> 編寫:[Andrwyw](https://github.com/Andrwyw) - 原文:[http://developer.android.com/training/gestures/viewgroup.html](http://developer.android.com/training/gestures/viewgroup.html) 因為很多時候event的目標是ViewGroup的孩子，并不是ViewGroup本身，所以處理[ViewGroup](http://developer.android.com/reference/android/view/ViewGroup.html)中的觸摸事件需要特別注意。為了確保每個view能正確地接受到它們想要的觸摸事件，可以重載[onInterceptTouchEvent()](http://developer.android.com/reference/android/view/ViewGroup.html#onInterceptTouchEvent(android.view.MotionEvent))函數。 ### 在ViewGroup中截獲觸摸事件 每當在[ViewGroup](http://developer.android.com/reference/android/view/ViewGroup.html)的表面上檢測到一個觸摸事件，包括它子view的表面，`onInterceptTouchEvent()`都會被調用。如果`onInterceptTouchEvent()`返回`true`，[MotionEvent](http://developer.android.com/reference/android/view/MotionEvent.html)就被截獲了，這表示它不會被傳遞給孩子了，而是傳遞給該父view自身的[onTouchEvent()](http://developer.android.com/reference/android/view/View.html#onTouchEvent(android.view.MotionEvent))方法。 `onInterceptTouchEvent()`方法讓父view能夠在它的子view之前處理觸摸事件。如果你讓`onInterceptTouchEvent()`返回`true`，則之前處理觸摸事件的子view會收到[ACTION_CANCEL](http://developer.android.com/reference/android/view/MotionEvent.html#ACTION_CANCEL)事件，并且該點之后的事件會被發送給該父view自身的`onTouchEvent()`函數，進行常規處理。`onInterceptTouchEvent()`也可以返回`false`，這樣事件沿view層級分發到目標前，父view可以簡單地觀察該事件。這里的目標是指，通過`onTouchEvent()`處理消息事件的view。 接下來的代碼段中，`MyViewGroup`繼承自[ViewGroup](http://developer.android.com/reference/android/view/ViewGroup.html)。`MyViewGroup`有多個子view。如果你水平地拖動手指經過某個子view，該子view不會接收到觸摸事件，而是`MyViewGroup`處理這些觸摸事件來滾動它的內容。然而，如果你點擊子view中的button，或垂直地滾動子view，則父view不會截獲這些觸摸事件，因為子view本就是預定目標。在這些情況下，`onInterceptTouchEvent()`應該返回`false`，`MyViewGroup`的`onTouchEvent()`也不會被調用。 ~~~ 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。當子view不想該父view和祖先view通過`onInterceptTouchEvent()`截獲它的觸摸事件時，可調用[ViewGroup](http://developer.android.com/reference/android/view/ViewGroup.html)的該方法。 ### 使用ViewConfiguration的常量 上面的代碼段中使用了當前的[ViewConfiguration](http://developer.android.com/reference/android/view/ViewConfiguration.html)來初始化`mTouchSlop`變量。你可以使用[ViewConfiguration](http://developer.android.com/reference/android/view/ViewConfiguration.html)類來獲取Android系統常用的一些距離、速度、時間值。 “Touch slop”是指在被識別為移動的手勢前，用戶觸摸可移動的那一段像素距離。Touch slop通常用來預防用戶在做一些其他觸摸操作時，出現意外地滑動，例如觸摸屏幕上的元素。 另外兩個常用的[ViewConfiguration](http://developer.android.com/reference/android/view/ViewConfiguration.html)函數是[getScaledMinimumFlingVelocity()](http://developer.android.com/reference/android/view/ViewConfiguration.html#getScaledMinimumFlingVelocity())和[getScaledMaximumFlingVelocity()](http://developer.android.com/reference/android/view/ViewConfiguration.html#getScaledMaximumFlingVelocity())。這兩個函數會返回初始化一個快速滑動(fling)的最小、最大速度（分別地），以像素每秒為測量單位。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 ~~~ ### 擴展view的可觸摸區域 Android提供了[TouchDelegate](http://developer.android.com/reference/android/view/TouchDelegate.html)類，讓父view擴展子view的可觸摸區域，擴展后的區域可超過子view本身的邊界。這在子view很小，但需要一個更大的觸摸區域時非常有用。如果需要，你也可以使用這種方式來實現對子view的觸摸區域的收縮。 在下面的例子中，[ImageButton](http://developer.android.com/reference/android/widget/ImageButton.html)對象是所謂的"delegate view"（是指觸摸區域將被父view擴展的那個子view）。這是布局文件： ~~~ <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> ~~~ 下面的代碼段做了這樣幾件事： - 獲得父view對象并發送一段[Runnable](http://developer.android.com/reference/java/lang/Runnable.html)到UI線程。這會確保父view在調用[getHitRect()](http://developer.android.com/reference/android/view/View.html#getHitRect(android.graphics.Rect))函數前會布局它的子view。`getHitRect()`函數會獲得子view在父view坐標系中的點擊矩形（觸摸區域）。 - 找到[ImageButton](http://developer.android.com/reference/android/widget/ImageButton.html)子view，然后調用`getHitRect()`來獲得它的觸摸區域的邊界。 - 擴展[ImageButton](http://developer.android.com/reference/android/widget/ImageButton.html)的點擊矩形的邊界。 - 實例化一個[TouchDelegate](http://developer.android.com/reference/android/view/TouchDelegate.html)對象，并把擴展過的點擊矩形和[ImageButton](http://developer.android.com/reference/android/widget/ImageButton.html)子view作為參數傳遞給它。 - 設置父view的[TouchDelegate](http://developer.android.com/reference/android/view/TouchDelegate.html)，這樣在touch delegate邊界內的點擊就會傳遞到該子view上。 在[ImageButton](http://developer.android.com/reference/android/widget/ImageButton.html)子view的touch delegate范圍內，父view會接收到所有的觸摸事件。如果觸摸事件發生在子view自身的點擊矩形中，父view會把觸摸事件交給子view處理。 ~~~ 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); } } }); } } ~~~