如何在Android中调用CustomView的onDraw(Canvas canvas)函数？

public class CustomView extends View {
    // 构造函数
    public CustomView(Context context) {
        super(context);
    }

    // 重写onDraw方法
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        // 在canvas上进行绘制操作，例如绘制一个圆形
        Paint paint = new Paint();
        paint.setColor(Color.RED);
        canvas.drawCircle(getWidth() / 2, getHeight() / 2, 100, paint);
    }
}

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 找到CustomView的实例
        CustomView customView = findViewById(R.id.custom_view);
        // 调用invalidate()方法触发重绘
        customView.invalidate();
    }
}

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <com.example.myapplication.CustomView
        android:id="@+id/custom_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>