自定义控件 实现圆形跟随鼠标点击移动

public class MainActivity extends AppCompatActivity {
    private int screenW;        //屏幕宽度
    private int screenH;        //屏幕高度
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Display dis = this.getWindowManager().getDefaultDisplay();
        // 设置全屏
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
                WindowManager.LayoutParams.FLAG_FULLSCREEN);
        // 获取屏幕宽度
        screenW = dis.getWidth();
        // 获取屏幕高度
        screenH = dis.getHeight();

        setContentView(new BallView(this));
    }

    //自定义绘图类
    class BallView extends View {
        private Paint paint;        //定义画笔
        private float cx = 50;      //圆点默认X坐标
        private float cy = 50;      //圆点默认Y坐标
        private int radius = 80;
        //定义颜色数组
        private int colorArray[] = {Color.RED};
        private int paintColor = colorArray[0]; //定义画笔默认颜色

        public BallView(Context context) {
            super(context);
            //初始化画笔
            initPaint();
        }
        private void initPaint(){
            paint = new Paint();
            //设置消除锯齿
            paint.setAntiAlias(true);
            //设置画笔颜色
            paint.setColor(paintColor);
        }

        //重写onDraw方法实现绘图操作
        @Override
        protected void onDraw(Canvas canvas) {
            super.onDraw(canvas);
            //将屏幕设置为白色
            canvas.drawColor(Color.WHITE);
            //修正圆点坐标
            revise();
            //绘制小圆作为小球
            canvas.drawCircle(cx, cy, radius, paint);
        }

        //修正圆点坐标
        private void revise(){
            if(cx <= radius){
                cx = radius;
            }else if(cx >= (screenW-radius)){
                cx = screenW-radius;
            }
            if(cy <= radius){
                cy = radius;
            }else if(cy >= (screenH-radius)){
                cy = screenH-radius;
            }
        }

        @Override
        public boolean onTouchEvent(MotionEvent event) {
            switch (event.getAction()) {
                case MotionEvent.ACTION_DOWN:
                    // 按下
                    cx = (int) event.getX();
                    cy = (int) event.getY();
                    // 通知重绘
                    postInvalidate();   //该方法会调用onDraw方法，重新绘图
                    break;
                case MotionEvent.ACTION_MOVE:
                    // 移动
                    cx = (int) event.getX();
                    cy = (int) event.getY();
                    // 通知重绘
                    postInvalidate();
                    break;
                case MotionEvent.ACTION_UP:
                    // 抬起
                    cx = (int) event.getX();
                    cy = (int) event.getY();
                    // 通知重绘
                    postInvalidate();
                    break;
            }

            /*
             * 备注1：此处一定要将return super.onTouchEvent(event)修改为return true，原因是：
             * 1）父类的onTouchEvent(event)方法可能没有做任何处理，但是返回了false。
             * 2)一旦返回false，在该方法中再也不会收到MotionEvent.ACTION_MOVE及MotionEvent.ACTION_UP事件。
             */
            //return super.onTouchEvent(event);
            return true;
        }
    }
}