Why I Cannot Draw On SurfaceView ?

SurfaceView srf;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Init();

    srf = (SurfaceView)findViewById(R.id.surfaceView1);
    Canvas cv = new Canvas();
    Paint pn = new Paint();
    pn.setAntiAlias(true);
    pn.setStrokeWidth(6f);
    pn.setColor(Color.BLUE);
    pn.setStyle(Paint.Style.STROKE);
    pn.setStrokeJoin(Paint.Join.ROUND);
    cv.drawLine(2, 2, 100, 100,pn);
    srf.draw(cv);

}