# 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：
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