Dagger2中@Binds与@Provides批注的用例是什么

问题描述

对于Dagger2的 @Bind注释，我不确定./a>

I am not certain on the purpose for Dagger2's @Bind annotation.

根据我在网上阅读的内容，我仍然不清楚，但这是一个示例:

From what i have read online im still not clear but here is an example:

@Module
public abstract class HomeModule {

  @Binds
  public abstract HomePresenter bindHomePresenter(HomePresenterImp   
    homePresenterImp);
}

和类定义如下:

public interface HomePresenter {
    Observable<List<User>> loadUsers();
}

public class HomePresenterImp implements HomePresenter {

    public HomePresenterImp(){
    }  

    @Override
    public Observable<List<User>> loadUsers(){
        //Return user list observable
    }
}

如果我只能使用提供注释，为什么我需要使用@Binds:

why would i need to use @Binds if i can just use provides annotation as follows:

@Provides
public HomePresenter provideHomePresenter() {
    return new HomePresenterImp();
}

@Binds而不是@Provides的用例是什么?如果我使用@Binds，我是否仍需要在我的appcomponent中声明它(当我使用@Binds时，它是一个抽象类)?

what is the usecase for @Binds instead of @Provides ? if i use @Binds do i still need to declare it in my appcomponent (its an abstract class when i use @Binds)?

推荐答案

@Binds可以完全等同于@Provides注释的方法，如下所示:

@Binds can be perfectly equivalent to a @Provides-annotated method like this:

@Provides
public HomePresenter provideHomePresenter() {
    return new HomePresenterImp();
}

...尽管您可能更喜欢使用HomePresenterImp作为方法参数的变体，该变体允许Dagger实例化HomePresenterImp(假设它具有@Inject构造函数)，包括传递所需的任何依赖项.您还可以创建static，因此Dagger无需实例化Module实例即可调用它.

...though you'd probably prefer a variant that takes HomePresenterImp as a method parameter, which lets Dagger instantiate HomePresenterImp (assuming it has an @Inject constructor) including passing any dependencies it needs. You can also make this static, so Dagger doesn't need to instantiate your Module instance to call it.

@Provides
public static HomePresenter provideHomePresenter(HomePresenterImp presenter) {
    return presenter;
}

那为什么要选择@Binds呢? Dagger对此有常见问题解答，但归结为以下原因:

So why would you choose @Binds instead? Dagger has a FAQ about it, but it boils down do these reasons:

• @Binds(略)更紧凑:您可以跳过实现.
• @Binds在接口和抽象类中工作，而@BindsOptionalOf和@ContributesAndroidInjector之类的Dagger功能则严格要求这些接口和抽象类.
• @Binds帮助您的代码保持高效. @Provides方法可以是实例方法，需要Dagger实例化您的Module才能调用它们.使您的@Provides方法static也可以完成此操作，但是如果您忘记了static，您的@Provides方法仍然可以编译. @Binds方法不会.
• @Binds防止Dagger进行代码生成并为对象保留单独的Factory/Provider，因为Java不允许Dagger访问其实现是多么简单.在您的情况下，Dagger可以将Provider<HomePresenterImp>强制转换为Provider<HomePresenter>，并且只保留一个，而不是为HomePresenter保留一个，而只为HomePresenterImp调用一个.

• @Binds is (slightly) more compact: You can skip the implementation.
• @Binds works in interfaces and abstract classes, which are strictly required for Dagger features like @BindsOptionalOf and @ContributesAndroidInjector.
• @Binds helps your code stay efficient. @Provides methods can be instance methods, which require Dagger to instantiate your Module in order to call them. Making your @Provides method static will also accomplish this, but your @Provides method will still compile if you forget the static. @Binds methods will not.
• @Binds prevents Dagger from having to codegen and keep a separate Factory/Provider for the object, since Java doesn't give Dagger access to know that the implementation is as simple as it is. In your case, Dagger can cast the Provider<HomePresenterImp> to a Provider<HomePresenter> and only keep one, rather than keeping one for HomePresenter that does nothing but call the one for HomePresenterImp.

因此，整个事情都可以很好地表示为:

Thus, the entire thing would be well-represented as:

@Binds abstract HomePresenter bindHomePresenter(HomePresenterImp presenter);

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