Android自定义View如何实现拖拽效果

public class CustomView extends View {
 
 private int lastX;
 private int lastY;
 private int startLeft;
 private int startRight;
 private int startTop;
 private int startBottom;
 
 public CustomView(Context context, @Nullable AttributeSet attrs) {
 super(context, attrs);
 }
 
 @Override
 protected void onDraw(Canvas canvas) {
 super.onDraw(canvas);
 //画一个小球
 Paint paint = new Paint();
 paint.setStrokeWidth(30);
 paint.setColor(Color.parseColor("#FF0000"));
 paint.setAntiAlias(true);
 paint.setStyle(Paint.Style.FILL_AND_STROKE);
 canvas.drawCircle(getWidth()/2,getHeight()/2,30,paint);
 }
 
 @Override
 public boolean onTouchEvent(MotionEvent event) {
 int action = event.getAction();
 //获取手机触摸的坐标
 int x = (int) event.getX();
 int y = (int) event.getY();
 switch (action){
 case MotionEvent.ACTION_DOWN://按下,获取小球初始的位置
 startLeft = getLeft();
 startRight = getRight();
 startTop = getTop();
 startBottom = getBottom();
 lastX = x;
 lastY = y;
 break;
 case MotionEvent.ACTION_MOVE://移动,小球跟随手指的移动
 int offsetX = x - lastX;
 int offsetY = y - lastY;
 layout(getLeft()+offsetX,getTop()+offsetY,
 getRight()+offsetX,getBottom()+offsetY);
 break;
 case MotionEvent.ACTION_UP://当手指抬起时,回到小球初始的位置
 layout(startLeft, startTop, startRight, startBottom);
 break;
 }
 return true;
 }
}