Dagger 2-@Singleton注释类的目的是什么 [英] Dagger 2 - what is the purpose of a @Singleton annotation class

问题描述

在匕首2中文档我注意到您可以拥有 @Singleton 带注释的类。我试图在代码中将类标记为 @Singleton 的目的是什么，但未生成单例对象。我尚不清楚使用此注释标记班级的用途。

From the dagger 2 Documentation I noticed that you can have a @Singleton annotated class. What is the purpose of marking a class as @Singleton as I have tried to do this in my code but a singleton object is NOT produced. I'm not clear on what use marking my class with this annotation serves.

在文档中，请重点关注以下语句：

From the documentation please focus on the following statement:

@Singleton注射类的注解也可用作
文档。它提醒潜在的维护者，此类可能是由多个线程共享的
。*

The @Singleton annotation on an injectable class also serves as documentation. It reminds potential maintainers that this class may be shared by multiple threads.*

@Singleton
class CoffeeMaker {
    // ...
}

更新：查看froger_mcs答案后，我看到在Dagger 2中，您可以通过模块或构造函数注入来提供注入。因此可以注入以下类，尽管不在模块中：

UPDATE: After reviewing froger_mcs answer I see that in Dagger 2 you can provide injections either by a module OR by a constructor injection. So the following class, although not in a module, can be injected:

@Singleton
public class MyClass {
    @Inject
    public MyClass() {

    }
}

在此版本中，为我们注入了构造函数，并且在Android活动中，您只需执行以下操作即可得到它：

In this version the constructor is injected for us and in an Android activity you would just do the following and it will get provided:

@Inject
MyClass myClass;
//then in onCreate actually inject(this) from your graph of course.

推荐答案

@Singleton （以及任何其他作用域注释）使您的类成为依赖关系图中的单个实例（这意味着只要Component对象存在，此实例将是单个）。

@Singleton (and any other scope annotation) makes your class a single instance in your dependencies graph (it means that this instance will be "singleton" as long as Component object exists).

简而言之-每次注入 @Singleton 带注释的类（带有 @Inject 注释），只要您从相同的组件注入它，它将是相同的实例。

In short - everytime you're injecting @Singleton annotated class (with @Inject annotation) it will be the same instance as long as you inject it from the same Component.

有关更多信息，我指的是我的博客文章关于 @Singleton 和其他作用域注释在Dagger 2中的工作方式： http://frogermcs.github.io/dependency-injection-with-dagger-2-custom-scopes/

For more I'm referring my blog post about how @Singleton and other scopes annotations works in Dagger 2: http://frogermcs.github.io/dependency-injection-with-dagger-2-custom-scopes/

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