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运用依赖注入

回到 TodoController，添加一些代码以使用 ITodoItemService:

public class TodoController : Controller
{
    private readonly ITodoItemService _todoItemService;

    public TodoController(ITodoItemService todoItemService)
    {
        _todoItemService = todoItemService;
    }

    public IActionResult Index()
    {
        // Get to-do items from database

        // Put items into a model

        // Pass the view to a model and render
    }
}

既然 ITodoItemService 在命名空间 Services 里，你同样需要在文件顶部添加一个 using 语句：

using AspNetCoreTodo.Services;

这个类的第一行声明了一个私有变量，保存 ITodoItemService 的引用。这个变量可以让你在后面的 Index 方法里使用该服务（具体方法，稍后便知）。

public TodoController(ITodoItemService todoItemService) 这一行给类定义了一个 构造函数(constructor)。构造函数是一个特殊的方法，它会在为（本例中是 TodoController）类创建一个新的实例的时候被调用。在构造函数中加入的 ITodoItemService 参数，表示你做出如下声明：要创建一个 TodoController，你必须提供一个能匹配 ITodoItemService 接口的对象。

接口如此有用的原因就在于，因为它们有助于解耦（分离）你程序里的逻辑。既然这个控制器依赖于 ITodoItemService 接口，而不是任何 特定的 类，它就不知道也不必关心实际使用的是哪个具体的类。它可以是 FakeTodoItemService，或者是其它读写数据库的类，或者别的什么类。只要它符合该接口的要求，控制器就能工作。这使你可以轻而易举地，独立测试程序的各部分。（我会在 自动化测试 一章讲解测试相关的内容。）

现在，你终于可以在 action 方法里，（通过你声明的那个私有变量）使用 ITodoItemService 从服务层获取 待办事项 了：

public IActionResult Index()
{
    var items = await _todoItemService.GetIncompleteItemsAsync();

    // ...
}

还记得吗？ GetIncompleteItemsAsync 方法返回一个 Task<TodoItem[]>。“返回一个 Task”的意思是说，该方法不能立刻给出一个结果，但是你可以使用关键字 await，以确保你的代码暂停，直到结果就绪才继续执行。

当你编写代码访问数据库或者外部 API 服务的时候，Task 模式是很常见的，因为在数据库（或者网络）响应之前，它不可能给出实际的结果。如果你在 JavaScript 或者其它语言里使用过 promise 或者 回调函数，Task 与之如出一辙：承诺你，肯定会给出一个结果——在未来的某个时候。

如果你在老式 JavaScript 里应付过 “回调地狱”，那你现在走运了。在 .NET 里使用 Task 跟依附代码打交道要容易得多，这归功于神奇的关键字 await。 await 把代码暂停在 异步(async) 操作上，而后，在底层数据库或者网络请求结束时，从暂停的地方恢复执行。就是说，你的程序并没有卡住或者阻塞住，因为它可以处理其它的请求。如果现在想不通也别担心，跟着做下去就行！

目前的重点就是修改 Index 方法的签名，以返回一个 Task<IActionResult>，代替之前的 IActionResult，并标记为 async：

public async Task<IActionResult> Index()
{
    var items = await _todoItemService.GetIncompleteItemsAsync();

    // Put items into a model

    // Pass the view to a model and render
}

胜利在望！你已经让 TodoController 依赖于 ITodoItemService 接口，但你还没告诉 ASP.NET Core，你想把 FakeTodoItemService 作为幕后的实际服务。可能你觉得这是理所当然的，因为你的ITodoItemService仅有这一个实现，但你后面会为同一个接口提供多个实现，所以，有必要明确指定实现。

要声明（或者“关联”）具体的类到每个接口上，需要写在 StartUp 类的 ConfigureServices 方法里，在当前情况里，应该这么写：

Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    // (... some code)

    services.AddMvc();
}

ConfigureServices 方法负责的是把东西添加到 服务容器 里，或者 ASP.NET Core 的说法是服务的集合。 services.AddMvc 这行添加了一些服务，它们是 ASP.NET Core 系统内部依赖的（你可以试试，把这行代码注释掉）。你在应用里所需的任何其它服务，也都要在这个地方添加到服务容器里。

把下面这行添加到ConfigureServices中的任意位置：

services.AddSingleton<ITodoItemService, FakeTodoItemService>();

这一行告知 ASP.NET Core，在任何时候，只要 ITodoItemService 被一个构造函数（或其它什么地方）被请求，就用这个 FakeTodoItemService。

AddSingleton 把你的服务作为 singleton 添加进服务容器。这意味着，只有一个FakeTodoItemService的实例被创建，并在每次被请求的时候都被复用。在后面，当你写另一个服务去跟数据库交互时，你会采用一个不同的方式（叫做 scoped）。我会在 运用数据库 一章里说明原因。

