看懂这篇文章你需要有 依赖注入 以及 dagger 相关的知识点

为了方便管理，将所有的依赖注入的代码放在一个包，包括Module ，component，注解

ApplicationMoudle

提供全局的注入 ，如 Retrofit 的 service ，DataRepository 等， 其中的@provides 均标记为 @Singleton

@Module
public class ApplicationModule {
    protected final Application mApplication;

    public ApplicationModule(Application application) {
        mApplication = application;
    }

    @Provides
    @Singleton
    Application provideApplication() {
        return mApplication;
    }

    @Provides
    @Singleton
    Context provideContext() {
        return mApplication;
    }

    @Provides
    @Singleton
    ThreadExecutor provideThreadExecutor(JobExecutor jobExecutor) {
        return jobExecutor;
    }

    @Provides
    @Singleton
    PostExecutionThread providePostExecutionThread(UIThread uiThread) {
        return uiThread;
    }

    @Provides
    @Singleton
    JiandanService provideRibotsService() {
        return JiandanService.Creator.newJiandanService();
    }

    @Provides
    @Singleton
    DataRepository provideDataRepository(DataManager dataManager) {
        return dataManager;
    }
}

注： 可以看到 PostExecutionThread 和 UIThread、DataRepository 和DataManager。
并不是同一个对象，其实前者是接口，后者是前者的实现，接口都定义在 domain 层，实现定义在别的层，所以 domain 层可以是纯 java 的实现，通过 dagger 的注入实现了三层的完全解耦

ApplicationComponent

将 ApplicationMoudle提供的构造对象暴露出来

@Singleton
@Component(modules = ApplicationModule.class)
public interface ApplicationComponent {
    //Exposed to sub-graphs.
    Activity activity();

    JiandanService jiandanService();

    ThreadExecutor threadExecutor();

    PostExecutionThread postExecutionThread();

    DataRepository userRepository();

    VODataMapper provideVODataMapper();

}

ActivityModule

提供 activity 的依赖注入，这个维度是 @PerActivity的

@Module
public class ActivityModule {

    private Activity mActivity;

    public ActivityModule(Activity activity) {
        mActivity = activity;
    }

    @Provides
    @PerActivity
    Activity provideActivity() {
        return mActivity;
    }

}

ActivityComponent

@PerActivity
@Component(dependencies = ApplicationComponent.class, modules = ActivityModule.class)
public interface ActivityComponent {
    //Exposed to sub-graphs.
    Activity activity();

    JiandanService jiandanService();

    ThreadExecutor threadExecutor();

    PostExecutionThread postExecutionThread();

    DataRepository userRepository();

    VODataMapper provideVODataMapper();

}

这里并没有 inject方法，以后的Activity维度的Component 需要继承这个Component。在@PerFragment维度 要依赖ActivityComponent

举个栗子

@PerActivity
@Component(dependencies = ApplicationComponent.class, modules = CommentMoudle.class)
public interface CommentActivityComponent {
    void inject(CommentActivity commentActivity);

}

BoringPicMoudle

具体的 usecae 的构造方法的提供

@Module
public class PicModule {

    @Provides
    @PerFragment
    GetPicsCase provideMorePicsUseCase(
            DataRepository userRepository, ThreadExecutor threadExecutor,
            PostExecutionThread postExecutionThread) {
        return new GetPicsCase(threadExecutor, postExecutionThread, userRepository);
    }

}

DataRepository 、ThreadExecutor、PostExecutionThread ，在ApplicationComponent.class中都暴露出了，dagger 会自己在 applicationMoudle 中找到对应的构造方法

BoringPicComponent

@PerFragment，依赖ActivityComponent.class，在ActivityComponent.class中已经将ApplicationComponent中的构造方法给暴露出来了，除此之外，还依赖PicModule.class，其中暴露了需要用的 usecae

@PerFragment
@Component(dependencies = ActivityComponent.class, modules = PicModule.class)
public interface BoringPicComponent extends ActivityComponent {
    void inject(PicNewFragment PicNewFragment);
}```


在 fragment 中注入

```java
BoringPicComponent picComponent = DaggerBoringPicComponent.builder()
                .activityComponent(getActivitynComponent())
                .picModule(new PicModule())
                .build();
        picComponent.inject(this);
        
//     baseFragment   --------------
          protected ActivityComponent getActivitynComponent() {
        return DaggerActivityComponent.builder()
                .activityModule(new ActivityModule(getActivity()))
                .applicationComponent(getApplicationComponent())
                .build();
    }
    
    protected ApplicationComponent getApplicationComponent() {
        return ((JiandanApplication)getActivity().getApplication()).getComponent();
    }
    

presenter

虽然没有在module的 provide方法中暴提供 presenter 的方法，其实是在presenter类的构造方法

    @Inject
    public BoringPicPresenter(GetPicsCase getPicsCase) {
        this.getMorePics = getPicsCase;
    }