在 ViewGroup 中管理触摸事件

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在 ViewGroup 中处理触摸事件需要特别小心，因为 ViewGroup 通常会有子视图，而这些子视图会成为与 ViewGroup 本身不同的触摸事件的目标。为了确保每个视图正确接收为其设计的触摸事件，请重写 onInterceptTouchEvent() 方法。

在 ViewGroup 中拦截触摸事件

onInterceptTouchEvent() 方法在检测到 ViewGroup 表面（包括其子视图表面）发生触摸事件时调用。如果 onInterceptTouchEvent() 返回 true，则会拦截 MotionEvent，这意味着它不会传递给子视图，而是传递给父视图的 onTouchEvent() 方法。

onInterceptTouchEvent() 方法让父视图有机会在其子视图之前看到触摸事件。如果您从 onInterceptTouchEvent() 返回 true，则先前处理触摸事件的子视图会收到一个 ACTION_CANCEL 事件，并且从那时起的所有事件都将发送到父视图的 onTouchEvent() 方法进行常规处理。onInterceptTouchEvent() 也可以返回 false，并在事件沿着视图层级结构向下传递到其常规目标时进行监听，常规目标会使用自己的 onTouchEvent() 来处理事件。

在以下代码段中，MyViewGroup 类扩展了 ViewGroup。MyViewGroup 包含多个子视图。如果您在子视图上水平拖动手指，子视图将不再获得触摸事件，并且 MyViewGroup 将通过滚动其内容来处理触摸事件。但是，如果您点击子视图中的按钮，或垂直滚动子视图，父视图不会拦截这些触摸事件，因为子视图是预期的目标。在这些情况下，onInterceptTouchEvent() 返回 false，并且不调用 MyViewGroup 类的 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.
        ...
    }
}

请注意，ViewGroup 还提供了一个 requestDisallowInterceptTouchEvent() 方法。当子视图不希望父视图及其祖先使用 onInterceptTouchEvent() 拦截触摸事件时，ViewGroup 会调用此方法。

处理 ACTION_OUTSIDE 事件

如果 ViewGroup 收到带有 ACTION_OUTSIDE 的 MotionEvent，默认情况下该事件不会分发给其子视图。要处理带有 ACTION_OUTSIDE 的 MotionEvent，可以重写 dispatchTouchEvent(MotionEvent event) 以分发给相应的 View，或者在相关的 Window.Callback（例如 Activity）中处理。

使用 ViewConfiguration 常量

前面的代码段使用当前的 ViewConfiguration 初始化了一个名为 mTouchSlop 的变量。您可以使用 ViewConfiguration 类访问 Android 系统使用的常见距离、速度和时间。

“触摸斜率 (Touch slop)”是指用户的触摸可以在手势被解释为滚动之前偏移的像素距离。触摸斜率通常用于防止用户在执行其他触摸操作（例如触摸屏幕元素）时意外滚动。

另外两个常用的 ViewConfiguration 方法是 getScaledMinimumFlingVelocity() 和 getScaledMaximumFlingVelocity()。这些方法分别返回以像素/秒为单位衡量的启动抛掷动作所需的最小和最大速度。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.
    }
}

扩展子视图的可触摸区域

Android 提供了 TouchDelegate 类，使父视图能够将子视图的可触摸区域扩展到子视图的边界之外。当子视图必须很小但需要较大的触摸区域时，这非常有用。您也可以使用此方法缩小子视图的触摸区域。

在以下示例中，ImageButton 是_委托视图_，即父视图扩展其触摸区域的子视图。以下是布局文件

<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>

以下代码段完成了这些任务

  • 获取父视图并在 UI 线程上发布一个 Runnable。这确保父视图在调用 getHitRect() 方法之前布置其子视图。getHitRect() 方法获取子视图在父视图坐标中的_命中矩形_（或可触摸区域）。
  • 找到 ImageButton 子视图并调用 getHitRect() 以获取子视图可触摸区域的边界。
  • 扩展 ImageButton 子视图的命中矩形边界。
  • 实例化一个 TouchDelegate，将扩展后的命中矩形和 ImageButton 子视图作为参数传入。
  • 在父视图上设置 TouchDelegate，以便将触摸委派边界内的触摸路由到子视图。

作为 ImageButton 子视图的触摸委托，父视图接收所有触摸事件。如果触摸事件发生在子视图的命中矩形内，父视图会将触摸事件传递给子视图进行处理。

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, 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);
                }
            }
        });
    }
}