深入理解net core中的依赖注入、Singleton、Scoped、Transient（四）

1.1依赖

1.2 什么是注入

注入体现的是一个IOC（控制反转的的思想）。在反转之前 ，我们先看看正转。

AccountController自己来实例化需要的依赖。

private ILoginService<ApplicationUser> _loginService;
public AccountController()
{
  _loginService = new EFLoginService()
}

大师说，这样不好。你不应该自己创建它，而是应该由你的调用者给你。于是你通过构造函数让外界把这两个依赖传给你。

 public 
 AccountController(ILoginService<ApplicationUser> loginService)
{
  _loginService = loginService;
}

把依赖的创建丢给其它人，自己只负责使用，其它人丢给你依赖的这个过程理解为注入。

1.3 为什么要反转？

为了在业务变化的时候尽少改动代码可能造成的问题。

比如我们现在要把从EF中去验证登录改为从Redis去读，于是我们加了一个 RedisLoginService。这个时候我们只需要在原来注入的地方改一下就可以了。

 var controller = new AccountController(new EFLoginService());
controller.Login(userName, password);

// 用Redis来替换原来的EF登录

var controller = new AccountController(new RedisLoginService());
controller.Login(userName, password);

var serviceCollection = new ServiceCollection()
  .AddTransient<ILoginService, EFLoginService>()
  .AddSingleton<ILoginService, EFLoginService>()
  .AddScoped<ILoginService, EFLoginService>();

public interface IServiceCollection : IList<ServiceDescriptor>
{
}

private static IServiceCollection Add(
  IServiceCollection collection,
  Type serviceType,
  Type implementationType,
  ServiceLifetime lifetime)
{
  var descriptor = 
  new ServiceDescriptor(serviceType, implementationType, lifetime);
  collection.Add(descriptor);
  return collection;
}

public enum ServiceLifetime
{
  Singleton,
  Scoped,
  Transient
}

public interface IOperation
{
        Guid OperationId { get; }
}
public interface IOperationSingleton : IOperation { }
public interface IOperationTransient : IOperation{}
public interface IOperationScoped : IOperation{}

public class Operation : 
  IOperationSingleton,
  IOperationTransient,
  IOperationScoped
{
    private Guid _guid;

    public Operation() {
        _guid = Guid.NewGuid();
    }

    public Operation(Guid guid)
    {
        _guid = guid;
    }

    public Guid OperationId => _guid;
}

在程序内我们可以多次调用ServiceProvider的GetService方法，获取到的都是同一个实例。

var services = new ServiceCollection();
// 默认构造
services.AddSingleton<IOperationSingleton, Operation>();
// 自定义传入Guid空值
services.AddSingleton<IOperationSingleton>(
  new Operation(Guid.Empty));
// 自定义传入一个New的Guid
services.AddSingleton <IOperationSingleton>(
  new Operation(Guid.NewGuid()));

var provider = services.BuildServiceProvider();

// 输出singletone1的Guid
var singletone1 = provider.GetService<IOperationSingleton>();
Console.WriteLine($"signletone1: {singletone1.OperationId}");

// 输出singletone2的Guid
var singletone2 = provider.GetService<IOperationSingleton>();
Console.WriteLine($"signletone2: {singletone2.OperationId}");
Console.WriteLine($"singletone1 == singletone2 ? : { singletone1 == singletone2 }");

我们对IOperationSingleton注册了三次，最后获取两次，大家要注意到我们获取到的始终都是我们最后一次注册的那个给了一个Guid的实例，前面的会被覆盖。

