevnettest D/dhn﹕ ViewGroupA dispatchTouchEvent2
01-19 18:00:02.838 23804-23804/com.dhn.touchevnettest D/dhn﹕ ViewGroupA dispatchTouchEvent1

分析：

MyGroupViewA.dispatchTouchEvent()方法中code1第九行intercepted = onInterceptTouchEvent(ev)直接将intercepted置为true,向下执行code3第一行if判断失败，就不会进入事件分发的for循环code3第32行，也就意味着不进行事件在MyGroupViewA这里被截断，不继续往MyGroupViewB分发。直接进入code6。

实验三：

MyView的onTouchEvent()直接返回true。

结果：

01-19 18:12:04.688 29980-29980/com.dhn.touchevnettest D/dhn﹕ ViewGroupA dispatchTouchEvent0
01-19 18:12:04.688 29980-29980/com.dhn.touchevnettest D/dhn﹕ ViewGroupA onInterceptTouchEvent0
01-19 18:12:04.688 29980-29980/com.dhn.touchevnettest D/dhn﹕ ViewGroupB dispatchTouchEvent0
01-19 18:12:04.688 29980-29980/com.dhn.touchevnettest D/dhn﹕ ViewGroupB onInterceptTouchEvent0
01-19 18:12:04.688 29980-29980/com.dhn.touchevnettest D/dhn﹕ View dispatchTouchEvent0
01-19 18:12:04.688 29980-29980/com.dhn.touchevnettest D/dhn﹕ View onTouchEnent0

分析：

该实验事件分发同实验一，不同的是当执行到MyView.onTouchEvent()方法时，直接返回true。正如code6第8行所示，result被设置为true，导致dispatchTouchEvent()返回true，返回到MyGroupViewB.dispatchTouchEvent()方法时会进入for循环（code3第65行），设置mFirstTouchTarget != null,这样code6第一行的判断就为假，不会调用dispatchTransformedTouchEvent(ev, canceled, TouchTarget.ALL_POINTER_IDS)也就不会调用MyGroupViewB.onTouchEvent()，接着MyGroupViewB.dispatchTouchEvent()返回true，返回进入MyGroupViewA.dipatchTouchEvent()时同样不会调用onTouchEvent()方法。

参考：《Android群英传》-徐宜生