java如何在DragListener中获取画布项以便在其中绘制？

Question

我尝试在画布上绘制一些形状，如果项目被放置到画布上：

public class MainActivity extends AppCompatActivity {

    private String selectedItem=null;
    private Button circle=null;
    private Button rectangle=null;
    private Button triangle=null;

    private MyTouchListener touch= new MyTouchListener();

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

        rectangle=(Button)findViewById(R.id.rectangle);
        rectangle.setOnTouchListener(touch);
        rectangle.setOnDragListener(new MyDragListener());

        circle=(Button)findViewById(R.id.circle);
        circle.setOnTouchListener(touch);
        circle.setOnDragListener(new MyDragListener());

        triangle=(Button)findViewById(R.id.triangle);
        triangle.setOnTouchListener(touch);
        triangle.setOnDragListener(new com.example.dragtest.MainActivity.MyDragListener());

    }

    private final class MyTouchListener implements View.OnTouchListener {
        public boolean onTouch(View view, MotionEvent motionEvent) {
            if (motionEvent.getAction() == MotionEvent.ACTION_DOWN) {
                // selectedButton
                final int id = view.getId();
                switch(id){
                    case R.id.circle:
                        selectedItem="circle";
                        break;
                    case R.id.rectangle:
                        selectedItem="rectangle";
                        break;
                    case R.id.triangle:
                        selectedItem="triangle";
                        break;
                }

                return true;
            }

            return false;
        }
    }

    private final class MyDragListener implements View.OnDragListener {
        public boolean onDrag(View v, DragEvent event) {
            final int id = v.getId();
            switch (event.getAction()) {
                case DragEvent.ACTION_DRAG_STARTED:
                    // do nothing
                    break;
                case DragEvent.ACTION_DRAG_ENTERED:
                    break;
                case DragEvent.ACTION_DRAG_EXITED:
                    break;
                case DragEvent.ACTION_DROP:
                    // Dropped, reassign View to ViewGroup
                    if(id != R.id.dropzone || !(v instanceof Canvas)){
                        return true;
                    }
                    final float x = v.getX();
                    final float y = v.getY();
                    final Canvas c = (Canvas)v;
                    switch(selectedItem){
                        case "rectangle":
                            Paint p = new Paint();
                            p. setARGB(0, 255, 0, 0);
                            c.drawRect(x,y,x+5,y+5,p);
                            break;
                        case "circle":
                            Paint p = new Paint();
                            p. setARGB(0, 255, 0, 128);
                            c.drawCircle(x,y,100,p);
                            break;
                        case "triangle":
                            Paint p = new Paint();
                            p. setARGB(0, 255, 144, 128);

                            Point a = new Point(x,y);
                            Point b = new Point(x-5, y+5);
                            Point c = new Point(x+5, y+5);

                            Path path = new Path();
                            path.setFillType(Path.FillType.EVEN_ODD);
                            path.lineTo(b.x, b.y);
                            path.lineTo(c.x, c.y);
                            path.lineTo(a.x, a.y);
                            path.close();

                            c.drawPath(path, p);
                            break;
                    }

                    break;
                case DragEvent.ACTION_DRAG_ENDED:

                default:
                    break;
            }
            return true;

        }
    }
}

但在第86行：

 final Canvas c = (Canvas)v;

我得到以下错误：

error: incompatible types: View cannot be converted to Canvas

据我所知，视图获取事件的源代码，如果项目被删除，我希望执行一些代码

因此，我如何检测物品是否掉落：

• 在哪里下车
• 画布中的哪个位置被放下

实际上，我想检查按钮是否落在画布内。你知道我怎么做吗