處理 ViewGroup 中的觸控事件需要特別留意，因為 ViewGroup 擁有為不同觸控事件指定的子項，而非 ViewGroup 本身。為確保每個檢視畫面正確收到適用的觸控事件，請覆寫 onInterceptTouchEvent() 方法。
請參閱下列相關資源：
攔截 ViewGroup 中的觸控事件
只要在 ViewGroup 的表面 (包括其子項的表面) 偵測到觸控事件，系統就會呼叫 onInterceptTouchEvent() 方法。如果 onInterceptTouchEvent() 傳回 true，則 MotionEvent 會遭到攔截，表示該值不會傳遞至子項，而是傳遞至父項的onTouchEvent()方法。
onInterceptTouchEvent() 方法可讓父項在子項之前查看觸控事件。如果您從 onInterceptTouchEvent() 傳回 true，先前用於處理觸控事件的子項檢視區塊會收到 ACTION_CANCEL，而從該時點開始的事件會傳送至父項的 onTouchEvent() 方法，以便進行一般處理。onInterceptTouchEvent() 也可以傳回 false，並在事件沿著檢視區塊階層前往常用的目標時進行監視，這些目標會使用自己的 onTouchEvent() 處理事件。
在以下程式碼片段中，MyViewGroup 類別會擴充 ViewGroup。MyViewGroup 包含多個子檢視畫面。如果您將手指水平拖曳子項檢視畫面，子項檢視畫面就不會再收到觸控事件，而 MyViewGroup 會透過捲動內容來處理觸控事件。不過，如果您輕觸子檢視畫面中的按鈕，或垂直捲動子檢視畫面，父項就不會攔截這些觸控事件，因為子項是預定的目標。在這些情況下，onInterceptTouchEvent() 會傳回 false，且不會呼叫 MyViewGroup 類別的 onTouchEvent()。
Kotlin
class MyViewGroup @JvmOverloads constructor(
context: Context,
private val mTouchSlop: Int = ViewConfiguration.get(context).scaledTouchSlop
) : ViewGroup(context) {
...
override fun onInterceptTouchEvent(ev: MotionEvent): Boolean {
// This method only determines whether you want to intercept the motion.
// If this method returns true, onTouchEvent is called and you can do
// the actual scrolling there.
return when (ev.actionMasked) {
// Always handle the case of the touch gesture being complete.
MotionEvent.ACTION_CANCEL, MotionEvent.ACTION_UP -> {
// Release the scroll.
mIsScrolling = false
false // Don't intercept the touch event. Let the child handle it.
}
MotionEvent.ACTION_MOVE -> {
if (mIsScrolling) {
// You're currently scrolling, so intercept the touch event.
true
} else {
// If the user drags their finger horizontally more than the
// touch slop, start the scroll.
// Left as an exercise for the reader.
val xDiff: Int = calculateDistanceX(ev)
// Touch slop is calculated using ViewConfiguration constants.
if (xDiff > mTouchSlop) {
// Start scrolling!
mIsScrolling = true
true
} else {
false
}
}
}
...
else -> {
// In general, don't intercept touch events. The child view
// handles them.
false
}
}
}
override fun onTouchEvent(event: MotionEvent): Boolean {
// Here, you actually handle the touch event. For example, if the action
// is ACTION_MOVE, scroll this container. This method is only called if
// the touch event is intercepted in onInterceptTouchEvent.
...
}
}
Java
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 only determines whether you want to intercept the motion.
// If this method returns true, onTouchEvent is called and you can 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; // Don't intercept touch event. Let the child handle it.
}
switch (action) {
case MotionEvent.ACTION_MOVE: {
if (mIsScrolling) {
// You're currently scrolling, so intercept the touch event.
return true;
}
// If the user drags their finger horizontally more than the
// touch slop, start the scroll.
// Left as an exercise for the reader.
final int xDiff = calculateDistanceX(ev);
// Touch slop is calculated using ViewConfiguration constants.
if (xDiff > mTouchSlop) {
// Start scrolling.
mIsScrolling = true;
return true;
}
break;
}
...
}
// In general, don't intercept touch events. The child view handles them.
return false;
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
// Here, you actually handle the touch event. For example, if the
// action is ACTION_MOVE, scroll this container. This method is only
// called if the touch event is intercepted in onInterceptTouchEvent.
...
