好的。这件事几乎让我发疯。

几天前，我可以实现下面的代码，当我尝试向右或向左滑动时，toast 被成功调用。

但是，现在不能调用了，因为e1一直是null!

怎么会这样？我在模拟器上尝试了这段代码，但一位用户向我报告说，它在真实设备上也不起作用。

public class SwipeGestureListener extends GestureDetector.SimpleOnGestureListener{

    private static final int SWIPE_MIN_DISTANCE = 150;
    private static final int SWIPE_MAX_OFF_PATH = 100;

    private static final int SWIPE_THRESHOLD_VELOCITY = 100;
    private final Activity activity;
    protected MotionEvent mLastOnDownEvent = null;

    public SwipeGestureListener(Activity activity) {
        this.activity = activity;
    }

    @Override
    public boolean onDown(MotionEvent e) {
        mLastOnDownEvent = e;
        System.out.println(e);
        return super.onDown(e);
    }

    @Override   
    public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
        System.out.println(e1 + " " + e2);
        if (e1==null)
            e1 = mLastOnDownEvent;
        if (e1==null || e2==null)
            return false;

        float dX = e2.getX()-e1.getX();
        float dY = e2.getY()-e1.getY();

        if (Math.abs(dY)<SWIPE_MAX_OFF_PATH && Math.abs(velocityX)>=SWIPE_THRESHOLD_VELOCITY && Math.abs(dX)>=SWIPE_MIN_DISTANCE ) {

            if (dX>0) {
                Toast.makeText(activity.getApplicationContext(), "Right Swipe", Toast.LENGTH_SHORT).show();
                activity.fetchPrevious();
            } else {
                Toast.makeText(activity.getApplicationContext(), "Left Swipe", Toast.LENGTH_SHORT).show();
                activity.fetchNext();
            }

            return true;
        } 

        return false;
    }

}

实现手势的代码:

final GestureDetector gdt = new GestureDetector(this, new SwipeGestureListener(this));
listview.setOnTouchListener(new OnTouchListener(){

    @Override
    public boolean onTouch(View v, MotionEvent event) {
        gdt.onTouchEvent(event);
        return false;
        }

});

最佳答案

我尝试了您的代码，但确实没有用，然后我将 GestureDetector 更改为较新/更新的 API“GestureDetectorCompat”，并进一步更改了一些内容: 使用 GestureDetector.OnGestureListener 而不是 OnSimpleGestureListener， onDown() 应该返回 true 然后它起作用了。我无法向您解释，确切的问题是什么。但是变更线似乎有问题。也许有人可以解释潜在的问题。查看正在运行的代码:

public class MainActivity extends Activity {
   ImageView imageView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        imageView= (ImageView) findViewById(R.id.imageView1);

        final GestureDetectorCompat gdt = new GestureDetectorCompat(this, new SwipeGestureListener(this));
        imageView.setOnTouchListener(new OnTouchListener(){

            @Override
            public boolean onTouch(View v, MotionEvent event) {
                return gdt.onTouchEvent(event);
                }

        });

    }




    public class SwipeGestureListener implements GestureDetector.OnGestureListener{

        private static final int SWIPE_MIN_DISTANCE = 150;
        private static final int SWIPE_MAX_OFF_PATH = 100;

        private static final int SWIPE_THRESHOLD_VELOCITY = 100;
        private final Activity activity;
        protected MotionEvent mLastOnDownEvent = null;

        public SwipeGestureListener(Activity activity) {
            this.activity = activity;
    }

        @Override
        public boolean onDown(MotionEvent e) {
            mLastOnDownEvent = e;
            System.err.println("ondown");
            //System.out.println(e);
            return true;
        }

        @Override   
        public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
            System.err.println("onFling");
            System.out.println(e1 + " " + e2);
            if (e1==null)
                e1 = mLastOnDownEvent;
            if (e1==null || e2==null)
                return false;

            float dX = e2.getX()-e1.getX();
            float dY = e2.getY()-e1.getY();

            if (Math.abs(dY)<SWIPE_MAX_OFF_PATH && Math.abs(velocityX)>=SWIPE_THRESHOLD_VELOCITY && Math.abs(dX)>=SWIPE_MIN_DISTANCE ) {

                if (dX>0) {
                    Toast.makeText(activity.getApplicationContext(), "Right Swipe", Toast.LENGTH_SHORT).show();
                   // activity.fetchPrevious();
                } else {
                    Toast.makeText(activity.getApplicationContext(), "Left Swipe", Toast.LENGTH_SHORT).show();
                   // activity.fetchNext();
                }

                return true;
            } 

            return false;
        }

        @Override
        public void onLongPress(MotionEvent e) {
            // TODO Auto-generated method stub

    }

        @Override
        public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) { 
            // TODO Auto-generated method stub
            return false;
    }

        @Override
        public void onShowPress(MotionEvent e) {
            // TODO Auto-generated method stub

    }

       @Override
       public boolean onSingleTapUp(MotionEvent e) {
          // TODO Auto-generated method stub
           return false;
       }

       }

   }

请试试吧!

关于android - OnFling MotionEvent e1 为空？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/17390873/