2.3 实例生命周期之Tranisent 

这次我们获取到的IOperationTransient为两个不同的实例。

var services = new ServiceCollection();
services.AddTransient<IOperationTransient, Operation>();
    
var provider = services.BuildServiceProvider();

var transient1 = provider.GetService<IOperationTransient>();
Console.WriteLine($"transient1: {transient1.OperationId}");

var transient2 = provider.GetService<IOperationTransient>();
Console.WriteLine($"transient2: {transient2.OperationId}");
Console.WriteLine($"transient1 == transient2 ? : 
  { transient1 == transient2 }");

 var services = new ServiceCollection()
.AddScoped<IOperationScoped, Operation>()
.AddTransient<IOperationTransient, Operation>()
.AddSingleton<IOperationSingleton, Operation>();

var provider = services.BuildServiceProvider();
using (var scope1 = provider.CreateScope())
{
    var p = scope1.ServiceProvider;

    var scopeobj1 = p.GetService<IOperationScoped>();
    var transient1 = p.GetService<IOperationTransient>();
    var singleton1 = p.GetService<IOperationSingleton>();

    var scopeobj2 = p.GetService<IOperationScoped>();
    var transient2 = p.GetService<IOperationTransient>();
    var singleton2 = p.GetService<IOperationSingleton>();

    Console.WriteLine(
        $"scope1: { scopeobj1.OperationId }," +
        $"transient1: {transient1.OperationId}, " +
        $"singleton1: {singleton1.OperationId}");

    Console.WriteLine($"scope2: { scopeobj2.OperationId }, " +
        $"transient2: {transient2.OperationId}, " +
        $"singleton2: {singleton2.OperationId}");
}

接下来

scope1: 200d1e63-d024-4cd3-88c9-35fdf5c00956, 
transient1: fb35f570-713e-43fc-854c-972eed2fae52, 
singleton1: da6cf60f-670a-4a86-8fd6-01b635f74225

scope2: 200d1e63-d024-4cd3-88c9-35fdf5c00956, 
transient2: 2766a1ee-766f-4116-8a48-3e569de54259, 
singleton2: da6cf60f-670a-4a86-8fd6-01b635f74225

如果再创建一个新的Scope运行，

scope1: 29f127a7-baf5-4ab0-b264-fcced11d0729, 
transient1: 035d8bfc-c516-44a7-94a5-3720bd39ce57, 
singleton1: da6cf60f-670a-4a86-8fd6-01b635f74225

scope2: 29f127a7-baf5-4ab0-b264-fcced11d0729, 
transient2: 74c37151-6497-4223-b558-a4ffc1897d57, 
singleton2: da6cf60f-670a-4a86-8fd6-01b635f74225

三、DI在ASP.NET Core中的应用

3.1在Startup类中初始化

public void ConfigureServices(IServiceCollection services)
{
    services.AddTransient<ILoginService<ApplicationUser>, 
      EFLoginService>();
    services.AddMvc();
)

public static IMvcBuilder AddMvc(this IServiceCollection services)
{
    if (services == null)
    {
        throw new ArgumentNullException(nameof(services));
    }

    var builder = services.AddMvcCore();

    builder.AddApiExplorer();
    builder.AddAuthorization();
    AddDefaultFrameworkParts(builder.PartManager);
    ...
}

 private ILoginService<ApplicationUser> _loginService;
public AccountController(
  ILoginService<ApplicationUser> loginService)
{
  _loginService = loginService;
}

@using MilkStone.Services;
@model MilkStone.Models.AccountViewModel.LoginViewModel
@inject ILoginService<ApplicationUser>  loginService
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head></head>
<body>
  @loginService.GetUserName()
</body>
</html>

HttpContext.RequestServices.GetService<ILoginService<ApplicationUser>>();

builder.RegisterGeneric(typeof(LoggingBehavior<,>)).As(typeof(IPipelineBehavior<,>));
 
builder.RegisterGeneric(typeof(ValidatorBehavior<,>)).As(typeof(IPipelineBehavior<,>));

public IServiceProvider ConfigureServices(
  IServiceCollection services){
    services.AddMvc();
    // Add other framework services

    // Add Autofac
    var containerBuilder = new ContainerBuilder();
    containerBuilder.RegisterModule<DefaultModule>();
    containerBuilder.Populate(services);
    var container = containerBuilder.Build();
    return new AutofacServiceProvider(container);
}

4.1 有何变化 

其中很大的一个变化在于，Autofac 原来的一个生命周期InstancePerRequest，将不再有效。正如我们前面所说的，整个request的生命周期被ASP.NET Core管理了，所以Autofac的这个将不再有效。我们可以使用 InstancePerLifetimeScope ，同样是有用的，对应了我们ASP.NET Core DI 里面的Scoped。