# 第3章：依赖注入 #### 任务16：介绍 1、依赖注入概念详解 * 从UML和软件建模来理解 * 从单元测试来理解 2、ASP.NET Core 源码解析 #### 任务17：从UML角度来理解依赖 **1、什么是依赖** 当一个类A完成某个任务需要另一个类B来帮助时，A就对B产生了依赖例如CustomerController需要对customer进行新增或查找时用到EF,则对EF的Context产生了依赖 ``` var context = new CustomerContext(new DbContextOptions{}); ``` **2、显示依赖与隐式依赖** 显示依赖：把一个类用到的所有外部组件放到一个类最上面，在构造函数里面初始化 ``` private CustomerContext _context; public CustomerController() { _context = new CustomerContext(new DbContextOptions{}); } ``` 隐式依赖：需要用到的地方再初始化，不推荐 ``` var context = new CustomerContext(new DbContextOptions{}); ``` **3、依赖倒置原则** 依赖高层业务，不依赖低层业务的具体实现，而依赖于具体的抽象 CustomerController是高层业务的一个组件，依赖于CustomerContext是一个低层数据库的实现，如果现在需要把EF换成一个内存的实现或者mysql，需要修改CustomerController类，风险很大，所以应该依赖于低层业务的抽象把低层业务方法抽象，比如查找，新增，抽象出一个接口，当不需要使用EF的时候，使用内存的实现替换 ``` private ICustomerRepository _customerRepository; public CustomerController() { _customerRepository = new EfCustomerRepository( new CustomerContext(new DbContextOptions{})); } ``` #### 任务18：控制反转实现依赖注入的方式不由自己决定，而是交给一个IOC容器，需要什么由容器传入，比如生产环境需要使用EF，则由容器传入一个EfCustomerRepository，而测试环境需要使用内存级别的，则传入一个MemoryCustomerRepository ``` private ICustomerRepository _customerRepository; public CustomerController(ICustomerRepository customerRepository) { _customerRepository = customerRepository; } ``` #### 任务19：单元测试 ``` var repository = new Data.MemoryCustomerRepository(); var controller = new CustomerController(repository);// 通过外部控制Controller里面的依赖 var customer = new Model.Customer() { FirstName = "Mingson", LastName = "Zheng", Phone = "123456789", }; var result = controller.Add(customer); Assert.IsType(result);// 正确结果 var resultBad = controller.Add(customer); Assert.IsType(resultBad);// 错误结果 ``` 通过单元测试可以得知修改Bug过程中是否误删代码，导致原来通过的测试现在无法通过。 #### 任务20：DI初始化的源码解读 Microsoft.AspNetCore.Hosting.WebHostBuilder ``` ///

/// Builds the required services and an which hosts a web application. ///

public IWebHost Build() { ...... // 第一步，build IServiceCollection serviceCollection1 = this.BuildCommonServices(out hostingStartupErrors); // 第二步，获取ServiceCollection,ServiceProvider IServiceCollection serviceCollection2 = serviceCollection1.Clone(); IServiceProvider providerFromFactory = GetProviderFromFactory(serviceCollection1); ...... // 第三步，new一个WebHost，传入ServiceCollection，ServiceProvider WebHost webHost = new WebHost(serviceCollection2, providerFromFactory, this._options, this._config, hostingStartupErrors); ...... // 第四步，webHost初始化方法Initialize webHost.Initialize(); ...... } ``` 第一步BuildCommonServices中new一个ServiceCollection就是在startup接口中使用 ``` private IServiceCollection BuildCommonServices( out AggregateException hostingStartupErrors) { ...... ServiceCollection services = new ServiceCollection(); // new完之后添加一些初始化操作 ...... return (IServiceCollection) services; } ``` IStartup接口 ``` namespace Microsoft.AspNetCore.Hosting { public interface IStartup { IServiceProvider ConfigureServices(IServiceCollection services); void Configure(IApplicationBuilder app);// 配置管道 } } ``` 第四步，webHost初始化方法Initialize ``` public void Initialize() { ...... this.EnsureApplicationServices(); ...... } private void EnsureApplicationServices() { ...... this.EnsureStartup(); this._applicationServices = this._startup.ConfigureServices(this._applicationServiceCollection); } private void EnsureStartup() { if (this._startup != null) return; this._startup = this._hostingServiceProvider.GetService(); if (this._startup == null) throw new InvalidOperationException(string.Format("No startup configured. Please specify startup via WebHostBuilder.UseStartup, WebHostBuilder.Configure, injecting {0} or specifying the startup assembly via {1} in the web host configuration.", (object) "IStartup", (object) "StartupAssemblyKey")); } ``` #### 任务21：依赖注入的使用了解ASP.NET Core 依赖注入，看这篇就够了：