コンポーネント間通信とDI

コンポーネント間の通信パターン

親から子：Parameters

@* Child: Alert.razor *@
<div class="alert alert-@Type">
    @Message
</div>

@code {
    [Parameter] public string Type { get; set; } = "info"; // primary, warning, danger
    [Parameter, EditorRequired] public string Message { get; set; } = "";
}

@* Parent usage: *@
<Alert Type="danger" Message="Something went wrong!" />
<Alert Message="Record saved." />

子から親：EventCallback

@* Child: QuantityPicker.razor *@
<button @onclick="() => OnChanged.InvokeAsync(Value - 1)">-</button>
<span>@Value</span>
<button @onclick="() => OnChanged.InvokeAsync(Value + 1)">+</button>

@code {
    [Parameter] public int Value { get; set; } = 1;
    [Parameter] public EventCallback<int> OnChanged { get; set; }
}

@* Parent: *@
<QuantityPicker Value="@_qty" OnChanged="q => _qty = q" />
<p>Quantity: @_qty</p>

親を介した兄弟コンポーネント間の通信

@* Parent holds shared state *@
<CategoryFilter OnChanged="c => _category = c" />
<ProductList Category="_category" />

@code {
    private string _category = "All";
}

@* CategoryFilter raises an event -> Parent updates _category
   -> ProductList re-renders with new Category *@

カスケードパラメーター

@* Wrap a subtree *@
<CascadingValue Value="_theme">
    <MainLayout />
</CascadingValue>

@code { private Theme _theme = new Theme { PrimaryColor = "#6366f1" }; }

@* Any descendant can receive it: *@
@code {
    [CascadingParameter]
    public Theme AppTheme { get; set; } = default!;
}

<h1 style="color: @AppTheme.PrimaryColor">Hello</h1>

@inject によるサービスの注入

@inject IProductService ProductSvc
@inject ILogger<ProductList> Logger
@inject NavigationManager Nav

@code {
    private List<Product> _products = new();

    protected override async Task OnInitializedAsync()
    {
        Logger.LogInformation("Loading products");
        _products = await ProductSvc.GetAllAsync();
    }

    private void GoToProduct(int id)
        => Nav.NavigateTo($"/products/{id}");
}

Blazor Server の Scoped サービス

// Blazor Server: scoped = per circuit (connection lifetime)
builder.Services.AddScoped<CartService>();
// All components in the same browser tab share the SAME CartService

// This is GOOD for cart state (user's items persist across pages)
// This is BAD for request-scoped data — treat as session-like scope

// For true per-render-cycle scope, create your own:
using var scope = ScopeFactory.CreateScope();
var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();

ルーティングのための NavigationManager

@inject NavigationManager Nav

@code {
    private void GoHome()         => Nav.NavigateTo("/");
    private void GoToProduct(int id) => Nav.NavigateTo($"/products/{id}");
    private void ForceReload()    => Nav.NavigateTo("/", forceLoad: true);

    protected override void OnInitialized()
    {
        // Read current URL
        var uri = Nav.Uri;
        var path = new Uri(uri).AbsolutePath;
    }
}

Blazor WASM での HttpClient

// Program.cs (Blazor WASM)
builder.Services.AddScoped(sp => new HttpClient
{
    BaseAddress = new Uri(builder.HostEnvironment.BaseAddress)
});

// Component:
@inject HttpClient Http

@code {
    private List<Product> _products = new();

    protected override async Task OnInitializedAsync()
    {
        _products = await Http.GetFromJsonAsync<List<Product>>("/api/products")
                   ?? new();
    }
}

実践例：ユーザーコンテキストサービス

public class UserContextService
{
    public string? Name { get; private set; }
    public bool IsAdmin { get; private set; }

    public async Task InitializeAsync(AuthenticationState auth)
    {
        var user = auth.User;
        Name = user.Identity?.Name;
        IsAdmin = user.IsInRole("Admin");
    }
}

// Registered as scoped:
builder.Services.AddScoped<UserContextService>();

// Used in any component:
@inject UserContextService UserCtx
<p>Hello, @UserCtx.Name @(UserCtx.IsAdmin ? "(Admin)" : "")</p>

クイックチェック

まとめ：コンポーネント間通信と DI