Android中view的Touch事件传递顺序

先看一下ViewGroup的dispatchTouchEvent的关键源码：

public boolean dispatchTouchEvent(MotionEvent ev) {
...
if (action == MotionEvent.ACTION_DOWN) {
...
if (disallowIntercept || !onInterceptTouchEvent(ev)) {
final View[] children = mChildren;
final int count = mChildrenCount;
for (int i = count - 1; i >= 0; i--) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE
|| child.getAnimation() != null) {
...
if (child.dispatchTouchEvent(ev))  {
// Event handled, we have a target now.
mMotionTarget = child;
return true;
}
}
}
}
}
...
final View target = mMotionTarget;
if (target == null) {
...
return super.dispatchTouchEvent(ev);
}
if (!disallowIntercept && onInterceptTouchEvent(ev)) {
...
if (!target.dispatchTouchEvent(ev)) {
// target didn't handle ACTION_CANCEL. not much we can do
// but they should have.
}
// clear the target
mMotionTarget = null;
// Don't dispatch this event to our own view, because we already
// saw it when intercepting; we just want to give the following
// event to the normal onTouchEvent().
return true;
}
...
return target.dispatchTouchEvent(ev);
}

根据以上代码可知，如果onInterceptTouchEvent方法返回true，会直接调用当前ViewGroup的onTouch方法，否则则会依次调用其内包含的子控件的dispatchTouchEvent方法。我写了一个测试用例，ViewGroup的onInterceptTouchEvent方法与所有View的onTouch方法都打了Log：

<?xml version="1.0" encoding="utf-8"?>
<org.gavin.test.view.MyScrollView xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent" >

<org.gavin.test.view.MyLinearLayout
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical">
<org.gavin.test.view.MyImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/ic_launcher"></org.gavin.test.view.MyImageView>
</org.gavin.test.view.MyLinearLayout>

</org.gavin.test.view.MyScrollView>

Log信息：
I/MyScrollView(26001): onInterceptTouchEvent:MotionEvent{404ea7c8 action=0 x=34.215885 y=51.963867 pressure=0.14117648 size=0.1}
I/MyLinearLayout(26001): onInterceptTouchEvent:MotionEvent{404ea7c8 action=0 x=34.215885 y=51.963867 pressure=0.14117648 size=0.1}
I/MyImageView(26001): onTouchEvent:MotionEvent{404ea7c8 action=0 x=34.215885 y=51.963867 pressure=0.14117648 size=0.1}
I/MyLinearLayout(26001): onTouchEvent:MotionEvent{404ea7c8 action=0 x=34.215885 y=51.963867 pressure=0.14117648 size=0.1}
I/MyScrollView(26001): onTouchEvent:MotionEvent{404ea7c8 action=0 x=34.215885 y=51.963867 pressure=0.14117648 size=0.1}

如果MyLinearLayout的onInterceptTouchEvent方法返回true：

11-04 14:05:08.389: I/MyScrollView(26297): onInterceptTouchEvent:MotionEvent{404e9ed0 action=0 x=24.439919 y=41.980873 pressure=0.13333334 size=0.15}
11-04 14:05:08.389: I/MyLinearLayout(26297): onInterceptTouchEvent:MotionEvent{404e9ed0 action=0 x=24.439919 y=41.980873 pressure=0.13333334 size=0.15}
11-04 14:05:08.397: I/MyLinearLayout(26297): onTouchEvent:MotionEvent{404e9ed0 action=0 x=24.439919 y=41.980873 pressure=0.13333334 size=0.15}
11-04 14:05:08.397: I/MyScrollView(26297): onTouchEvent:MotionEvent{404e9ed0 action=0 x=24.439919 y=41.980873 pressure=0.13333334 size=0.15}

OK，再看这段代码：

if (child.dispatchTouchEvent(ev))  {
// Event handled, we have a target now.
mMotionTarget = child;
return true;
}

从这段代码可以看出，如果某个子控件的dispatchTouchEvent方法返回true，则中断以后的消息传递。并在下一个非ACTION_DOWN事件直接调用此子控件的dispatchTouchEvent方法。一个View的dispatchTouchEvent返回true，主要可能是onTouch方法返回true。如果我们把MyImageView的onTouchEvent返回值改为true：
I/MyScrollView(26393): onInterceptTouchEvent:MotionEvent{404eb2b0 action=0 x=32.749493 y=51.05632 pressure=0.11764707 size=0.1}
/MyLinearLayout(26393): onInterceptTouchEvent:MotionEvent{404eb2b0 action=0 x=32.749493 y=51.05632 pressure=0.11764707 size=0.1}
I/MyImageView(26393): onTouchEvent:MotionEvent{404eb2b0 action=0 x=32.749493 y=51.05632 pressure=0.11764707 size=0.1}