c# - 扩展 HttpClient

Question

我正在尝试创建一个扩展 httpclient 但仍然是抽象的类。

public abstract class Repository : HttpClient
{
    public override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        //retry logic here
        var response = await base.SendAsync(request, cancellationToken);
        while (!IsValid(response))
        {
            Attempts++;
            response = await Retry(request, cancellationToken);
        }
        Attempts = 0;
        return response;
    }
    public int Attempts { get; set; }
    public abstract bool IsValid(HttpResponseMessage response);
    public abstract Task<HttpResponseMessage> Retry(HttpRequestMessage request, CancellationToken cancellationToken);
    public abstract Type GetPageForUri(Uri uri);
    public abstract Task<object> GetItems(Uri uri);
}

但是当子类调用 getStringAsync 时它不会调用 Repository 中的 SendAsync

public class EmailClient : Repository
{
    public override bool IsValid(HttpResponseMessage response)
    {
        return response.IsSuccessStatusCode;
    }
    public override Task<HttpResponseMessage> Retry(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        return base.SendAsync(request, cancellationToken);
    }
    public override Type GetPageForUri(Uri uri)
    {
        return typeof(EmailPage);
    }
    public override async Task<object> GetItems(Uri uri)
    {
        var emails = await GetStringAsync(uri);
        return emails;
    }
}

我做错了什么？我应该制作一个 HttpClientHandler 并将其包含在 HttpClient 中吗？如果有任何不同，我会尝试阅读有关虚拟的内容。但我不明白为什么它没有被调用。谁能帮帮我？

Answer 1

HttpClient 是围绕 HttpMessageHandler 实现(通常是 HttpClientHandler，大多数运行时实现都支持的特性的属性)的薄包装器

实际上有一个 HttpMessageHandler 基类用于装饰器，例如您的重试场景 DelegatingHandler，您可以像这样实现它:

类似于:

public class RetryMessageHandler : DelegatingHandler
{
    readonly int retryCount;

    public RetryMessageHandler(int retryCount)
        : this(new HttpClientHandler(), retryCount)
    {
    }

    public RetryMessageHandler(HttpMessageHandler innerHandler, int retryCount)
        : base(innerHandler)
    {
        this.retryCount = retryCount;
    }

    public override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        HttpResponseMessage response = null;

        for (int i = 0; i<=retryCount; i++)
        {
            response = await base.SendAsync(request, cancellationToken);

            if (response.IsSuccessStatusCode)
            {
                return response;
            }
        }

        return response;
    }
}

然后你会像这样使用它:

HttpClient httpClient = HttpClientFactory.CreatePipeline(new HttpClientHandler(), 
    new [] { new RetryMessageHandler(3) });

var response = await httpClient.SendAsync(request);

此外，我建议您的 EmailClient 包装/依赖于 HttpClient，而不是扩展它。 (有关原因的更多信息，请谷歌“组合与继承”)