public class TurnTableView extends View {

    private int mHeight;
    private int mWidth;
    private int x;
    private int y;
    private boolean mOnBall;
    private int mRadius = 90;

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

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

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

    @Override//測量
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        //獲取當前控制元件的寬高
        mWidth = this.getWidth();
        mHeight = this.getHeight();
        //獲取螢幕的正中心點
        x = mWidth / 2;
        y = mHeight / 2;
    }

    @Override//繪製
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Paint paint = new Paint();//畫筆
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher);//設定圖片
        canvas.drawBitmap(bitmap,x,y,paint);//放入圖片

    }
    //手勢監聽器,可以得到使用者手指在螢幕上滑動的座標
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()){
            case MotionEvent.ACTION_DOWN:
                float newX = event.getX();
                float newY = event.getY();
                //進行判斷,判斷使用者的手指是否按在了圖上
                mOnBall  = isOnBall(newX,newY);
                Toast.makeText(getContext(), "使用者的手指是否點到圖上了嗎？"+mOnBall, Toast.LENGTH_SHORT).show();
                break;
            case MotionEvent.ACTION_MOVE:
                //只用使用者點到圖上時,我才讓它動
                if (mOnBall){
                    x = (int) event.getX();
                    y = (int) event.getY();
                    //回撥OnDrawer方法
                    postInvalidate();
                }
                break;
            case MotionEvent.ACTION_UP:

                break;
        }
        return true;
    }
    //,判斷使用者的手指是否按在了圖上
    private boolean isOnBall(float newX, float newY) {
        //勾股定理的絕對值
        double sqrt = Math.sqrt((newX - x) * (newX - x) + (newY - y) * (newY - y));
        //判斷絕對值是否小於等於半徑
        if(sqrt <= mRadius){
            return true;
        }
        return false;
    }
}