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:
1 public class MyGroupViewA extends LinearLayout {
2
3 public MyGroupViewA(Context context) {
4 super(context);
5 }
6
7 public MyGroupViewA(Context context, AttributeSet attrs) {
8 super(context, attrs);
9 }
10
11 public MyGroupViewA(Context context, AttributeSet attrs,
12 int defStyleAttr) {
13 super(context, attrs, defStyleAttr);
14 }
15
16 @Override
17 public boolean dispatchTouchEvent(MotionEvent ev) {
18 Log.d("dhn", "ViewGroupA dispatchTouchEvent" + ev.getAction());
19 return super.dispatchTouchEvent(ev);
20 }
21
22 @Override
23 public boolean onInterceptTouchEvent(MotionEvent ev) {
24 Log.d("dhn", "ViewGroupA onInterceptTouchEvent" + ev.getAction());
25 return super.onInterceptTouchEvent(ev);
26 }
27
28 @Override
29 public boolean onTouchEvent(MotionEvent event) {
30 Log.d("dhn", "ViewGroupA onTouchEvent" + event.getAction());
31 return super.onTouchEvent(event);
32 }
33 }
MyGroupViewB:
1 public class MyGroupViewB extends LinearLayout {
2
3 public MyGroupViewB(Context context) {
4 super(context);
5 }
6
7 public MyGroupViewB(Context context, AttributeSet attrs) {
8 super(context, attrs);
9 }
10
11 public MyGroupViewB(Context context, AttributeSet attrs,
12 int defStyleAttr) {
13 super(context, attrs, defStyleAttr);
14 }
15
16 @Override
17 public boolean dispatchTouchEvent(MotionEvent ev) {
18 Log.d("dhn", "ViewGroupB dispatchTouchEvent" + ev.getAction());
19 return super.dispatchTouchEvent(ev);
20 }
21
22 @Override
23 public boolean onInterceptTouchEvent(MotionEvent ev) {
24 Log.d("dhn", "ViewGroupB onInterceptTouchEvent" + ev.getAction());
25 return super.onInterceptTouchEvent(ev);
26 }
27
28 @Override
29 public boolean onTouchEvent(MotionEvent event) {
30 Log.d("dhn", "ViewGroupB onTouchEvent" + event.getAction());
31 return true;
32 }
33 }
MyView:
1 public class MyView extends View {
2 public MyView(Context context) {
3 super(context);
4 }
5
6 public MyView(Context context, AttributeSet attrs) {
7 super(context, attrs);
8 }
9
10 public MyView(Context context, AttributeSet attrs, int defStyleAttr) {
11 super(context, attrs, defStyleAttr);
12 }
13
14 @Override
15 public boolean onTouchEvent(MotionEvent event) {
16 Log.d("dhn", "View onTouchEnent" + event.getAction());
17 return super.onTouchEvent(event);
18 }
19
20 @Override
21 public boolean dispatchTouchEvent(MotionEvent event) {
22 Log.d("dhn", "View dispatchTouchEvent" + event.getAction());
23 return super.dispatchTouchEvent(event);
24 }
25 }
布局文件：activity_main.xml
1 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 2 xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" 3 android:layout_height="match_parent"> 4 5 <com.dhn.touchevnettest.MyGroupViewA 6 android:layout_width="match_parent" 7 android:layout_height="match_parent" 8 android:background="@android:color/holo_blue_bright"> 9 10 <com.dhn.touchevnettest.MyGroupViewB 11 android:layout_width="300dp" 12 android:layout_height="300dp" 13 android:background="@android:color/holo_green_dark"> 14 15 <com.dhn.touchevnettest.MyView 16 android:id="@+id/myView" 17 android:layout_width="100dp" 18 android:layout_height="100dp" 20 android:background="@android:color/darker_gray"/> 21 22 23 </com.dhn.touchevnettest.MyGroupViewB> 24 25 </com.dhn.touchevnettest.MyGroupViewA> 26 27 </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:
1 final boolean intercepted;
2 if (actionMasked == MotionEvent.ACTION_DOWN
3 || mFirstTouchTarget != null) {
4 final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
5 if (!disallowIntercept) {
6 intercepted = onInterceptTouchEvent(ev);
7 ev.setAction(action); // restore action in case it was changed
8 } else {
9 intercepted = false;
10 }
11 } else {
12 // There are no touch targets and this action is not an initial down
13 // so this view group continues to intercept touches.
