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在 ViewGroup 中管理轻触事件

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。

请参阅以下相关资源:

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 ViewGroupMyViewGroup 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 MyViewGrouponTouchEvent()

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_OUTSIDEMotionEvent,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_OUTSIDEMotionEvent,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()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);
                    }
                }
            });
        }
    }