我想检测一扔事件一个简单的Andr​​oid应用程序，但第一个 MotionEvent 参数总是空。为什么被称为一个空参数onFling方法？该Android文档说，当一个一扔事件发生下来MotionEvent初始和匹配起来它被称为MotionEvent。

 类MyGestureDetector扩展SimpleOnGestureListener {
    @覆盖
    公共布尔onFling（MotionEvent E1，E2 MotionEvent，浮velocityX，浮velocityY）{
        // E1总是== null，则返回初空引用异常
        如果（E1 == NULL）{
            返回false;
        }
        如果（e1.getY（） -  e2.getY（）＆≥120）{
            handleFlingEvent（）;
            返回true;
        }
        返回false;
    }
}
 

主要活动有这样的onTouchEvent方法：

  GestureDetector flingDetector =新GestureDetector（新MyGestureDetector（））;

@覆盖
公共布尔的onTouchEvent（MotionEvent事件）{
    如果（flingDetector.onTouchEvent（事件））{
        返回true;
    }
    返回super.onTouchEvent（事件）;
}
 

解决方案

我已经得到了类似的问题。它发生在当GestureDetector和onFling方法是在自定义视图类，这是该活动的用户界面的一部分。经过一系列的测试中我发现，只有主活动可以检测到扫动手势良好。所以，我解决他们替换活动的问题。但我没有找到这个问题的根本原因。

I am trying to detect a fling event on a simple Android application, but the first MotionEvent argument is always null. Why is the onFling method being called with a null argument? The Android documentation says that it gets called when a fling event occurs with the initial on down MotionEvent and the matching up MotionEvent.

class MyGestureDetector extends SimpleOnGestureListener {
    @Override
    public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
        // e1 always == null, return early a null reference exception
        if (e1 == null) {
            return false;
        }
        if (e1.getY() - e2.getY() > 120) {
            handleFlingEvent();
            return true;
        }
        return false;
    }
}

The main Activity has this onTouchEvent method:

GestureDetector flingDetector = new GestureDetector(new MyGestureDetector());

@Override
public boolean onTouchEvent(MotionEvent event) {
    if (flingDetector.onTouchEvent(event)) {
        return true;
    }
    return super.onTouchEvent(event);
}

I have got the similar problem. It happens when the GestureDetector and the onFling method was in a custom view class, which is a part of the activity UI. After a series of testing I found that only the main Activity can detect the swipe gesture well. So I solve the problem by replace them in the activity. But I didn't find the root cause of this problem.