好了，当一个请求进来，将会被发送到 TodoController，当控制器需要一个ITodoItemService 时，ASP.NET Core 会在 可用服务集合 里查找并自动给出 FakeTodoItemService。因为服务是从服务容器里“注入(injected)”的，这个模式被称为 依赖注入(dependency injection)。

Use dependency injection

Back in the TodoController, add some code to work with the ITodoItemService:

public class TodoController : Controller
{
    private readonly ITodoItemService _todoItemService;

    public TodoController(ITodoItemService todoItemService)
    {
        _todoItemService = todoItemService;
    }

    public IActionResult Index()
    {
        // Get to-do items from database

        // Put items into a model

        // Pass the view to a model and render
    }
}

Since ITodoItemService is in the Services namespace, you'll also need to add a using statement at the top:

using AspNetCoreTodo.Services;

The first line of the class declares a private variable to hold a reference to the ITodoItemService. This variable lets you use the service from the Index action method later (you'll see how in a minute).

The public TodoController(ITodoItemService todoItemService) line defines a constructor for the class. The constructor is a special method that is called when you want to create a new instance of a class (the TodoController class, in this case). By adding an ITodoItemService parameter to the constructor, you've declared that in order to create the TodoController, you'll need to provide an object that matches the ITodoItemService interface.

Interfaces are awesome because they help decouple (separate) the logic of your application. Since the controller depends on the ITodoItemService interface, and not on any specific class, it doesn't know or care which class it's actually given. It could be the FakeTodoItemService, a different one that talks to a live database, or something else! As long as it matches the interface, the controller can use it. This makes it really easy to test parts of your application separately. I'll cover testing in detail in the Automated testing chapter.

Now you can finally use the ITodoItemService (via the private variable you declared) in your action method to get to-do items from the service layer:

public IActionResult Index()
{
    var items = await _todoItemService.GetIncompleteItemsAsync();

    // ...
}

Remember that the GetIncompleteItemsAsync method returned a Task<TodoItem[]>? Returning a Task means that the method won't necessarily have a result right away, but you can use the await keyword to make sure your code waits until the result is ready before continuing on.

The Task pattern is common when your code calls out to a database or an API service, because it won't be able to return a real result until the database (or network) responds. If you've used promises or callbacks in JavaScript or other languages, Task is the same idea: the promise that there will be a result - sometime in the future.

If you've had to deal with "callback hell" in older JavaScript code, you're in luck. Dealing with asynchronous code in .NET is much easier thanks to the magic of the await keyword! await lets your code pause on an async operation, and then pick up where it left off when the underlying database or network request finishes. In the meantime, your application isn't blocked, because it can process other requests as needed. This pattern is simple but takes a little getting used to, so don't worry if this doesn't make sense right away. Just keep following along!

The only catch is that you need to update the Index method signature to return a Task<IActionResult> instead of just IActionResult, and mark it as async:

public async Task<IActionResult> Index()
{
    var items = await _todoItemService.GetIncompleteItemsAsync();

    // Put items into a model

    // Pass the view to a model and render
}

You're almost there! You've made the TodoController depend on the ITodoItemService interface, but you haven't yet told ASP.NET Core that you want the FakeTodoItemService to be the actual service that's used under the hood. It might seem obvious right now since you only have one class that implements ITodoItemService, but later you'll have multiple classes that implement the same interface, so being explicit is necessary.

Declaring (or "wiring up") which concrete class to use for each interface is done in the ConfigureServices method of the Startup class. Right now, it looks something like this:

Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    // (... some code)

    services.AddMvc();
}

The job of the ConfigureServices method is adding things to the service container, or the collection of services that ASP.NET Core knows about. The services.AddMvc line adds the services that the internal ASP.NET Core systems need (as an experiment, try commenting out this line). Any other services you want to use in your application must be added to the service container here in ConfigureServices.

Add the following line anywhere inside the ConfigureServices method:

services.AddSingleton<ITodoItemService, FakeTodoItemService>();

This line tells ASP.NET Core to use the FakeTodoItemService whenever the ITodoItemService interface is requested in a constructor (or anywhere else).

AddSingleton adds your service to the service container as a singleton. This means that only one copy of the FakeTodoItemService is created, and it's reused whenever the service is requested. Later, when you write a different service class that talks to a database, you'll use a different approach (called scoped) instead. I'll explain why in the Use a database chapter.

That's it! When a request comes in and is routed to the TodoController, ASP.NET Core will look at the available services and automatically supply the FakeTodoItemService when the controller asks for an ITodoItemService. Because the services are "injected" from the service container, this pattern is called dependency injection.