Shell | ViewModel Lifecycle

Shiny Shell provides lifecycle management for your ViewModels by implementing simple interfaces. These work similarly to Prism Library — implement the interface and the behavior is automatic.

IPageLifecycleAware

Notifies your ViewModel when the page becomes visible or hidden.

using Shiny;

public class MyViewModel : IPageLifecycleAware
{
    public void OnAppearing()
    {
        // Page is now visible — start timers, refresh data, etc.
    }

    public void OnDisappearing()
    {
        // Page is being hidden or popped — pause timers, save state, etc.
    }
}

INavigationConfirmation

Allows you to prevent navigation away from the page (e.g. unsaved changes).

using Shiny;

public class EditViewModel : INavigationConfirmation
{
    public bool HasUnsavedChanges { get; set; }

    public async Task<bool> CanNavigate()
    {
        if (!HasUnsavedChanges)
            return true;

        // Return false to block navigation
        return await ShowConfirmDialog("You have unsaved changes. Discard?");
    }
}

If CanNavigate() returns false, the navigation is cancelled and the user stays on the current page.

INavigationAware

Allows you to add or modify navigation arguments before leaving the page.

using Shiny;

public class FormViewModel : INavigationAware
{
    public string DraftText { get; set; }

    public void OnNavigatingFrom(IDictionary<string, object> parameters)
    {
        // Attach data that the previous page can receive
        parameters["DraftText"] = DraftText;
    }
}

The parameters dictionary is delivered to the target ViewModel via IQueryAttributable.ApplyQueryAttributes.

IDisposable

When a page is removed from the navigation stack entirely, Dispose() is called on the ViewModel. Use this to clean up event handlers, subscriptions, or other resources.

public class StreamViewModel : IDisposable
{
    private readonly IDisposable _subscription;

    public StreamViewModel(IDataService dataService)
    {
        _subscription = dataService.Stream.Subscribe(OnData);
    }

    public void Dispose()
    {
        _subscription.Dispose();
    }
}

IQueryAttributable

This is a built-in .NET MAUI interface (not from Shiny) that receives navigation arguments. Implement it to handle arguments from NavigateTo, GoBack, and PopToRoot.

using Microsoft.Maui.Controls;

public class DetailViewModel : IQueryAttributable
{
    public int Id { get; set; }
    public string Name { get; set; }

    public void ApplyQueryAttributes(IDictionary<string, object> query)
    {
        if (query.TryGetValue("Id", out var id))
            Id = (int)id;

        if (query.TryGetValue("Name", out var name))
            Name = (string)name;
    }
}

Complete Example

A ViewModel implementing all lifecycle interfaces:

using Shiny;
using Microsoft.Maui.Controls;

public class FullViewModel :
    IPageLifecycleAware,
    INavigationConfirmation,
    INavigationAware,
    IQueryAttributable,
    IDisposable
{
    private IDisposable _subscription;
    public int ItemId { get; set; }
    public bool IsDirty { get; set; }

    // Receive navigation arguments
    public void ApplyQueryAttributes(IDictionary<string, object> query)
    {
        if (query.TryGetValue("ItemId", out var id))
            ItemId = (int)id;
    }

    // Page became visible
    public void OnAppearing()
    {
        _subscription = SomeService.Subscribe(OnUpdate);
    }

    // Page being hidden
    public void OnDisappearing()
    {
        _subscription?.Dispose();
        _subscription = null;
    }

    // Guard navigation
    public Task<bool> CanNavigate()
    {
        return Task.FromResult(!IsDirty);
    }

    // Attach outgoing arguments
    public void OnNavigatingFrom(IDictionary<string, object> parameters)
    {
        parameters["LastViewedId"] = ItemId;
    }

    // Final cleanup
    public void Dispose()
    {
        _subscription?.Dispose();
    }
}