14 intercepted = true;
15 }
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:
1 public boolean onInterceptTouchEvent(MotionEvent ev) {
2 return false;
3 }
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:
1 if (!canceled && !intercepted) {
2
3 // If the event is targeting accessiiblity focus we give it to the
4 // view that has accessibility focus and if it does not handle it
5 // we clear the flag and dispatch the event to all children as usual.
6 // We are looking up the accessibility focused host to avoid keeping
7 // state since these events are very rare.
8 View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
9 ? findChildWithAccessibilityFocus() : null;
10
11 if (actionMasked == MotionEvent.ACTION_DOWN
12 || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
13 || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
14 final int actionIndex = ev.getActionIndex(); // always 0 for down
15 final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
16 : TouchTarget.ALL_POINTER_IDS;
17
18 // Clean up earlier touch targets for this pointer id in case they
19 // have become out of sync.
20 removePointersFromTouchTargets(idBitsToAssign);
21
22 final int childrenCount = mChildrenCount;
23 if (newTouchTarget == null && childrenCount != 0) {
24 final float x = ev.getX(actionIndex);
25 final float y = ev.getY(actionIndex);
26 // Find a child that can receive the event.
27 // Scan children from front to back.
28 final ArrayList<View> preorderedList = buildOrderedChildList();
29 final boolean customOrder = preorderedList == null
30 && isChildrenDrawingOrderEnabled();
31 final View[] children = mChildren;
32 for (int i = childrenCount - 1; i >= 0; i--) {
33 final int childIndex = customOrder
34 ? getChildDrawingOrder(childrenCount, i) : i;
35 final View child = (preorderedList == null)
36 ? children[childIndex] : preorderedList.get(childIndex);
37
38 // If there is a view that has accessibility focus we want it
39 // to get the event first and if not handled we will perform a
40 // normal dispatch. We may do a double iteration but this is
41 // safer given the timeframe.
42 if (childWithAccessibilityFocus != null) {
43 if (childWithAccessibilityFocus != child) {
44 continue;
45 }
46 childWithAccessibilityFocus = null;
47 i = childrenCount - 1;
48 }
49
50 if (!canViewReceivePointerEvents(child)
51 || !isTransformedTouchPointInView(x, y, child, null)) {
52 ev.setTargetAccessibilityFocus(false);
53 continue;
54 }
55
56 newTouchTarget = getTouchTarget(child);
57 if (newTouchTarget != null) {
58 // Child is already receiving touch within its bounds.
59 // Give it the new pointer in addition to the ones it is handling.
60 newTouchTarget.pointerIdBits |= idBitsToAssign;
61 break;
62 }
63
64 resetCancelNextUpFlag(child);
65 if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
66 // Child wants to receive touch within its bounds.
67 mLastTouchDownTime = ev.getDownTime();
68 if (preorderedList != null) {
69 // childIndex points into presorted list, find original index
70 for (int j = 0; j < childrenCount; j++) {
71 if (children[childIndex] == mChildren[j]) {
72 mLastTouchDownIndex = j;
73 break;
74 }
75 }
76 } else {
77 mLastTouchDownIndex = childIndex;
78 }
79 mLastTouchDownX = ev.getX();
80 mLastTouchDownY = ev.getY();
81 newTouchTarget = addTouchTarget(child, idBitsToAssign);
82 alreadyDispatchedToNewTouchTarget = true;
83 break;
84 }
85
86 // The accessibility focus didn‘t handle the event, so clear
87 // the flag and do a normal dispatch to all children.
88 ev.setTargetAccessibilityFocus(false);
89 }
90 if (preorderedList != null) preorderedList.clear();
91 }
92
93 if (newTouchTarget == null && mFirstTouchTarget != null) {
94 // Did not find a child to receive the event.
95 // Assign the pointer to the least recently added target.
96 newTouchTarget = mFirstTouchTarget;
97 while (newTouchTarget.next != null) {
98 newTouchTarget = newTouchTarget.next;
99 }
100 newTouchTarget.pointerIdBits |= idBitsToAssign;
101 }
102 }
103 }
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:
1 if (child == null) {
2 handled = super.dispatchTouchEvent(event);
3 } else {
4 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：
1 ListenerInfo li = mListenerInfo;
2 if (li != null && li.mOnTouchListener != null
3 && (mViewFlags & ENABLED_MASK) == ENABLED
4 && li.mOnTouchListener.onTouch(this, event)) {
5 result = true;
6 }
7
8 if (!result && onTouchEvent(event)) {
9 result = true;
10 }
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:
1 if (mFirstTouchTarget == null) {
2 // No touch targets so treat this as an ordinary view.
3 handled = dispatchTransformedTouchEvent(ev, canceled, null,
4 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