第一次调用Edittext.SelectAll()在Xamarin android中不起作用

EditText editText;

protected override void OnCreate(Bundle savedInstanceState)
{
    base.OnCreate(savedInstanceState);
    // 设置布局等操作

    editText = FindViewById<EditText>(Resource.Id.editText);

    // 设置OnGlobalLayout监听
    ViewTreeObserver viewTreeObserver = editText.ViewTreeObserver;
    viewTreeObserver.AddOnGlobalLayoutListener(new MyGlobalLayoutListener(editText));
}

private class MyGlobalLayoutListener : Java.Lang.Object, ViewTreeObserver.IOnGlobalLayoutListener
{
    private EditText editText;
    private bool isLoaded = false;

    public MyGlobalLayoutListener(EditText editText)
    {
        this.editText = editText;
    }

    public void OnGlobalLayout()
    {
        if (!isLoaded)
        {
            editText.SelectAll();
            isLoaded = true;
        }
    }
}