在基础和进阶两篇文章中，讨论很多的理论知识，不知道你们怎么样，反正我是有点懵逼了，程序猿么，终归是要垒代码。来吧，我们用代码说话，看一看Dagger2到底是怎么个用法。

本文代码地址 Dagger2 示例 app-mvp-dagger2
示例中采用了MVP架构，使用Retrofit进行网络通信，并融入了 Rxjava RxBus 等。Dagger2 使用示例在 app-mvp-dagger2 分支，请自行chekcout。

依赖标注

@Inject 标注

@Inject 注解 BaseActivity 和 BaseFragment 的 Presenter 实例。

public abstract class BaseActivity<T extends BasePresenter> extends SupportActivity implements BaseView {
    @Inject 
    protected T mPresenter
    ...
}

public abstract class BaseFragment<T extends BasePresenter> extends SupportFragment implements BaseView {
    @Inject
    public T mPresenter;
    ...
}

@Inject标注依赖构造函数

@Inject
public MainPresenter(RetrofitHelper retrofitHelper) {
    this.retrofitHelper = retrofitHelper;
    registerEvent();
}

 @Inject
public RepositoryPresenter(RetrofitHelper retrofitHelper){
     this.retrofitHelper = retrofitHelper;
}

@Moudle 工厂类

AppModule 提供全局依赖实例

@Module
public class AppModule {
    private final ArchiApplication application;

    public AppModule(ArchiApplication application){
        this.application = application;
    }

    @Provides
    @Singleton
    ArchiApplication provideApplicationContext(){
        return application;
    }

    @Provides
    @Singleton
    RetrofitHelper provideRetrofitHelper(RepositoriesService repositoriesService, UserInfoService userInfoService){
        return new RetrofitHelper(repositoriesService, userInfoService);
    }

}

ActivityModule 为 Activity 提供 this 实例

@Module
public class ActivityModule {
    private Activity mActivity;
    public ActivityModule(Activity activity) {
        this.mActivity = activity;
    }
    @Provides
    @ActivityScope
    public Activity provideActivity() {
        return mActivity;
    }
}

FragmentModule 为 Fragment 提供 Activity 实例

@Module
public class FragmentModule {
    private Fragment fragment;
    public FragmentModule(Fragment fragment) {
        this.fragment = fragment;
    }
    @Provides
    @FragmentScope
    public Activity provideActivity() {
        return fragment.getActivity();
    }
}

Component组织

Component 是依赖和被依赖的的桥梁，注入依赖。

AppComponent

定义 AppComponent 接口，@Singleton 标注，增加可读性

@Singleton
// modules 定义提供依赖实例的 Module 
@Component(modules = {AppModule.class, HttpModule.class})
public interface AppComponent {
    ArchiApplication getContext();  // 提供App的Context

    RetrofitHelper getRetrofitHelper();

}

创建 AppComponent 单例

public class ArchiApplication extends Application {
    
    public static AppComponent appComponent;
    
    public static AppComponent getAppComponent(){
        if (appComponent == null) {
            appComponent = DaggerAppComponent.builder()
                    .appModule(new AppModule(mInstance))
                    .httpModule(new HttpModule())
                    .build();
        }
        return appComponent;
    }
}

ActivityComponent

自定义 ActivityScope 注解

@Scope
@Retention(RetentionPolicy.RUNTIME)
public @interface ActivityScope {
}

定义 ActivityComponent 使用 @ActivityScope 标注

@ActivityScope
@Component(dependencies = AppComponent.class,modules = ActivityModule.class)
public interface ActivityComponent {
    Activity getActivity();

    void inject(RepositoryActivity repositoryActivity);
}

@ActivityScope 标注 Module 中的依赖实例提供方法

@Provides
@ActivityScope
public Activity provideActivity() {
    return mActivity;
}

Build ActivityComponent实例

public abstract class BaseActivity<T extends BasePresenter> extends SupportActivity implements BaseView {
    
    ...
    
    protected ActivityComponent getActivityComponent(){
        return DaggerActivityComponent.builder()
                // 添加依赖 component
                .appComponent(ArchiApplication.getAppComponent())
                .activityModule(new ActivityModule(this))
                .build();
    }
}

注入实例

public class RepositoryActivity extends BaseActivity<RepositoryPresenter> implements RepositoryContract.View{
    ...
    @Override
    protected void initInject() {
        getActivityComponent().inject(this);
    }
    ...
}

FragmentComponent

自定义 FragmentComponent 注解

@Scope
@Retention(RetentionPolicy.RUNTIME)
public @interface FragmentScope {
}

定义 FragmentComponent 使用 @FragmentScope 标注

@FragmentScope
@Component(dependencies = AppComponent.class,modules = FragmentModule.class)
public interface FragmentComponent {

