Xamarin：ViewModel 不更新 UI

Question

我第一次尝试实现 ViewModel 绑定，发现绑定到视图模型中的属性的标签没有更新以反映该值 我认为我正在做的事情很愚蠢，但希望有人看看并提供帮助。

视图模型：

public class MainPageViewModel : INotifyPropertyChanged
{

    public MainPageViewModel()
    {

    }

    public MainPageViewModel(List<Player> _players, GameType _selectedGame)
    {

        this.Players = _players;
        this.SelectedGame = _selectedGame;
        //Device.BeginInvokeOnMainThread(() =>
        //{
        //    Players = _players;
        //    SelectedGame = _selectedGame;
        //});
        
    }

    private List<Player> players;

    public List<Player> Players
    {
        get { return players; }
        set 
        { 
            players = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("Players")); //might need collectionchanged here
        }
    }

    private GameType selectedGame;

    public GameType SelectedGame
    {
        get { return selectedGame; }
        set 
        {
            selectedGame = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("SelectedGame"));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    
}

游戏类型：

public class GameType : INotifyPropertyChanged
{
    private string _name = "";
    public string Name
    { 
        get { return _name; } 
        set { _name = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("Name"));
        } 
    }

    public event PropertyChangedEventHandler PropertyChanged;

    //will have scoring and rules

}

主页控制：

        <StackLayout x:Name="stackLayoutScoring" Grid.Column="3" Grid.RowSpan="4">
        <Label x:Name="labelGameType" Text="{Binding SelectedGame.Name}" HorizontalTextAlignment="Center" />

MainPageViewModel（有一些我尝试过的东西）：

public class MainPageViewModel : INotifyPropertyChanged
{

    public MainPageViewModel()
    {

    }

    public MainPageViewModel(List<Player> _players, GameType _selectedGame)
    {

        this.Players = _players;
        this.SelectedGame = _selectedGame;
        //Device.BeginInvokeOnMainThread(() =>
        //{
        //    Players = _players;
        //    SelectedGame = _selectedGame;
        //});
        
    }

    private List<Player> players;

    public List<Player> Players
    {
        get { return players; }
        set 
        { 
            players = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("Players")); 
        }
    }

    private GameType selectedGame;

    public GameType SelectedGame
    {
        get { return selectedGame; }
        set 
        {
            selectedGame = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("SelectedGame"));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    
}

}

游戏开始事件参数：

public class GameStartedEventArgs : EventArgs, INotifyPropertyChanged
{
    public GameStartedEventArgs(Game game)
    {
        _Game = game;
    }


    private Game _game;
    public Game _Game
    {
        get { return _game; }
        set { _game = value; PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("_Game")); }
    }

    public event PropertyChangedEventHandler PropertyChanged; //PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("Name")); 
}

主页代码：

 public void InitGame(Game game)
    {
        //initialize points and setup first player
        stackLayoutScoring.BindingContext = viewModel;
        viewModel.Players = game.Players;
        viewModel.SelectedGame = game.SelectedGame;
        //stackLayoutScoring.BindingContext = viewModel;
    }

现在 viewModel 在 MainPage.cs 中接收到正确的值，但 labelGameType.Text 永远不会更新。 我不确定从异步方法调用 InitGame() 是否会导致问题：

async void GameStartedAlert(object sender, GameStartedEventArgs e)
    {
        await DisplayAlert("Success!", Statics.CurrentGame.SelectedGame.Name + ": Game Started!", "OK");
        InitGame(e._Game);
        
    }

我正在努力找出问题所在。我希望这是愚蠢的事情。

谢谢，

迈克

Answer 1

我的问题是由于页面之间导航不正确而引起的。

我正在使用

async void NavigateToMainPage(object sender, EventArgs e)
    {
        await Navigation.PushAsync(new MainPage());
    }

它创建 MainPage 的新实例，与具有数据绑定的实例并行。我用以下方法修复了它：

public void NavigateToMainPage()
    {
        App.Current.MainPage.Navigation.PopAsync();
    }

感谢 Jason 发现我的代码的另一个问题。