}
}
請注意，ViewGroup 也提供 requestDisallowInterceptTouchEvent() 方法。如果子項不希望父項及其祖系透過 onInterceptTouchEvent() 攔截觸控事件，ViewGroup 會呼叫這個方法。
處理 ACTION_OUTSIDE 事件
如果 ViewGroup 收到含有 ACTION_OUTSIDE 的 MotionEvent，則根據預設，事件不會調派至其子項。如要使用 ACTION_OUTSIDE 處理 MotionEvent，請覆寫 dispatchTouchEvent(MotionEvent event) 以調度至適當的 View，或是在相關的 Window.Callback 中處理，例如 Activity。
使用 ViewConfiguration 常數
上述程式碼片段會使用目前的 ViewConfiguration 來初始化名為 mTouchSlop 的變數。您可以使用 ViewConfiguration 類別存取 Android 系統使用的常見距離、速度和時間。
「觸控滑桿」是指使用者輕觸手勢在系統將手勢解讀為捲動之前的距離 (以像素為單位)。觸控滑動空間通常用於防止使用者執行其他觸控操作 (例如觸碰螢幕元素) 時，不小心捲動畫面。
其他兩種常用的 ViewConfiguration 方法為 getScaledMinimumFlingVelocity() 和 getScaledMaximumFlingVelocity()。這些方法會分別傳回最小和最大速度，以便以每秒像素為單位啟動甩動。例如：
Kotlin
private val vc: ViewConfiguration = ViewConfiguration.get(context)
private val mSlop: Int = vc.scaledTouchSlop
private val mMinFlingVelocity: Int = vc.scaledMinimumFlingVelocity
private val mMaxFlingVelocity: Int = vc.scaledMaximumFlingVelocity
...
MotionEvent.ACTION_MOVE -> {
...
val deltaX: Float = motionEvent.rawX - mDownX
if (Math.abs(deltaX) > mSlop) {
// A swipe occurs, do something.
}
return false
}
...
MotionEvent.ACTION_UP -> {
...
if (velocityX in mMinFlingVelocity..mMaxFlingVelocity && velocityY < velocityX) {
// The criteria are satisfied, do something.
}
}
Java
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 occurs, do something.
}
...
case MotionEvent.ACTION_UP: {
...
} if (mMinFlingVelocity <= velocityX && velocityX <= mMaxFlingVelocity
&& velocityY < velocityX) {
// The criteria are satisfied, do something.
}
}
展開子項檢視畫面的可觸控區域
Android 提供 TouchDelegate 類別，可讓父項將子項檢視區塊的可觸控區域延伸至子項邊界之外。如果子項必須很小，但需要較大的觸控區域，這種做法就非常實用。您也可以使用這個方法縮小子項的觸控區域。
在以下範例中，ImageButton 是「委派」檢視畫面_，也就是父項會擴充觸控區域的子項。以下是版面配置檔案：
<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>
下列程式碼片段已完成這些工作：
- 取得父項檢視畫面，並在 UI 執行緒上發布
Runnable。這樣可確保父項在呼叫getHitRect()方法前，先將子項排版。getHitRect()方法會在父項的座標中取得子項的命中矩形 (或可觸控區域)。 - 找出
ImageButton子項檢視畫面，並呼叫getHitRect()以取得子項可觸控區域的邊界。 - 延伸
ImageButton子項檢視畫面的命中矩形邊界。 - 將展開的命中矩形和
ImageButton子檢視區塊做為參數，並以此例項化TouchDelegate。 - 在父項檢視畫面上設定
TouchDelegate，以便將觸控委派函式邊界內的觸控動作重新導向至子項。
在容量為 ImageButton 子項檢視畫面的觸控委派時，父項檢視畫面會接收所有觸控事件。如果觸控事件發生在子項的命中矩形內，父項會將觸控事件傳遞給子項處理。
Kotlin
public class MainActivity : Activity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
// Post in the parent's message queue to make sure the parent lays out
// its children before you call getHitRect().
findViewById<View>(R.id.parent_layout).post {
// The bounds for the delegate view, which is an ImageButton in this
// example.
val delegateArea = Rect()
val myButton = findViewById<ImageButton>(R.id.button).apply {
isEnabled = true
setOnClickListener {
Toast.makeText(
this@MainActivity,
"Touch occurred within ImageButton touch region.",
Toast.LENGTH_SHORT
).show()
}
// The hit rectangle for the ImageButton.
getHitRect(delegateArea)
}
// Extend the touch area of the ImageButton beyond its bounds on the
// right and bottom.
delegateArea.right += 100
delegateArea.bottom += 100
// Set the TouchDelegate on the parent view so that touches within
// the touch delegate bounds are routed to the child.
(myButton.parent as? View)?.apply {
// 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 receives
// motion events.
touchDelegate = TouchDelegate(delegateArea, myButton)
}
}
}
}
Java
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, which is 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 receives
// motion events.
TouchDelegate touchDelegate = new TouchDelegate(delegateArea,
myButton);
// Set the TouchDelegate on the parent view so that touches
// within the touch delegate bounds are routed to the child.
if (View.class.isInstance(myButton.getParent())) {
((View) myButton.getParent()).setTouchDelegate(touchDelegate);
}
}
});
}
}