    Activity getActivity();

    void inject(MainFragment mainFragment);
}

@FragmentScope 标注 Module 中的依赖实例提供方法

@Provides
@FragmentScope
public Activity provideActivity() {
    return fragment.getActivity();
}

Build FragmentComponent

public abstract class BaseFragment<T extends BasePresenter> extends SupportFragment implements BaseView {
    
    ...
    
    protected FragmentComponent getFragmentComponent(){
        return DaggerFragmentComponent.builder()
                // 添加依赖 component 
                .appComponent(ArchiApplication.getAppComponent())
                .fragmentModule(new FragmentModule(this))
                .build();
    }
}

注入实例

public class MainFragment extends BaseFragment<MainPresenter> implements MainContract.View {
    ...
    @Override
    protected void initInject() {
        getFragmentComponent().inject(this);
    }
    ...
}

RetrofitHelper 注入流程

MainPresenter 和 RepositoryPresenter 的构造函数中，有一个参数为 RetrofitHelper，根据Dagger2的注入流程，我们可以知道，RetrofitHelper会优先创建注入。
在 AppComponent 中管理的 Modules 中有一个 HttpModule，该 Module 主要作用是提供网络访问实例。
我们来看一下 RetrofitHelper 的注入流程。

Presenter 的构造函数参数为 RetrofitHelper，所以 RetrofitHelper 会优先创建。

RetrofitHelper 构造函数参数为网络访问实例 RepositoriesService, UserInfoService。

public RetrofitHelper(RepositoriesService repositoriesService, UserInfoService userInfoService){
        this.repositoriesService = repositoriesService;
        this.userInfoService = userInfoService;
    }

RepositoriesService 和 UserInfoService 实例由 HttpModule 提供

@Module
public class HttpModule {

    /**
     * 提供builder
     * */
    @Singleton
    @Provides
    Retrofit.Builder provideRetrofitBuilder (){
        return new Retrofit.Builder();
    }

    @Singleton
    @Provides
    OkHttpClient.Builder provideOkHttpClientBuilder(){
        return new OkHttpClient.Builder();
    }



    /**
     * 提供客户端
     * */
    @Singleton
    @Provides
    @RespositoriesUrl
    Retrofit provideRepositoriesRetrofit(Retrofit.Builder builder,OkHttpClient client){
        return createRetrofit(builder, client, RepositoriesService.HOST);
    }

    @Singleton
    @Provides
    @UserInfoUrl
    Retrofit provideUserInfoRetrofit(Retrofit.Builder builder,OkHttpClient client){
        return createRetrofit(builder, client, RepositoriesService.HOST);
    }

    @Singleton
    @Provides
    OkHttpClient provideClient(OkHttpClient.Builder builder) {
        return builder.connectTimeout(60000L, TimeUnit.MILLISECONDS)
                .readTimeout(60000L, TimeUnit.MILLISECONDS)
                .writeTimeout(60000L, TimeUnit.MILLISECONDS)
                .build();
    }


    /**
     * 提供API
     * */
    @Singleton
    @Provides
    RepositoriesService provideRepositoriesService(@RespositoriesUrl Retrofit retrofit){
        return retrofit.create(RepositoriesService.class);
    }

    @Singleton
    @Provides
    UserInfoService provideUserInfoService(@UserInfoUrl Retrofit retrofit){
        return retrofit.create(UserInfoService.class);
    }

    private Retrofit createRetrofit(Retrofit.Builder builder, OkHttpClient client, String host) {
        return builder
                .baseUrl(host)
                .client(client)
                .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
                .addConverterFactory(GsonConverterFactory.create())
                .build();
    }
}

RetrofitHelper 完成创建后由 AppComponent 注入，为全局单一实例。

@Qualifier 使用

@Qualifier 的作用是使 Dagger2 避免依赖混乱。

定义 Qualifier 以 RespositoriesUrl 为例。

@Qualifier
@Documented
@Retention(RetentionPolicy.RUNTIME)
public @interface RespositoriesUrl {
}

使用 @RespositoriesUrl 标注依赖实例提供方法，该方法和其他方法的返回相同。

@Singleton
@Provides
@RespositoriesUrl
Retrofit provideRepositoriesRetrofit(Retrofit.Builder builder,OkHttpClient client){
    return createRetrofit(builder, client, RepositoriesService.HOST);
}

使用 @RespositoriesUrl 标注 provideRepositoriesRetrofit 的返回值

@Singleton
@Provides
RepositoriesService provideRepositoriesService(@RespositoriesUrl Retrofit retrofit){
    return retrofit.create(RepositoriesService.class);
}

author @ygwang