Xamarin:使用ViewModel的实例设置labelText不是更新UI

public class MyViewModel : INotifyPropertyChanged
{
    private string _labelText;

    public string LabelText
    {
        get { return _labelText; }
        set
        {
            if (_labelText != value)
            {
                _labelText = value;
                OnPropertyChanged(nameof(LabelText));
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

<Label Text="{Binding LabelText}" />

public partial class MainPage : ContentPage
{
    public MainPage()
    {
        InitializeComponent();

        MyViewModel viewModel = new MyViewModel();
        this.BindingContext = viewModel;
    }
}

((MyViewModel)this.BindingContext).LabelText = "新的文本";