I have a Maui application with tabs. On the DashboardMainPage, items are displayed in a CollectionView. From this page, I can navigate to a subpage (GoalsPage) where all items can be seen in more detail. In GoalsPage, I can navigate to another subpage (CreateGoalPage) where new goals can be saved to the local database.

The navigation flow (using shell navigation, Shell.Current.GoToAsync):

DashboardMainPage -> GoalsPage -> CreateGoalPage

public partial class DashboardMainViewModel : BaseViewModel
{
    // The repository to handle CRUD to local db
    private readonly IUnitOfWork _unitOfWork;
    public ObservableCollection<Goal> Goals { get; } = new ObservableCollection<Goal>();

    public DashboardMainViewModel(IUnitOfWork unitOfWork, INavigationService navigationService) : base(navigationService)
    {
        _unitOfWork = unitOfWork;
    }

    // This is called from OnAppearing of DashboardMainPage
    internal async Task InitializeAsync()
    {
        if (IsInitialized)  // Prevents LoadDataAsync running multiple times when navigate on and navigate back from subpage
        {
            return;
        }

        await LoadDataAsync();
    }
    
    private async Task LoadDataAsync()
    {
        Goals.Clear();

        var goalsTask =  _unitOfWork.Goal.GetAllAsync(_ => _.Completed == false);
        Goals.AddRange(goalsTask);
    }

    [RelayCommand]
    private async Task GoToGoalsPage() => await NavigationService.GoToAsync(nameof(GoalsPage));
}

public partial class GoalsViewModel : BaseViewModel
{
    private readonly IUnitOfWork _unitOfWork;
    public ObservableCollection<Goal> OnGoingGoals { get; } = new ObservableCollection<Goal>();
    public ObservableCollection<Goal> CompletedGoals { get; } = new ObservableCollection<Goal>();

    public GoalsViewModel(IUnitOfWork unitOfWork, INavigationService navigationService) : base(navigationService)
    {
        _unitOfWork = unitOfWork;     
    }
    
    // This is called from OnAppearing of DashboardMainPage
    internal async Task InitializeAsync()
    {
        if (IsInitialized) // Prevents LoadDataAsync running multiple times when navigate on and navigate back from subpage
        {
            return;
        }

        await LoadDataAsync();
    }

    private async Task LoadDataAsync()
    {
        OnGoingGoals.Clear();
        CompletedGoals.Clear();

        var goals = await _unitOfWork.Goal.GetAllAsync();
        var completed = goals.Where(_ => _.IsCompleted).OrderByDescending(_ => _.Completed.Date);

        OnGoingGoals.AddRange(goals.Where(_ => !_.IsCompleted));
        CompletedGoals.AddRange(completed);
    }

    [RelayCommand]
    private async Task GoToGoalCreatePage()
    {
        var goal = new Goal();
        
        var parameters = new Dictionary<string, object> {
            { nameof(Goal), goal }
        };
        await NavigationService.GoToAsync(nameof(GoalCreatePage), parameters);
    }
}

[QueryProperty(nameof(Goal), nameof(Goal))]
[QueryProperty(nameof(IsEdit), nameof(IsEdit))]
public partial class GoalCreateViewModel : BaseViewModel
{
    // The repository to handle CRUD to local db
    private readonly IUnitOfWork _unitOfWork;

    [ObservableProperty]
    private Goal _goal;

    public GoalCreateViewModel(IUnitOfWork unitOfWork, INavigationService navigationService) : base(navigationService)
    {
        _unitOfWork = unitOfWork;
    }

    [RelayCommand]
    private async Task Save()
    {
        var savedGoal = await _unitOfWork.Goal.SaveAsync(Goal);
        if (savedGoal is not null)
        {
            // should I trigger here a weakreference to call item in the both parent vms?
            
            // Navigate back to GoalsPage
            await NavigationService.GoBackAsync();
        }
    }
}

QUESTIONS
When a new goal is added in the sub-sub page, how is it possible to update the two parent pages?

• Should I use weakreference (seems against mvvm)?
• The fact I am using twice the same list, is there a way I can make a single observable collection which can be reached in all VM? (DashboardMainViewModel , GoalsViewModel and CreateGoalViewModel) and when locally is saved I could just add as Goals.Add(NewGoal)? Should it be part of the BaseViewModel?