如何在当前窗口中绘制画布？(不使用setContentView)

import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.os.Bundle;
import android.view.View;
import android.view.Window;
import android.view.WindowManager;

public class MyCanvasActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        
        // 设置全屏
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
                WindowManager.LayoutParams.FLAG_FULLSCREEN);
        
        // 创建自定义的View类的实例
        MyCanvasView canvasView = new MyCanvasView(this);
        
        // 将自定义的View类添加到当前窗口的内容视图中
        setContentView(canvasView);
    }

    // 自定义的View类
    private static class MyCanvasView extends View {
        private Paint paint;

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

        @Override
        protected void onDraw(Canvas canvas) {
            super.onDraw(canvas);
            
            // 在画布上绘制一个矩形
            canvas.drawRect(100, 100, 500, 500, paint);
        }
    }
}