什么都没画

Question

此代码在运行时在屏幕上不显示任何内容。我需要在任何地方画一个简单的圆圈。

public class Touchevent extends AppCompatActivity {
    float mLastTouchX = 0;
    float mLastTouchY = 0;
    Canvas zex=new Canvas();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_touchevent);
        RelativeLayout mylayout = (RelativeLayout)findViewById(R.id.test);

        mylayout.setOnTouchListener(
            new RelativeLayout.OnTouchListener() {
                public boolean onTouch(View v, MotionEvent m) {
                    onTouchEvent(m);
                    onDraw(zex);
                    return true;
                }
            }
        );
    }

    public void onDraw(Canvas canvasx) {
        canvasx.drawColor(Color.WHITE);
        Paint paint = new Paint();
        paint.setColor(Color.BLUE);
        canvasx.drawCircle(mLastTouchX,mLastTouchY,100,paint);
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        mLastTouchX = ev.getX();
        mLastTouchY = ev.getY();
        return true;
    }
}

代码显示没有错误。我是否需要添加自定义视图？那么如何实现点击监听器？