Fetch And Send Data

@page "/products/{CategoryId:int}"
@implements IAsyncDisposable
@inject ProductService ProductService
@inject ILogger<Products> Logger

@if (error is not null)
{
    <div class="alert alert-danger">
        <p>@error</p>
        <button @onclick="LoadAsync">Retry</button>
    </div>
}
else if (products is null)
{
    <p>Loading…</p>
}
else
{
    @if (isLoading)
    {
        <p><em>Refreshing…</em></p>
    }
    @foreach (var p in products)
    {
        <p>@p.Name — @p.Price.ToString("C")</p>
    }
}

@code {
    [Parameter] public int CategoryId { get; set; }
    [SupplyParameterFromQuery] public string? ViewMode { get; set; } // UI-only

    private CancellationTokenSource? cts;
    private int? loadedCategoryId;
    private List<Product>? products;
    private bool isLoading;
    private string? error;

    protected override async Task OnParametersSetAsync()
    {
        if (CategoryId == loadedCategoryId)
        {
            return; // Only ViewMode changed — no reload
        }

        loadedCategoryId = CategoryId;
        await LoadAsync();
    }

    private async Task LoadAsync()
    {
        if (cts is not null)
        {
            await cts.CancelAsync();
            cts.Dispose();
        }

        cts = new CancellationTokenSource();
        var cancellationToken = cts.Token; // Capture locally before await

        error = null;
        isLoading = true;

        try
        {
            var result = await ProductService.GetByCategoryAsync(CategoryId, cancellationToken);
            products = result;
        }
        catch (OperationCanceledException ex) when (!cancellationToken.IsCancellationRequested)
        {
            Logger.LogWarning(ex, "Timed out loading category {CategoryId}", CategoryId);
            error = "The request timed out. Please try again.";
        }
        finally
        {
            isLoading = false;
        }
    }

    public async ValueTask DisposeAsync()
    {
        if (cts is not null)
        {
            await cts.CancelAsync();
            cts.Dispose();
        }
    }
}

var response = await http.PostAsJsonAsync("products", newProduct);
response.EnsureSuccessStatusCode();

var response = await http.PutAsJsonAsync($"products/{id}", updated);
response.EnsureSuccessStatusCode();

var response = await http.DeleteAsync($"products/{id}");
response.EnsureSuccessStatusCode();

public abstract class ProductServiceBase
{
    public abstract Task<Product[]> GetAllAsync(CancellationToken ct = default);
}

// Server — direct database access
public class ServerProductService(AppDbContext db) : ProductServiceBase
{
    public override async Task<Product[]> GetAllAsync(CancellationToken ct = default) =>
        await db.Products.ToArrayAsync(ct);
}

// Client — calls API
public class ClientProductService(HttpClient http) : ProductServiceBase
{
    public override async Task<Product[]> GetAllAsync(CancellationToken ct = default) =>
        await http.GetFromJsonAsync<Product[]>("api/products", ct) ?? [];
}