ViewModel不会处理EventAggregator事件

Question

我似乎只能处理来自ShellViewModel的ShellViewModel事件，但我想从LoginViewModel处理它。

ShellViewModel将LoginViewModel构造为它的活动项。我还将它设置为从IHandle<AuthenticatedMessage>继承作为事件发布是否有效的测试。它能够处理那个事件。为了简洁起见，我没有在代码中显示任何取消订阅事件。

public class ShellViewModel : Conductor<IScreen>.Collection.OneActive, IHandle<AuthenticatedMessage>    
{
    public ShellViewModel(IEventAggregator eventAggregator, LoginViewModel loginViewModel)
    {
        _eventAggregator = eventAggregator;
        _loginViewModel = loginViewModel;
    }
    
    protected override async Task OnActivateAsync(CancellationToken cancellationToken)
    {
        await base.OnActivateAsync(cancellationToken);
        _eventAggregator.SubscribeOnPublishedThread(this);
    
        await ActivateItemAsync(_loginViewModel);
    }
    
    public async Task HandleAsync(AuthenticatedMessage authCode, CancellationToken cancellationToken)
    {
        // this is reached! So the event is publishing successfully.
        await Task.CompletedTask;
    }
}

LoginViewModel也订阅了这个事件，但是它的Handle方法没有被调用。

Login方法负责创建发布事件的LoginWindowViewModel窗口(显示在下面)。

public class LoginViewModel : Screen, IHandle<AuthenticatedMessage>
{
    private IEventAggregator _eventAggregator;
    private readonly IWindowManager _windowManager;
    private readonly ILoginWindowViewModelFactory _LoginWindowViewModelFactory;

    public LoginViewModel(IEventAggregator eventAggregator, 
        IWindowManager windowManager, 
        ILoginWindowViewModelFactory loginWindowViewModelFactory)
    {
        _eventAggregator = eventAggregator; 
        _windowManager = windowManager;
        _LoginWindowViewModelFactory = loginWindowViewModelFactory;
    }
    
    protected override async Task OnActivateAsync(CancellationToken cancellationToken)
    {
        await base.OnActivateAsync(cancellationToken);
        
        _eventAggregator.SubscribeOnPublishedThread(this);
    }
    
    // This is bound to a button click event. It creates a window.
    public async void Login()
    {
        Uri loginUri = new Uri(_api.BaseLoginUrl);

        await _windowManager.ShowWindowAsync(
            _ndLoginWindowViewModelFactory.Create(loginUri, _eventAggregator));
    }       
    
    
    public async Task HandleAsync(AuthenticatedMessage authCode, CancellationToken cancellationToken)
    {
        // why is this is never reached?
        await Task.CompletedTask;
    }
}

发布AuthenticatedMessage事件的AuthenticatedMessage：

public class LoginWindowViewModel : Screen
{
    private readonly IEventAggregator _eventAggregator;
    private readonly Uri _initialUri;         

    public NDLoginWindowViewModel(Uri initialUri, IEventAggregator eventAggregator)
    {
        _initialUri = initialUri;
        _eventAggregator = eventAggregator;
    }       
    
    // bound to the WebView2 (browser control) event
    public async void NavigationStarting(object sender, CoreWebView2NavigationStartingEventArgs e)
    {            
            string authCode = HttpUtility.ParseQueryString(new Uri(e.Uri).Query).Get("code");
            
            // Publish event here. LoginViewModel should handle this, but currently only ShellViewModel can.
            await _eventAggregator.PublishOnUIThreadAsync(new AuthenticatedMessage(authCode));                              
        }
    }
}

Answer 1

在将eventAggregator.SubscribeOnPublishedThread(this);移动到LoginViewModel构造函数而不是OnActivateAsync()方法之后，我解决了这个问题。

从这里：

protected override async Task OnActivateAsync(CancellationToken cancellationToken)
{
    await base.OnActivateAsync(cancellationToken);
    
    _eventAggregator.SubscribeOnPublishedThread(this);
}

到这里：

public LoginViewModel(IEventAggregator eventAggregator, 
    IWindowManager windowManager, 
    ILoginWindowViewModelFactory loginWindowViewModelFactory)
{
    _eventAggregator = eventAggregator; 
    _windowManager = windowManager;
    _LoginWindowViewModelFactory = loginWindowViewModelFactory;

    _eventAggregator.SubscribeOnPublishedThread(this);
}

编辑：

当视图第一次在ShellViewModel中创建时，不会调用ShellViewModel方法，因为它是我的根屏幕和导体。所以订阅从来没有发生过。

public class ShellViewModel : Conductor<IScreen>.Collection.OneActive
{
...
    protected override async Task OnActivateAsync(CancellationToken cancellationToken)
    {
        await base.OnActivateAsync(cancellationToken);            
    
        await ActivateItemAsync(_loginViewModel);
        // IsActive = false here, therefore the child Screen `_loginViewModel` 
        // is also not active. Result is that OnActivateAsync 
        // in this view model does not get called.
    }
}

它与this problem and answer直接相关。

这就解释了为什么将它移到构造函数中解决了这个问题。

最后的解决方案是在Constructor和OnActivateAsync方法中添加OnActivateAsync。这允许我重新订阅事件后，我导航离开这个视图模型，并回到它。