Gestion de l’état dans Blazor

Difficultés de gestion de l’état dans Blazor

État simple avec un service à portée

public class CartState
{
    private readonly List<CartItem> _items = new();
    public IReadOnlyList<CartItem> Items => _items;
    public int Count => _items.Sum(i => i.Quantity);

    public event Action? OnChanged;

    public void Add(Product p)
    {
        var existing = _items.FirstOrDefault(i => i.ProductId == p.Id);
        if (existing is not null) existing.Quantity++;
        else _items.Add(new CartItem { ProductId = p.Id, Name = p.Name, Price = p.Price });
        OnChanged?.Invoke();
    }
}

S’abonner aux changements d’état

@inject CartState Cart
@implements IDisposable

<p>Cart: @Cart.Count items</p>

@code {
    protected override void OnInitialized()
    {
        Cart.OnChanged += OnCartChanged;
    }

    private void OnCartChanged()
    {
        InvokeAsync(StateHasChanged);
    }

    public void Dispose() =>
        Cart.OnChanged -= OnCartChanged;
}

CascadingValue comme conteneur d’état

@* MainLayout.razor *@
@inject CartState Cart

<CascadingValue Value="Cart">
    @Body
</CascadingValue>

@* Any nested component: *@
@code {
    [CascadingParameter]
    public CartState Cart { get; set; } = default!;

    private void Buy(Product p) => Cart.Add(p);
}

Fluxor : le modèle Flux pour Blazor

// dotnet add package Fluxor.Blazor.Web

// State record
[FeatureState]
public record CounterState(int Count = 0);

// Actions
public record IncrementAction(int Amount = 1);

// Reducers
public static class CounterReducers
{
    [ReducerMethod]
    public static CounterState Reduce(CounterState state, IncrementAction action)
        => state with { Count = state.Count + action.Amount };
}

// Registration:
builder.Services.AddFluxor(opt =>
    opt.ScanAssemblies(typeof(Program).Assembly));

Utiliser Fluxor dans un composant

@inherits FluxorComponent
@inject IState<CounterState> CounterState
@inject IDispatcher Dispatcher

<p>Count: @CounterState.Value.Count</p>
<button @onclick="Increment">+1</button>
<button @onclick="() => Dispatcher.Dispatch(new IncrementAction(5))">+5</button>

@code {
    private void Increment()
        => Dispatcher.Dispatch(new IncrementAction(1));
}

Persistance dans LocalStorage

@inject IJSRuntime JS

@code {
    private async Task SaveStateAsync()
    {
        var json = JsonSerializer.Serialize(_state);
        await JS.InvokeVoidAsync("localStorage.setItem", "appState", json);
    }

    protected override async Task OnInitializedAsync()
    {
        var json = await JS.InvokeAsync<string?>("localStorage.getItem", "appState");
        if (json is not null)
            _state = JsonSerializer.Deserialize<AppState>(json) ?? new();
    }

    private AppState _state = new();
}

Package Blazored.LocalStorage

// dotnet add package Blazored.LocalStorage
builder.Services.AddBlazoredLocalStorage();

@inject ILocalStorageService LocalStorage

@code {
    protected override async Task OnInitializedAsync()
    {
        _cart = await LocalStorage.GetItemAsync<List<CartItem>>("cart") ?? new();
    }

    private async Task SaveCartAsync()
    {
        await LocalStorage.SetItemAsync("cart", _cart);
    }

    private List<CartItem> _cart = new();
}

État dans l’URL avec NavigationManager

@inject NavigationManager Nav
@page "/products"

@code {
    [SupplyParameterFromQuery] public string? Search { get; set; }
    [SupplyParameterFromQuery] public int Page { get; set; } = 1;

    private void UpdateSearch(string s)
    {
        Nav.NavigateTo(
            Nav.GetUriWithQueryParameters(
                new Dictionary<string, object?>
                {
                    [nameof(Search)] = s,
                    [nameof(Page)]   = 1
                }));
    }
}

Cas réel : état d’un panier

public class CartState
{
    private readonly ILocalStorageService _storage;
    private List<CartItem> _items = new();
    public event Action? OnChanged;

    public CartState(ILocalStorageService storage) => _storage = storage;

    public async Task InitAsync()
    {
        _items = await _storage.GetItemAsync<List<CartItem>>("cart") ?? new();
    }

    public async Task AddAsync(Product p)
    {
        var item = _items.FirstOrDefault(i => i.ProductId == p.Id);
        if (item is not null) item.Quantity++;
        else _items.Add(new CartItem { ProductId = p.Id, Name = p.Name });
        await _storage.SetItemAsync("cart", _items);
        OnChanged?.Invoke();
    }
}

Vérification rapide

Récapitulatif : gestion de l’état dans Blazor