> 编写:[Andrwyw](https://github.com/Andrwyw) - 原文:[http://developer.android.com/training/gestures/viewgroup.html](http://developer.android.com/training/gestures/viewgroup.html) 因为很多时候event的目标是ViewGroup的孩子，并不是ViewGroup本身，所以处理[ViewGroup](http://developer.android.com/reference/android/view/ViewGroup.html)中的触摸事件需要特别注意。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()](http://developer.android.com/reference/android/view/ViewGroup.html#onInterceptTouchEvent(android.view.MotionEvent))函数。 ### 在ViewGroup中截获触摸事件 每当在[ViewGroup](http://developer.android.com/reference/android/view/ViewGroup.html)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](http://developer.android.com/reference/android/view/MotionEvent.html)就被截获了，这表示它不会被传递给孩子了，而是传递给该父view自身的[onTouchEvent()](http://developer.android.com/reference/android/view/View.html#onTouchEvent(android.view.MotionEvent))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](http://developer.android.com/reference/android/view/MotionEvent.html#ACTION_CANCEL)事件，并且该点之后的事件会被发送给该父view自身的`onTouchEvent()`函数，进行常规处理。`onInterceptTouchEvent()`也可以返回`false`，这样事件沿view层级分发到目标前，父view可以简单地观察该事件。这里的目标是指，通过`onTouchEvent()`处理消息事件的view。 接下来的代码段中，`MyViewGroup`继承自[ViewGroup](http://developer.android.com/reference/android/view/ViewGroup.html)。`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本就是预定目标。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](http://developer.android.com/reference/android/view/ViewGroup.html)也提供了[requestDisallowInterceptTouchEvent()](http://developer.android.com/reference/android/view/ViewGroup.html#requestDisallowInterceptTouchEvent(boolean))方法。当子view不想该父view和祖先view通过`onInterceptTouchEvent()`截获它的触摸事件时，可调用[ViewGroup](http://developer.android.com/reference/android/view/ViewGroup.html)的该方法。 ### 使用ViewConfiguration的常量 上面的代码段中使用了当前的[ViewConfiguration](http://developer.android.com/reference/android/view/ViewConfiguration.html)来初始化`mTouchSlop`变量。你可以使用[ViewConfiguration](http://developer.android.com/reference/android/view/ViewConfiguration.html)类来获取Android系统常用的一些距离、速度、时间值。 “Touch slop”是指在被识别为移动的手势前，用户触摸可移动的那一段像素距离。Touch slop通常用来预防用户在做一些其他触摸操作时，出现意外地滑动，例如触摸屏幕上的元素。 另外两个常用的[ViewConfiguration](http://developer.android.com/reference/android/view/ViewConfiguration.html)函数是[getScaledMinimumFlingVelocity()](http://developer.android.com/reference/android/view/ViewConfiguration.html#getScaledMinimumFlingVelocity())和[getScaledMaximumFlingVelocity()](http://developer.android.com/reference/android/view/ViewConfiguration.html#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 ~~~ ### 扩展view的可触摸区域 Android提供了[TouchDelegate](http://developer.android.com/reference/android/view/TouchDelegate.html)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](http://developer.android.com/reference/android/widget/ImageButton.html)对象是所谓的"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： - 获得父view对象并发送一段[Runnable](http://developer.android.com/reference/java/lang/Runnable.html)到UI线程。这会确保父view在调用[getHitRect()](http://developer.android.com/reference/android/view/View.html#getHitRect(android.graphics.Rect))函数前会布局它的子view。`getHitRect()`函数会获得子view在父view坐标系中的点击矩形（触摸区域）。 - 找到[ImageButton](http://developer.android.com/reference/android/widget/ImageButton.html)子view，然后调用`getHitRect()`来获得它的触摸区域的边界。 - 扩展[ImageButton](http://developer.android.com/reference/android/widget/ImageButton.html)的点击矩形的边界。 - 实例化一个[TouchDelegate](http://developer.android.com/reference/android/view/TouchDelegate.html)对象，并把扩展过的点击矩形和[ImageButton](http://developer.android.com/reference/android/widget/ImageButton.html)子view作为参数传递给它。 - 设置父view的[TouchDelegate](http://developer.android.com/reference/android/view/TouchDelegate.html)，这样在touch delegate边界内的点击就会传递到该子view上。 在[ImageButton](http://developer.android.com/reference/android/widget/ImageButton.html)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); } } }); } } ~~~