在 ViewGroup 中处理轻触事件时需要特别小心，因为 ViewGroup 通常会有子视图，而这些子视图是除 ViewGroup 本身以外的其他轻触事件的目标。为了确保每个视图都能够正确地收到专门发给它的轻触事件，请替换 onInterceptTouchEvent() 方法。
在 ViewGroup 中拦截轻触事件
只要在 ViewGroup 的 surface（包括其子视图的 surface）上检测到轻触事件，系统就会调用 onInterceptTouchEvent() 方法。如果 onInterceptTouchEvent() 返回 true，系统会拦截 MotionEvent，这意味着系统不会将其传递给子视图，而是传递给父视图的 onTouchEvent() 方法。
onInterceptTouchEvent() 方法可以让父视图先于子视图检测到任何轻触事件。如果您从 onInterceptTouchEvent() 返回 true，则之前处理轻触事件的子视图会收到 ACTION_CANCEL，并且从此时起，系统会将事件发送到父视图的 onTouchEvent() 方法以进行常规处理。onInterceptTouchEvent() 也可能会返回 false，并在事件按照视图层次结构传递至常规目标的过程中进行监视，而这些目标会使用自己的 onTouchEvent() 处理事件。
在以下代码段中，类 MyViewGroup 扩展了 ViewGroup。MyViewGroup 包含多个子视图。如果您在子视图上横向拖动手指，则子视图应该不再获取轻触事件，而 MyViewGroup 应该通过滚动内容来处理轻触事件。不过，如果您按子视图中的按钮，或纵向滚动子视图，则父视图不应该拦截这些轻触事件，因为子视图是预期目标。在这些情况下，onInterceptTouchEvent() 应该返回 false，并且系统不会调用 MyViewGroup 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 JUST determines whether we want to intercept the motion.
* If we return true, onTouchEvent will be called and we 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 // Do not intercept touch event, let the child handle it
}
MotionEvent.ACTION_MOVE -> {
if (mIsScrolling) {
// We're currently scrolling, so yes, intercept the
// touch event!
true
} else {
// If the user has dragged her finger horizontally more than
// the touch slop, start the scroll
// left as an exercise for the reader
val xDiff: Int = calculateDistanceX(ev)
// Touch slop should be calculated using ViewConfiguration
// constants.
if (xDiff > mTouchSlop) {
// Start scrolling!
mIsScrolling = true
true
} else {
false
}
}
}
...
else -> {
// In general, we don't want to intercept touch events. They should be
// handled by the child view.
false
}
}
}
override fun onTouchEvent(event: MotionEvent): Boolean {
// 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
...
}
}
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 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
...
}
}
请注意，ViewGroup 还提供了 requestDisallowInterceptTouchEvent() 方法。当子视图不想让父视图及其祖先视图使用 onInterceptTouchEvent() 拦截轻触事件时，ViewGroup 便会调用此方法。
处理 ACTION_OUTSIDE 事件
如果 ViewGroup 收到带有 ACTION_OUTSIDE 的 MotionEvent，则默认情况下系统不会将该事件发送给其子视图。如需处理带有 ACTION_OUTSIDE 的 MotionEvent，可以替换 dispatchTouchEvent(MotionEvent event) 以将其发送给相应的 View，或在相关 Window.Callback（例如 Activity）中对其进行处理。
使用 ViewConfiguration 常量
上面的代码段使用当前 ViewConfiguration 初始化名为 mTouchSlop 的变量。您可以使用 ViewConfiguration 类来获取 Android 系统常用的距离、速度和时间。
“Touch slop”是指在系统将手势解读为滚动之前，用户的轻触手势可以滑动的距离（以像素为单位）。Touch slop 通常用于防止在用户执行某些其他轻触操作（例如轻触屏幕上的元素）时发生意外滚动。
另外两个常用的 ViewConfiguration 方法是 getScaledMinimumFlingVelocity() 和 getScaledMaximumFlingVelocity()。这两个方法会分别返回发起滑动的最小速度和最大速度，以像素/秒为单位。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 occurred, do something
}
return false
}
...
MotionEvent.ACTION_UP -> {
...
if (velocityX in mMinFlingVelocity..mMaxFlingVelocity && velocityY < velocityX) {
// The criteria have been 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 occurred, do something
}
...
case MotionEvent.ACTION_UP: {
...
} if (mMinFlingVelocity <= velocityX && velocityX <= mMaxFlingVelocity
&& velocityY < velocityX) {
// The criteria have been satisfied, do something
}
}
扩展子视图的可轻触区域
Android 提供了 TouchDelegate 类，让父视图能够将子视图的可轻触区域扩展到子视图的边界之外。当子视图必须较小，同时又应该具有较大的轻触区域时，此方法很有用。您也可以在需要时使用此方法缩小子视图的轻触区域。
在以下示例中，ImageButton 是“代理视图”（即，父视图将扩展其轻触区域的子视图）。其布局文件如下：
<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>
以下代码段可实现以下操作：
- 获取父视图，并在界面线程上发布
Runnable。这样可以确保父视图先对子视图进行布局，然后再调用getHitRect()方法。getHitRect()方法以父视图坐标的形式获取子视图点击矩形（可轻触区域）。 - 查找
ImageButton子视图并调用getHitRect()以获取子视图可轻触区域的边界。 - 扩展
ImageButton点击矩形的边界。 - 实例化
TouchDelegate，将扩展的点击矩形和ImageButton子视图作为参数传入。 - 在父视图上设置
TouchDelegate，以便将轻触代理边界内的轻触操作传递给子视图。
作为 ImageButton 子视图的轻触代理，父视图会收到所有轻触事件。如果轻触事件发生在子视图的点击矩形内，父视图会将轻触事件传递给子视图处理。
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 (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
// Sets the TouchDelegate on the parent view, such 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 should receive 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 (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);
}
}
});
}
}