State Management - Do you use the AppState pattern with State Change Notification?

Implementing the INotifyPropertyChanged interface is one of the most popular and .NET native approaches to notify other components of changes to a shared state object.

Implementing the INotifyPropertyChanged interface allows listeners (other pages / components / classes) to be notified when the PropertyChanged event is invoked.

Listeners subscribe to the event by adding their own handling code to the PropertyChanged event.

In this example we made the BaseState class generic so that we can have a reusable abstraction that works for all types of state objects.

public abstract class BaseState<T> : INotifyPropertyChanged
{
    private T _state;

    public BaseState(T initialState)
    {
        _state = initialState;
    }

    protected T State => _state;

    public event PropertyChangedEventHandler? PropertyChanged = null!;

    protected void OnPropertyChanged([CallerMemberName] string name = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
    }
}

Figure: Generic State object that implements INotifyPropertyChanged interface

One of the main considerations with the BaseState abstraction is to keep the T State as a protected member and not expose it publicly. This restricts the possibility of external changes to our T State.

The next code snippet shows the Counter class which is a shared state object that is wrapped by the generic BaseState<Counter>. This enables us to notify listeners when the Counter state is explicitly changed.

The CounterState implementation will call the OnPropertyChanged() method whenever we explicitly changed the protected Counter.

public class Counter
{
    public int Count { get; set; }
}

public class CounterState : BaseState<Counter>
{
    public CounterState() : base(
        new Counter()
        {
            Count = 0
        })
    {
    }

    public void Reset()
    {
        State.Count = 0;
        OnPropertyChanged();
    }

    public void Increment()
    {
        ++State.Count;
        OnPropertyChanged();
    }
}

Figure: Implementation of the generic BaseState<T> object to notify listeners when the Counter object has been changed

In order for us to inject our CounterState object into a page or component, we must register it as a service (typically in Program.cs).

// register our CounterState object with a scoped lifetime
builder.Services.AddScoped<CounterState>();

Figure: Registering CounterState so that it can be injected to a page or component

The ideal time to add a state change handler is when the page/component is being initialized via OnInitializedAsync().

protected override async Task OnInitializedAsync()
{
    _state.PropertyChanged += async (s, p) => await InvokeAsync(StateHasChanged);

    await base.OnInitializedAsync();
}

Once a property is changed, the PropertyChanged event will be invoked (by BaseState<>) and our custom handler code will be executed.

The Counter page example below calls StateHasChanged() when the PropertyChanged event is invoked to refresh the view to display the latest state.

@page "/counterWithPropertyChangeNotification"
@implements IDisposable

@* Inject our scoped CounterState and use it in the view / code section *@
@inject CounterState _state

<PageTitle>Counter with Observed State</PageTitle>

<p class="h2">Counter with Observed State</p>
<p class="mb-4">Current count: @_state.Value.Count</p>

@* Note: Due to user interaction, the page will refresh and show updated state value, even though we have not called StateHasChanged *@
<button type="button" class="btn btn-primary" @onclick="IncrementCount">Click me</button>
<button type="button" class="btn btn-warning" @onclick="Reset">Reset</button>

@code {

    protected override async Task OnInitializedAsync()
    {
        _state.PropertyChanged += async (s, p) => await InvokeAsync(StateHasChanged);

        await base.OnInitializedAsync();
    }

    public void Dispose()
    {
        _state.PropertyChanged -= async (s, p) => await InvokeAsync(StateHasChanged);
    }

    private void IncrementCount()
    {
        _state.Increment();
    }

    private void Reset()
    {
        _state.Reset();
    }
}

Figure: Full example showing how to inject state, subscribe to state changes and how to unsubscribe from state changes

Note: Remember to unsubscribe from the PropertyChanged event to avoid any memory leaks. See rule about when to implement IDisposable.

Whenever the IncrementCount() or Reset() methods are invoked, any listeners on the page will invoke the handling code attached to the PropertyChanged event - and be able to invoke StateHasChanged in order to update their respective views.

The real value of implementing INotifyPropertyChanged (or by using an abstraction like BaseClass<T> above) is when the same shared state object is used multiple times on the same page and having the PropertyChanged event handlers invoked from a single interaction and automatically keeping the view up to date for all components.