WPF应用中使用IHttpClientFactory创建HttpClient时如何访问ProgressMessageHandler

我原本有一段能正常读取下载进度的HttpClient代码：

var handler = new HttpClientHandler() { AllowAutoRedirect = true };
var ph = new ProgressMessageHandler(handler);

ph.HttpReceiveProgress += (_, args) =>
{
    // 处理下载进度的逻辑
};

var client = new HttpClient(ph);
client.BaseAddress = new Uri("https://localhost:7145/");
HttpRequestMessage requestMessage = new(HttpMethod.Get, "DownloadFile");
requestMessage.Headers.Authorization = new AuthenticationHeaderValue("Bearer", _tokenConfig.AccessToken);
var response = await client.SendAsync(requestMessage);

现在我想移除类内的HttpClient初始化逻辑，改用微软的IHttpClientFactory注入，已经在启动类中完成了如下配置：

services.AddTransient<ProgressMessageHandler>();
services.AddHttpClient("api", opt =>
{
    opt.BaseAddress = new Uri(configuration.GetValue<string>("HttpClientConfig:BaseUrl"));
})
.AddHttpMessageHandler<ProgressMessageHandler>()
.ConfigurePrimaryHttpMessageHandler(() =>
{
    return new HttpClientHandler()
    {
        AllowAutoRedirect = false,
        UseDefaultCredentials = true
    };
});

现在可以通过_httpClientFactory.CreateClient("api")获取HttpClient，但不知道该如何及在何处访问HttpReceiveProgress事件。

方法一：自定义可订阅进度的Handler #

通过创建ProgressMessageHandler的子类，暴露进度事件供外部订阅，是最稳定可靠的方案：

1. 定义自定义Handler类：

public class SubscribableProgressMessageHandler : ProgressMessageHandler
{
    // 暴露自定义的进度事件
    public new event EventHandler<HttpProgressEventArgs>? HttpReceiveProgress;

    public SubscribableProgressMessageHandler(HttpMessageHandler innerHandler) : base(innerHandler)
    {
        // 转发基类事件到自定义事件
        base.HttpReceiveProgress += (sender, args) => HttpReceiveProgress?.Invoke(sender, args);
    }
}

2. 修改DI配置，替换原Handler为自定义类：

services.AddTransient<SubscribableProgressMessageHandler>();
services.AddHttpClient("api", opt =>
{
    opt.BaseAddress = new Uri(configuration.GetValue<string>("HttpClientConfig:BaseUrl"));
})
.AddHttpMessageHandler<SubscribableProgressMessageHandler>()
.ConfigurePrimaryHttpMessageHandler(() =>
{
    return new HttpClientHandler()
    {
        AllowAutoRedirect = false,
        UseDefaultCredentials = true
    };
});

3. 使用时获取Handler并订阅事件：

var client = _httpClientFactory.CreateClient("api");
// 从HttpClient的Handler链中获取自定义进度Handler
if (client.GetTypedClientHandler<SubscribableProgressMessageHandler>() is { } progressHandler)
{
    progressHandler.HttpReceiveProgress += (_, args) =>
    {
        // 处理下载进度逻辑，例如计算百分比
        var progressPercent = args.BytesTransferred * 100 / args.TotalBytes;
        Console.WriteLine($"下载进度：{progressPercent}%");
    };
}

// 发起下载请求
var requestMessage = new HttpRequestMessage(HttpMethod.Get, "DownloadFile");
requestMessage.Headers.Authorization = new AuthenticationHeaderValue("Bearer", _tokenConfig.AccessToken);
var response = await client.SendAsync(requestMessage);

方法二：直接从Handler链中获取原生ProgressMessageHandler #

如果不想自定义Handler，可以通过遍历Handler链找到目标实例，但该方案依赖HttpClient内部实现，版本更新可能失效：

var client = _httpClientFactory.CreateClient("api");
var currentHandler = client.DefaultRequestHeaders.GetType().Assembly
    .GetType("System.Net.Http.HttpClient+HttpHandlerFilter")?
    .GetField("_innerHandler", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)?
    .GetValue(client.DefaultRequestHeaders) as HttpMessageHandler;

// 遍历Handler链查找ProgressMessageHandler
while (currentHandler != null)
{
    if (currentHandler is ProgressMessageHandler progressHandler)
    {
        progressHandler.HttpReceiveProgress += (_, args) =>
        {
            // 处理下载进度逻辑
            Console.WriteLine($"已下载：{args.BytesTransferred} / {args.TotalBytes}");
        };
        break;
    }
    currentHandler = (currentHandler as DelegatingHandler)?.InnerHandler;
}

// 发起下载请求
var requestMessage = new HttpRequestMessage(HttpMethod.Get, "DownloadFile");
requestMessage.Headers.Authorization = new AuthenticationHeaderValue("Bearer", _tokenConfig.AccessToken);
var response = await client.SendAsync(requestMessage);

注意：方法二使用反射，存在版本兼容性风险，优先推荐方法一。

内容的提问来源于stack exchange，提问作者niks