“空”布局中的 GestureDetector NullPointerException

Question

首先，我的程序编译并启动。但是，一旦您在使用 GestureDetector 的 LinearLayout 上进行选项卡，它就会引发 NullPointerException。

我有一个 Activity，其中嵌套了一个名为“OrderFragment”的 Fragment。

public class OrderFragment extends Fragment {
View view;
Activity activty;
@Override
public View onCreateView(LayoutInflater inflater,ViewGroup container, Bundle savedInstanceState) {
    view = inflater.inflate(R.layout.orderfragment, container, false);
    return view;
}


@Override
public void onResume(){
    super.onResume();
    DragList orderContainer = (DragList) view.findViewById(R.id.orderContainer);
    for (int i=0;i<5;++i) {

        final TextView tv = new TextView(getActivity().getApplicationContext());
        tv.setTextColor(0x888888);
        tv.setText("wtf");
        orderContainer.addView(tv,new LinearLayout.LayoutParams(LinearLayout.LayoutParams.FILL_PARENT, LinearLayout.LayoutParams.WRAP_CONTENT));
    }

}

}

在这里，我想以编程方式将一些 TextView 添加到 DragView 中。这个 DragView 存在于 orderfragment.xml 中

在 DragView 类中，我尝试创建一些 TextView，用户可以通过这种方式进行拖动和交换。但我很难让拖动工作。请注意，onTouchEvent 函数是从安卓官方网站here逐字复制而来的

public class DragList extends LinearLayout {


public DragList(Context context, AttributeSet attrs) {
    super(context, attrs);
    init(context);
}

public DragList(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    init(context);
}

private void init(Context context) {
    //do stuff that was in your original constructor...
}

// The active pointer is the one currently moving our object.
private int INVALID_POINTER_ID = -1;
private int mActivePointerId = INVALID_POINTER_ID;
private float mLastTouchX;
private float mLastTouchY;
private GestureDetector mScaleDetector;

public DragList(Context context) {
    super(context);
}

@Override
public boolean onTouchEvent(MotionEvent ev) {
    // Let the ScaleGestureDetector inspect all events.

    mScaleDetector.onTouchEvent(ev);

    final int action = MotionEventCompat.getActionMasked(ev);

    switch (action) {
        case MotionEvent.ACTION_DOWN: {
            final int pointerIndex = MotionEventCompat.getActionIndex(ev);
            final float x = MotionEventCompat.getX(ev, pointerIndex);
            final float y = MotionEventCompat.getY(ev, pointerIndex);

            // Remember where we started (for dragging)
            mLastTouchX = x;
            mLastTouchY = y;
            // Save the ID of this pointer (for dragging)
            mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
            break;
        }

        case MotionEvent.ACTION_MOVE: {
            // Find the index of the active pointer and fetch its position
            final int pointerIndex =
                    MotionEventCompat.findPointerIndex(ev, mActivePointerId);

            final float x = MotionEventCompat.getX(ev, pointerIndex);
            final float y = MotionEventCompat.getY(ev, pointerIndex);

            // Calculate the distance moved
            final float dx = x - mLastTouchX;
            final float dy = y - mLastTouchY;

            float mPosX = dx;
            float mPosY = dy;



            // Remember this touch position for the next move event
            mLastTouchX = x;
            mLastTouchY = y;

            break;
        }

        case MotionEvent.ACTION_UP: {
            mActivePointerId = INVALID_POINTER_ID;
            break;
        }

        case MotionEvent.ACTION_CANCEL: {
            mActivePointerId = INVALID_POINTER_ID;
            break;
        }

        case MotionEvent.ACTION_POINTER_UP: {

            final int pointerIndex = MotionEventCompat.getActionIndex(ev);
            final int pointerId = MotionEventCompat.getPointerId(ev, pointerIndex);

            if (pointerId == mActivePointerId) {
                // This was our active pointer going up. Choose a new
                // active pointer and adjust accordingly.
                final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
                mLastTouchX = MotionEventCompat.getX(ev, newPointerIndex);
                mLastTouchY = MotionEventCompat.getY(ev, newPointerIndex);
                mActivePointerId = MotionEventCompat.getPointerId(ev, newPointerIndex);
            }
            break;
        }
    }
    return true;
}

}

错误：

    java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.view.GestureDetector.onTouchEvent(android.view.MotionEvent)' on a null object reference
        at com.example.jacobusconradi.versuchstestappv20.DragList.onTouchEvent(DragList.java:46)

而且我还可以确认，DragList 在视觉上是空的，或者不存在，因为我只看到给容器的白色背景色。当我使用调试模式时，它不会抛出任何东西，当我到达以编程方式添加的 TextViews 时。

感谢您的帮助和欢呼

Answer 1

请在您的 Constructor 部分初始化 mScaleDetector

http://developer.android.com/intl/es/reference/android/view/GestureDetector.html

mScaleDetector = new GestureDetector(context);

终于

    public DragList(Context context) {
    super(context);
    mScaleDetector = new GestureDetector(context);
    }

Answer 2

请实例化你的手势检测器

public DragList(Context context) {
    super(context);
    mScaleDetector = new GestureDetector(context);
}

希望这会有所帮助。

编辑

您不需要使用gestureDetector，您可以删除它，它应该可以工作。

如果您希望gestureDetector 能够执行某些操作，则需要传递一个OnGestureListener。