编写：Andrwyw
项目：Android Training中文版
原文：http://developer.android.com/training/gestures/viewgroup.html
处理ViewGroup中的触摸事件需要特别注意，因为通常情况下都是ViewGroup中的子view处理不同的触摸事件，而不是ViewGroup自己处理。为了确保每个view能正确地接受到它们想要的触摸事件，可以重载onInterceptTouchEvent()函数。
每当在ViewGroup的表面上检测到一个触摸事件，包括它子view的表面，onInterceptTouchEvent()都会被调用。如果onInterceptTouchEvent()返回true，MotionEvent就被截获了，表示它不再会被传递到子view了，而是传递给该父view的onTouchEvent()方法。
onInterceptTouchEvent()方法让父view能够在它的子view之前处理触摸事件。如果你让onInterceptTouchEvent()返回true，则之前处理触摸事件的子view会收到ACTION_CANCEL消息，并且该点之后的事件会被发送给该父view的onTouchEvent()函数，进行通常地处理。onInterceptTouchEvent()也可以返回false，这样在事件沿view层级分发到可通过onTouchEvent()处理它的目标前，父view可以简单地观察该事件。
接下来的代码段中，MyViewGroup继承自ViewGroup。MyViewGroup有多个子view。如果你水平地拖动手指经过某个子view，该子view不会接收到触摸事件，而是MyViewGroup处理这些触摸事件来滚动它的内容。然而，如果你点击子view中的button，或垂直地滚动子view，则父view不会截获这些触摸事件，因为子view本就是预订目标。在这些情况下，onInterceptTouchEvent()应该返回false，MyViewGroup的onTouchEvent()也不会被调用。
public class MyViewGroup extends ViewGroup {
private int mTouchSlop;
...
ViewConfiguration vc = ViewConfiguration.get(view.getContext());
mTouchSlop = vc.getScaledTouchSlop();
...
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
/*
* This method JUST determines whether we want to intercept the motion.
* If we return true, onTouchEvent will be called and we do the actual
* scrolling there.
*/
final int action = MotionEventCompat.getActionMasked(ev);
// Always handle the case of the touch gesture being complete.
if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {
// Release the scroll.
mIsScrolling = false;
return false; // Do not intercept touch event, let the child handle it
}
switch (action) {
case MotionEvent.ACTION_MOVE: {
if (mIsScrolling) {
// We're currently scrolling, so yes, intercept the
// touch event!
return true;
}
// If the user has dragged her finger horizontally more than
// the touch slop, start the scroll
// left as an exercise for the reader
final int xDiff = calculateDistanceX(ev);
// Touch slop should be calculated using ViewConfiguration
// constants.
if (xDiff > mTouchSlop) {
// Start scrolling!
mIsScrolling = true;
return true;
}
break;
}
...
}
// In general, we don't want to intercept touch events. They should be
// handled by the child view.
return false;
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
// Here we actually handle the touch event (e.g. if the action is ACTION_MOVE,
// scroll this container).
// This method will only be called if the touch event was intercepted in
// onInterceptTouchEvent
...
}
}
注意ViewGroup也提供了requestDisallowInterceptTouchEvent()方法。当它的子view不想该父view和祖先view通过onInterceptTouchEvent()截获它的触摸事件时，ViewGroup会调用改方法。
上面的代码段中使用了当前的ViewConfiguration来初始化mTouchSlop变量。你可以使用ViewConfiguration类来获取Android系统常用的一些距离、速度、时间值。
“Touch slop”是指在用户触摸事件可被识别为移动手势前,移动过的那一段像素距离。Touch slop通常用来预防用户在做一些其他操作时意外地滑动，例如触摸屏幕上的元素时。
另外两个常用的ViewConfiguration函数是getScaledMinimumFlingVelocity()和getScaledMaximumFlingVelocity()。这两个函数会返回初始化一个快速滑动(fling)的最小、最大速度（分别地），以像素每秒为测量单位。如：
ViewConfiguration vc = ViewConfiguration.get(view.getContext());
private int mSlop = vc.getScaledTouchSlop();
private int mMinFlingVelocity = vc.getScaledMinimumFlingVelocity();
private int mMaxFlingVelocity = vc.getScaledMaximumFlingVelocity();
...
case MotionEvent.ACTION_MOVE: {
...
float deltaX = motionEvent.getRawX() - mDownX;
if (Math.abs(deltaX) > mSlop) {
// A swipe occurred, do something
}
...
case MotionEvent.ACTION_UP: {
...
} if (mMinFlingVelocity <= velocityX && velocityX <= mMaxFlingVelocity
&& velocityY < velocityX) {
// The criteria have been satisfied, do something
}
}
Android提供了TouchDelegate类让父view扩展子view的可触摸区域，扩展后的区域可超过子view本身的边界。这在子view很小，但需要一个更大的触摸区域时非常有用。如果需要，你也可以使用这种方式来实现对子view的触摸区域的收缩。
在下面的例子中，ImageButton对象是这个”delegate view”（是指触摸区域将被父view扩展的那个子view）。这是布局文件：
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/parent_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity" >
<ImageButton android:id="@+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@null"
android:src="@drawable/icon" />
</RelativeLayout>
下面的代码段做了这样几件事：
在ImageButton子view的touch delegate范围内，父view会接收到所有的触摸事件。如果触摸事件发生在子view自身的点击矩形中，父view会把触摸事件交给子view处理。
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Get the parent view
View parentView = findViewById(R.id.parent_layout);
parentView.post(new Runnable() {
// Post in the parent's message queue to make sure the parent
// lays out its children before you call getHitRect()
@Override
public void run() {
// The bounds for the delegate view (an ImageButton
// in this example)
Rect delegateArea = new Rect();
ImageButton myButton = (ImageButton) findViewById(R.id.button);
myButton.setEnabled(true);
myButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Toast.makeText(MainActivity.this,
"Touch occurred within ImageButton touch region.",
Toast.LENGTH_SHORT).show();
}
});
// The hit rectangle for the ImageButton
myButton.getHitRect(delegateArea);
// Extend the touch area of the ImageButton beyond its bounds
// on the right and bottom.
delegateArea.right += 100;
delegateArea.bottom += 100;
// Instantiate a TouchDelegate.
// "delegateArea" is the bounds in local coordinates of
// the containing view to be mapped to the delegate view.
// "myButton" is the child view that should receive motion
// events.
TouchDelegate touchDelegate = new TouchDelegate(delegateArea,
myButton);
// Sets the TouchDelegate on the parent view, such that touches
// within the touch delegate bounds are routed to the child.
if (View.class.isInstance(myButton.getParent())) {
((View) myButton.getParent()).setTouchDelegate(touchDelegate);
}
}
});
}
}