public class Custom extends View{

    Paint paint;
    private float cx;      //圆点默认X坐标
    private float cy;      //圆点默认Y坐标
    private int radius = 40;
    private int mX1;
    private int mY1;
    private boolean mHasBall;
    int width;
    int height;

    Rect rect = new Rect();

    public Custom(Context context) {
        this(context,null);
    }

    public Custom(Context context, @Nullable AttributeSet attrs) {
        this(context,attrs,0);
    }

    public Custom(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        initData();
    }

    private void initData() {
        paint = new Paint();
        //设置消除锯齿
        paint.setAntiAlias(true);
        //设置画笔颜色
        paint.setColor(Color.RED);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        width = this.getWidth();
        height = this.getHeight();

        cx= width/2;
        cy=height/2;
    }

    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
    }

    @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();
        //随机设置画笔颜色
        //setPaintRandomColor();
        //绘制小圆作为小球
        canvas.drawCircle(cx, cy, radius, paint);
    }

    //修正圆点坐标
    private void revise() {

    }



    @Override
    public boolean onTouchEvent(MotionEvent event) {

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN: // 按下
                mX1 = (int) event.getX();
                mY1 = (int) event.getY();// 通知重绘
                mHasBall = isHasBall(mX1, mY1);
                break;
            case MotionEvent.ACTION_MOVE: // 移动
                if (mHasBall) {
                    this.cx = event.getX();
                    this.cy = event.getY();
                    postInvalidate();
                }
                break;
            case MotionEvent.ACTION_UP: // 抬起
                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;
    }

    private boolean isHasBall(int x1, int y1) {
        double sqrt = Math.sqrt((x1 - cx) * (x1 - cx) + (y1 - cy) * (y1 - cy));
        if (sqrt < radius) {
            return true;
        }
        return false;
    }

}