从（不同的）class - Xamarin.Forms 更改 TabbedPage 绑定属性

Question

My TabbedPage uses a Binding Property , which is defined in the tabbed page's ViewModel , for showing a Badge text.

I am setting the badge property when initializing the view (actually when it (re)appears). However, sometimes the badge text is changing from outside of my ViewModel(s), this is because I have a SignalR method which is called when a new message is being added by another application.

Though, when this happens the OnAppearing method of my tabbed viewmodel is obviously not called. So the question is, how can I 'notify' the tabbedpage viewmodel that the badge text should be changed.

I think the (best) way to do this is using somekind of Event. Since all of my ViewModels inherit from a 'ViewModelBase' I could implement the event notification / change in the ViewModelBase and override the property in my TabbedPage ViewModel. Though, sadly my knowledge about using Events / EventArgs is limited and the stuff I found about it is not working.

Is using EventArgs the best way to solve this problem? And if so, could anyone give any pointers how to implement it properly.

*On a side-note, I am also using Prism

My TabbedPage ViewModel:

public class RootTabbedViewModel : ViewModelBase, IPageLifecycleAware
{
    private readonly INavigationService _navigationService;
    private int _messageCount;

    public RootTabbedViewModel(INavigationService navigationService) 
        : base(navigationService)
    {
        _navigationService = navigationService;
    }


    public int MessageCount
    {
        get { return _messageCount; }
        set { SetProperty(ref _messageCount, value); }
    }

    public void OnDisappearing()
    {
       
    }

    void IPageLifecycleAware.OnAppearing()
    {
        // (omitted) Logic for setting the MessageCount property
    }
}

ViewModelVase:

public class ViewModelBase : BindableBase, IInitialize, IInitializeAsync, INavigationAware, IDestructible, IActiveAware
{
  
    public event EventHandler MessageAddedEventArgs; // this should be used to trigger the MessageCount change.. 
    protected INavigationService NavigationService { get; private set; }

    public ViewModelBase(INavigationService navigationService)
    {
        NavigationService = navigationService;
        Connectivity.ConnectivityChanged += Connectivity_ConnectivityChanged;

        IsNotConnected = Connectivity.NetworkAccess != NetworkAccess.Internet;
    }

    private bool _isNotConnected;
    public bool IsNotConnected
    {
        get { return _isNotConnected; }
        set { SetProperty(ref _isNotConnected, value); }
    }


    ~ViewModelBase()
    {
        Connectivity.ConnectivityChanged -= Connectivity_ConnectivityChanged;
    }

    async void Connectivity_ConnectivityChanged(object sender, ConnectivityChangedEventArgs e)
    {
        IsNotConnected = e.NetworkAccess != NetworkAccess.Internet;

        if (IsNotConnected == false)
        {
            await DataHubService.Connect();
        }
    }

    public virtual void Initialize(INavigationParameters parameters)
    {

    }

    public virtual void OnNavigatedFrom(INavigationParameters parameters)
    {

    }

    public virtual void OnNavigatedTo(INavigationParameters parameters)
    {

    }

    public virtual void Destroy()
    {

    }

    public virtual Task InitializeAsync(INavigationParameters parameters)
    {
        return Task.CompletedTask;
    }

}

SignalR Datahub which should trigger the event:

public static class DataHubService2
{

    // .. omitted some other SignalR specific code
    
    public static async Task Connect()
    {
        try
        {
            GetInstanse();

            hubConnection.On<Messages>("ReceiveMessage", async (message) =>
            {
                if(message != null)
                {
                    // event that message count has changed should be triggered here..
                }
                
            });



        }
        catch (Exception ex)
        {
            // ...
        }

    }

}

Answer 1

As pointed out by @Jason, this specific problem is a good use case for using the MessagingCenter.

In the end the implementation looks as following:

public static class DataHubService2
{

    // .. omitted some other SignalR specific code
    
    public static async Task Connect()
    {
        try
        {
            GetInstanse();

            hubConnection.On<Messages>("ReceiveMessage", async (message) =>
            {
                if(message != null)
                {
                     MessagingCenter.Send("UpdateMessageCount", "Update");
                }
                
            });



        }
        catch (Exception ex)
        {
            // ...
        }

    }

}


public class RootTabbedViewModel : ViewModelBase, IPageLifecycleAware
{
    private readonly INavigationService _navigationService;
    private int _messageCount;

    public RootTabbedViewModel(INavigationService navigationService) 
        : base(navigationService)
    {
        _navigationService = navigationService;

        MessagingCenter.Subscribe<string>("UpdateMessageCount", "Update", async (a) =>
        {
            await UpdateMessageCount();
        });
    }


    public int MessageCount
    {
        get { return _messageCount; }
        set { SetProperty(ref _messageCount, value); }
    }

    public void OnDisappearing()
    {
    }

    void IPageLifecycleAware.OnAppearing()
    {
        UpdateMessageCount();
    }

    async Task UpdateMessageCount()
    {
        int messageCount = await App.Database.GetNewMessageCountAsync();
        MessageCount = messageCount.ToString();
    }
}