Android -在画布上绘图在Runnable中不起作用

public class MyView extends View {
    private Paint paint;

    public MyView(Context context) {
        super(context);
        paint = new Paint();
        paint.setColor(Color.RED);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawCircle(100, 100, 50, paint);
    }
}

public class MainActivity extends AppCompatActivity {
    private MyView myView;

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

        myView = findViewById(R.id.my_view);

        // 在Runnable中绘制图形
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                // 在UI线程中执行绘图操作
                myView.post(new Runnable() {
                    @Override
                    public void run() {
                        myView.invalidate(); // 刷新界面
                    }
                });
            }
        };

        // 在后台线程中执行绘图操作
        new Thread(runnable).start();
    }
}