具有两个依赖服务的循环依赖

Question

我是 C# 和依赖注入的新手。目前我正在开展一个新项目，并希望在技术方面向前迈出一步。

在这个项目中，我遇到了三种导致循环依赖的情况。

我已经阅读了很多这方面的内容，并找到了像 Lazy<T> 和 IServiceProvider 这样的解决方案，但我想学习一个干净的解决方案来解决这个问题，并希望遵循最常见的建议来重构代码。

在这个例子中我们有四个服务：

AccountService -> 登录、注销等

HttpService -> 做 API-Stuff

LogService -> 做一些日志记录

LogRepository -> 用于 EF 的日志记录表/包装器的 CRUD

AccountService 使用HttpService 通过 API 进行身份验证。稍后，我想使用HttpService 通过 API 获取更多数据。 HttpService 现在需要AccountService 来获取用于验证请求的令牌。这会导致循环依赖错误。

账户服务

public interface IAccountService
{
    Identity Identity { get; }
    Task Login(Credentials Credentials);
    Task Logout();
}

public class AccountService : IAccountService
{
    public Identity Identity { get; private set; }
    
    private readonly IHttpService _httpService;
    private readonly ILogService _logService;
    
    public AccountService(
        IHttpService HttpService, ILogService LogService)
    {
        _httpService = HttpService;
        _logService = LogService;
    }

    public async Task Login(Credentials Credentials)
    {
        Identity = await _httpService.Post<Identity>(
            "api/rest/v1/user/authenticate", Credentials);
    }
}

HttpService

public interface IHttpService
{
    Task<T> Get<T>(string uri);
    Task Post(string uri, object value);
    Task<T> Post<T>(string uri, object value);
}

public class HttpService : IHttpService
{
    private readonly HttpClient _httpClient;
    private readonly IAccountService _accountService;
    private readonly ILogService _logService; 

    public HttpService(
        HttpClient HttpClient,
        IAccountService AccountService,
        ILogService ILogService)
    {
        _httpClient = HttpClient;
        _accountService = AccountService;
        _logService = LogService;
    }

    private async Task AddAuthentication(HttpRequestMessage Request)
    {
        Request.Headers.Authorization = new AuthenticationHeaderValue(
            "bearer", _accountService.Identity.SystemToken);
    }
}

解决或正确重新设计此问题的最佳做法是什么？

我有更多的循环依赖，例如在LogRepository 中使用LogService 或在HttpService 中使用LogService（因为HttpService 将日志条目发送到服务器）。

非常感谢您的帮助！

Answer 1

虽然你的对象图是循环的（AccountService -> HttpService -> AccountService）你的调用图不是。调用可能如下：

AccountService.Login
    -> HttpService.Post
        -> HttpService.AddAuthentication
            -> AccountService.Identity

循环对象图和非循环调用图经常发生在违反Single Responsibly Principle的组件上。功能（方法）类获得的越多，它们的对象图变得循环的机会就越大。将类拆分成更小、更集中的部分，不仅可以解决循环依赖问题，而且通常还可以改进应用程序的设计。

我认为您的情况实际上与我在section 6.3 的DIPP&P 中讨论的示例非常相似。该部分专门讨论了修复循环依赖关系。

长话短说，我认为最好的办法是将AccountService 拆分为（至少）两个服务：

• 一项负责登录和注销的服务
• 负责获取用户身份的第二个服务。

这两个服务都有自己的接口，与IAccountService 相比，这些新接口现在更宽。这可以提高您遵守Interface Segregation Principle 的机会。

下面是一个例子：

让我们从新的接口定义开始：

// Contains Login and Logout methods of old IAccountService
public interface IAuthenticationService
{
    Task Login(Credentials Credentials);
    Task Logout();
}

// Contains Identity property of old IAccountService
public interface IIdentityProvider
{
    // For simplicity I added a setter to the interface, because that keeps
    // the example simple, but it is possible to keep Identity read-only if
    // required.
    Identity Identity { get; set; }
}

// This interface is kept unchanged.
public interface IHttpService
{
    Task<T> Get<T>(string uri);
    Task Post(string uri, object value);
    Task<T> Post<T>(string uri, object value);
}

接下来让我们看看实现，从IAuthenticationService 实现开始：

// Old AccountService, now depending on IIdentityProvider
public class AuthenticationService : IAuthenticationService
{
    private readonly IHttpService _httpService;
    private readonly ILogService _logService;
    private readonly IIdentityProvider _identityProvider;
    
    public AccountService(
        IHttpService HttpService,
        ILogService LogService,
        IIdentityProvider IdentityProvider)
    {
        _httpService = HttpService;
        _logService = LogService;
        _identityProvider = IdentityProvider;
    }

    public async Task Login(Credentials Credentials)
    {
        _identityProvider.Identity = await _httpService.Post<Identity>(
            "api/rest/v1/user/authenticate", Credentials);
    }
}

这个“新的”AuthenticationService 包含AccountService 的部分代码，而旧的AccountService 逻辑的其余部分隐藏在注入AuthenticationService 的新IIdentityProvider 抽象后面。这种重构与Facade Service refactoring 非常相似（有关 Facade Service 重构的详细讨论，请参阅 DIPP&P 的section 6.1）。

IdentityProvider 实现了新的IIdentityProvider 接口并包含来自AccountService 的旧逻辑：

public class IdentityProvider : IIdentityProvider
{
    public Identity Identity { get; set; }
}

最后，HttpService 现在依赖于IIdentityProvider 而不是IAccountService：

// Now depends on IIdentityProvider instead of IAccountService
public class HttpService : IHttpService
{
    private readonly HttpClient _httpClient;
    private readonly IIdentityProvider _identityProvider;
    private readonly ILogService _logService; 

    public HttpService(
        HttpClient HttpClient,
        IIdentityProvider IdentityProvider,
        ILogService ILogService)
    {
        _httpClient = HttpClient;
        _identityProvider = IdentityProvider;
        _logService = LogService;
    }

    private async Task AddAuthentication(HttpRequestMessage Request)
    {
        // Now uses the new IIdentityProvider dependency instead
        // of the old IAccountService, which caused the cycle.
        Request.Headers.Authorization = new AuthenticationHeaderValue(
            "bearer", _identityProvider.Identity.SystemToken);
    }
}

使用这种新设计，对象图不再是循环的，可以如下构造：

var identity = new IdentityProvider();
var logger = new LogService();

new AccountService(
    new HttpService(
        new HttpClient(...),
        identity,
        logger),
    logger,
    identity);