Getting an unexpected error when I try to run my Android app with drag and drop

Question

I'm trying to make a pretty simple Android app (using Eclipse and Android 4.0.2, API 15) that implements drag & drop. It has an ImageView that needs to be dragged and dropped on a different ImageView. But I seem to be having some sort of problem. The app compiles correctly but I get a force close when I run it on an emulator and on a real device. My code has 3 classes: one for the (only) Activity, one for the "draggable" image listener and one for the "target" image listener:

Activity:

public class MainActivity extends Activity {

    ImageView imageToBeDragged = (ImageView)findViewById(R.id.imagetodrag);
    ImageView targetImage = (ImageView)findViewById(R.id.target);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        imageToBeDragged.setOnTouchListener(new ChoiceTouchListener());
        targetImage.setOnDragListener(new ChoiceDragListener());
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }
}

Draggable listener:

public final class ChoiceTouchListener implements OnTouchListener {

    @Override
    public boolean onTouch(View view, MotionEvent motionEvent) {
        if (motionEvent.getAction() == MotionEvent.ACTION_DOWN) {
            ClipData data = ClipData.newPlainText("", "");
            DragShadowBuilder shadowBuilder = new View.DragShadowBuilder(view);
            //start dragging the item touched
            view.startDrag(data, shadowBuilder, view, 0);
            return true;
        }
        else {
            return false;
        }
    }
}

Target listener:

public class ChoiceDragListener implements OnDragListener {

    @Override
    public boolean onDrag(View v, DragEvent event) {
        switch (event.getAction()) {
        case DragEvent.ACTION_DRAG_STARTED:
            //no action necessary
            break;
        case DragEvent.ACTION_DRAG_ENTERED:
            //no action necessary
            break;
        case DragEvent.ACTION_DRAG_EXITED:
            //no action necessary
            break;
        case DragEvent.ACTION_DROP:
            //handle the dragged view being dropped over a target view
            View view = (View) event.getLocalState();
            //stop displaying the view where it was before it was dragged
            view.setVisibility(View.INVISIBLE);
            //view dragged item is being dropped on
            ImageView dropTarget = (ImageView) v;
            //view being dragged and dropped
            ImageView dropped = (ImageView) view;
            //Dim the target image when the other ImageView is dropped on it
            dropTarget.setAlpha(100);
            break;
        case DragEvent.ACTION_DRAG_ENDED:
            //no action necessary
            break;
        default:
            break;
        }
        return true;
    }
}

What can be causing the error?

Thanks!

Answer 1

Shooting in the dark without seeing the error but your activity looked wrong. You can not find a view by its id until you have set the content view.

public class MainActivity extends Activity {

    ImageView imageToBeDragged;
    ImageView targetImage;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        imageToBeDragged = (ImageView)findViewById(R.id.imagetodrag);
        targetImage = (ImageView)findViewById(R.id.target);
        imageToBeDragged.setOnTouchListener(new ChoiceTouchListener());
        targetImage.setOnDragListener(new ChoiceDragListener());
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }
}