.NET Core 3.0之深入源码理解HttpClientFactory(二)
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写在前面
上一篇文章讨论了通过在ConfigureServices中调用services.AddHttpClient()方法，并基于此进一步探讨了DefaultHttpClientFactory是如何创建HttpClient实例和HttpMessageHandler实例的，并了解了DefaultHttpClientFactory内部维护者一个定时器和两个HttpMessageHandler对象集合，以定期清理无效的 HttpMessageHandler对象，详细的内容可以点击链接跳转，接下来我会接着前一篇文章继续展开相关讨论。
详细介绍
HttpMessageHandlerBuilder
该类是一个抽象类，起到生成器的作用，可用于用于配置HttpMessageHandler实例。HttpMessageHandlerBuilder会在ServiceCollection中被注册为Transient服务。调用方要为每个要创建的HttpMessageHandler实例检索一个新实例。实现者应该确保每个实例都只使用一次。
HttpMessageHandlerBuilder里面有三个比较重要的属性：
1: /// <summary>
2: /// 主HttpMessageHandler实例
3: /// </summary>
4: public abstract HttpMessageHandler PrimaryHandler { get; set; }
5:
6: /// <summary>
7: /// 这个是一个附加实例，用于配置HttpClient管道
8: /// </summary>
9: public abstract IList<DelegatingHandler> AdditionalHandlers { get; }
10:
11: /// <summary>
12: /// 可用于从依赖项注入容器解析服务的IServiceProvider
13: /// </summary>
14: public virtual IServiceProvider Services { get; }
<style></style>
这三个属性意味着每个HttpMessageHandlerBuilder都需要维护自身的HttpMessageHandler实例和管道。
其内部还有一个抽象方法：
1: public abstract HttpMessageHandler Build();<style></style>
当然，内部最核心的方法就是管道的创建过程了，需要传入主派生类自身的HttpMessageHandler和管道列表对象。它会将primaryHandler实例付给管道列表的第一个Item的InnerHandler，其他对象会依此后移，这也为我们自定义HttpMessageHandler(各种中间件)提供了无限可能。
相关实现如下：
1: var next = primaryHandler;
2: for (var i = additionalHandlersList.Count - 1; i >= 0; i--)
3: {
4: var handler = additionalHandlersList[i];
5: if (handler == null)
6: {
7: var message = Resources.FormatHttpMessageHandlerBuilder_AdditionalHandlerIsNull(nameof(additionalHandlers));
8: throw new InvalidOperationException(message);
9: }
10:
11: if (handler.InnerHandler != null)
12: {
13: var message = Resources.FormatHttpMessageHandlerBuilder_AdditionHandlerIsInvalid(
14: nameof(DelegatingHandler.InnerHandler),
15: nameof(DelegatingHandler),
16: nameof(HttpMessageHandlerBuilder),
17: Environment.NewLine,
18: handler);
19: throw new InvalidOperationException(message);
20: }
21:
22: handler.InnerHandler = next;
23: next = handler;
24: }
接下来我们看一下HttpMessageHandlerBuilder一个派生类DefaultHttpMessageHandlerBuilder，其构造函数会传入IServiceProvider实例，我们的自定义操作也可以参照这个类。
关于Build方法的实现如下，比较简单主要是调用了CreateHandlerPipeline方法：
1: public override HttpMessageHandler Build()
2: {
3: if (PrimaryHandler == null)
4: {
5: var message = Resources.FormatHttpMessageHandlerBuilder_PrimaryHandlerIsNull(nameof(PrimaryHandler));
6: throw new InvalidOperationException(message);
7: }
8:
9: return CreateHandlerPipeline(PrimaryHandler, AdditionalHandlers);
10: }<style></style>
ITypedHttpClientFactory
这是一个抽象工厂，该组件可以使用给定逻辑名称的自定义配置创建类型化HttpClient实例，与命名方式创建HttpClient具有相同的的功能。类型化客户端可能用于单个后端终结点，并封装此终结点的所有处理逻辑。 另一个优势是它们使用 DI 被注入到应用中需要的位置，下一篇文章会再次讨论相关功能。
我们首先看一下调用方式：
1: public static IHttpClientBuilder AddHttpClient<TClient>(this IServiceCollection services)
2: where TClient : class
3: {
4: if (services == null)
5: {
6: throw new ArgumentNullException(nameof(services));
7: }
8:
9: AddHttpClient(services);
10:
11: var name = TypeNameHelper.GetTypeDisplayName(typeof(TClient), fullName: false);
12: var builder = new DefaultHttpClientBuilder(services, name);
13: builder.AddTypedClient<TClient>();
14: return builder;
15: }<style></style>
可以看出此处的调用与普通的HttpClient没有什么太大区别，只是增加了一个泛型标记，而且该类型没有特殊的要求，只要是个类就行。其内部依然调用AddHttpClient(services），但它调用了另一个扩展方法，如下所示：
