Blazor WebAssembly 提供了一个缓存管理 API，可以帮助我们进行离线模式下的静态文件下载。首先，我们需要在 Program.cs 文件中启用离线模式，并为缓存管理器提供一个有意义的名称：

using System.Net.Http;
using Microsoft.AspNetCore.Components.WebAssembly.Hosting;
using Microsoft.Extensions.Caching.Memory;

public class Program
{
    public static async Task Main(string[] args)
    {
        var builder = WebAssemblyHostBuilder.CreateDefault(args);

        // 启用离线模式
        builder.Services.AddTransient(sp => new HttpClient { BaseAddress = new Uri(builder.HostEnvironment.BaseAddress) });
        builder.RootComponents.Add("app");
        builder.Services.AddMemoryCache();
        builder.Services.AddHttpClient("offline-cache")
            .ConfigureHttpClient(client => client.BaseAddress = new Uri(builder.HostEnvironment.BaseAddress))
            .ConfigurePrimaryHttpMessageHandler(() => new HttpClientHandler { AutomaticDecompression = DecompressionMethods.GZip })
            .AddHttpMessageHandler();

        await builder.Build().RunAsync();
    }
}

public class CacheHandler : DelegatingHandler
{
    private const string CacheHeaderName = "X-Blazor-Cache";

    private readonly IMemoryCache cache;

    public CacheHandler(IMemoryCache cache)
    {
        this.cache = cache;
    }

    protected override async Task SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // 检查是否离线模式
        if (!request.Headers.TryGetValues(CacheHeaderName, out var cacheHeaders))
        {
            return await base.SendAsync(request, cancellationToken);
        }

        var cacheKey = request.RequestUri.PathAndQuery;
        if (cache.TryGetValue(cacheKey, out var cachedResponse))
        {
            return (HttpResponseMessage)cachedResponse;
        }

        var result = await base.SendAsync(request, cancellationToken);
        if (result.IsSuccessStatusCode)
        {
            var cacheControl = cacheHeaders.SelectMany(x => x.Split(',', StringSplitOptions.RemoveEmptyEntries))
                .Select(x => x.Trim().ToLowerInvariant())
                .FirstOrDefault(x => x.StartsWith("max-age="));

            if (int.TryParse(cacheControl?.Substring("max-age