1: public static IHttpClientBuilder AddTypedClient<TClient>(this IHttpClientBuilder builder)
2: where TClient : class
3: {
4: if (builder == null)
5: {
6: throw new ArgumentNullException(nameof(builder));
7: }
8:
9: builder.Services.AddTransient<TClient>(s =>
10: {
11: var httpClientFactory = s.GetRequiredService<IHttpClientFactory>();
12: var httpClient = httpClientFactory.CreateClient(builder.Name);
13:
14: var typedClientFactory = s.GetRequiredService<ITypedHttpClientFactory<TClient>>();
15: return typedClientFactory.CreateClient(httpClient);
16: });
17:
18: return builder;
19: }<style></style>
可以看到最终的代码调用了ITypedHttpClientFactory的CreateClient方法，Microsoft.Extensions.Http包中有一个默认的ITypedHttpClientFactory派生类，DefaultTypedHttpClientFactory<TClient>，该类提供了了构造函数用于接收IServiceProvider实例，以及一个内部类声明的缓存对象，该对象十分重要，它被注册为singleton类型，已达到全局使用，并可以充当相关实例激活时的对象池。它也允许它的外部类注册为transient，这样它就不会在应用根服务提供程序上被关掉了。
相关代码如下：
1: public TClient CreateClient(HttpClient httpClient)
2: {
3: if (httpClient == null)
4: {
5: throw new ArgumentNullException(nameof(httpClient));
6: }
7:
8: return (TClient)_cache.Activator(_services, new object[] { httpClient });
9: }
内部缓存对象：
1: public class Cache
2: {
3: private readonly static Func<ObjectFactory> _createActivator = () => ActivatorUtilities.CreateFactory(typeof(TClient), new Type[] { typeof(HttpClient), });
4:
5: private ObjectFactory _activator;
6: private bool _initialized;
7: private object _lock;
8:
9: public ObjectFactory Activator => LazyInitializer.EnsureInitialized(
10: ref _activator,
11: ref _initialized,
12: ref _lock,
13: _createActivator);
14: }<style></style>
最后我们看一下源码中提供的范例：
1: class ExampleClient
2: {
3: private readonly HttpClient _httpClient;
4: private readonly ILogger _logger;
5: // typed clients can use constructor injection to access additional services
6: public ExampleClient(HttpClient httpClient, ILogger<ExampleClient> logger)
7: {
8: _httpClient = httpClient;
9: _logger = logger;
10: }
11: // typed clients can expose the HttpClient for application code to call directly
12: public HttpClient HttpClient => _httpClient;
13: // typed clients can also define methods that abstract usage of the HttpClient
14: public async Task SendHelloRequest()
15: {
16: var response = await _httpClient.GetAsync("/helloworld");
17: response.EnsureSuccessStatusCode();
18: }
19: }
20: //This sample shows how to consume a typed client from an ASP.NET Core middleware.
21: public void Configure(IApplicationBuilder app, ExampleClient exampleClient)
22: {
23: app.Run(async (context) =>
24: {
25: var response = await _exampleClient.GetAsync("/helloworld");
26: await context.Response.WriteAsync("Remote server said: ");
27: await response.Content.CopyToAsync(context.Response.Body);
28: });
29: }
30: //This sample shows how to consume a typed client from an ASP.NET Core MVC Controller.
31: public class HomeController : ControllerBase(IApplicationBuilder app, ExampleClient exampleClient)
32: {
33: private readonly ExampleClient _exampleClient;
34: public HomeController(ExampleClient exampleClient)
35: {
36: _exampleClient = exampleClient;
37: }
38: public async Task<IActionResult> Index()
39: {
40: var response = await _exampleClient.GetAsync("/helloworld");
41: var text = await response.Content.ReadAsStringAsync();
42: return Content("Remote server said: " + text, "text/plain");
43: };
44: }<style></style>
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