入门教程(推荐):
https://www.jianshu.com/p/26d9f99ea3bb

dagger2在Android中的应用

• 添加依赖

    implementation 'com.google.dagger:dagger:2.17'
    annotationProcessor 'com.google.dagger:dagger-compiler:2.17'

• AppComponent
  如果有SubComponent, 为SubComponent提供相应Builder

@Singleton
@Component(modules = {AppModule.class})
public interface AppComponent {
    void inject(App app);
    MainActivityComponent.Builder getMainActivityComponent();
}

• MainActivityComponent

@ActivityScope
@Subcomponent(modules = SubModule.class)
public interface MainActivityComponent{
    void inject(MainActivity main);

    @Subcomponent.Builder
    interface Builder{
        MainActivityComponent build();
    }
}
-----------------------------------------------------------------------------------------------
@Module
public class SubModule {
    @ActivityScope
    @Provides
    public ActivityEntity provideActivityEntity(){
        return new ActivityEntity();
    }
}
-----------------------------------------------------------------------------------------------
public class ActivityEntity {
    String name = "injectEntity";
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

• AppModule 提供注入的依赖的对象, 经测试, Module中不设置subComponent也没事儿, 只要AppComponent中返回相应的Builder就可以

@Module
public class AppModule {
    @Singleton
    @Provides
    public OkHttp provideOkhttp(){
        return new OkHttp();
    }
    @Singleton
    @Provides
    public RetrofitClient provideRetrofit(OkHttp okHttp){
        return new RetrofitClient(okHttp);
    }
}

-----------------------------------------------------------------------------------------------
public class OkHttp {
    String name = "okhttp";
    
    public String getName() {
        return name;
    }
    
    public void setName(String name) {
        this.name = name;
    }
}
-----------------------------------------------------------------------------------------------

public class RetrofitClient {
    public String name = "retrofit";
    public OkHttp okHttp;
    public RetrofitClient(OkHttp okHttp) {
        this.okHttp = okHttp;
    }
    
    public String getName() {
        return name;
    }
    
    public void setName(String name) {
        this.name = name;
    }
}

• App

public class App extends Application {
    
    private static AppComponent appComponent;
    
    @Override
    public void onCreate() {
        super.onCreate();
        appComponent = DaggerAppComponent.builder().appModule(new AppModule()).build();
        appComponent.inject(this);
    }
    
    public static AppComponent getAppComponent() {
        return appComponent;
    }
}

• MainActivity

public class MainActivity extends BaseActivity {
    
    private static final String TAG = "MainActivity";
    
    @Inject
    RetrofitClient client;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    
        App.getAppComponent().getMainActivityComponent().build().inject(this);
        
        Log.e(TAG, client.getName() );
        Log.e(TAG, client.okHttp.getName() );
        Log.e(TAG, String.valueOf(client.hashCode()));
    }
}

dagger.android在Android中的应用

• 添加依赖

   implementation 'com.google.dagger:dagger-android:2.17'
   implementation 'com.google.dagger:dagger-android-support:2.17'
   annotationProcessor 'com.google.dagger:dagger-android-processor:2.17'

• AppComponent
  modules中添加 AndroidInjectionModule.class

@Singleton
@Component(modules = {AppModule.class, AndroidInjectionModule.class, AppBindModule.class})
public interface AppComponent {
    void inject(App app);
}

• MainActivityComponent
  1. 类继承AndroidInjector<MainActivity>
  2. Builder extends AndroidInjector.Builder<MainActivity>

@ActivityScope
@Subcomponent(modules = SubModule.class)
public interface MainActivityComponent extends AndroidInjector<MainActivity>{
    @Subcomponent.Builder
    abstract class Builder extends AndroidInjector.Builder<MainActivity> {}
}

• SecondActivityComponent

@ActivityScope
@Subcomponent
public interface SecondActivityComponent extends AndroidInjector<SecondActivity>{
    @Subcomponent.Builder
    abstract class Builder extends AndroidInjector.Builder<SecondActivity> {}
}

• AppModule 提供注入的对象

@Module
public class AppModule {
    @Singleton
    @Provides
    public OkHttp provideOkhttp(){
        return new OkHttp();
    }
    @Singleton
    @Provides
    public RetrofitClient provideRetrofit(OkHttp okHttp){
        return new RetrofitClient(okHttp);
    }
}

• AppBindModule 负责绑定SubComponent, 如果新增了Activity只需要修改此处即可.

@Module
public abstract class AppBindModule {
    @ContributesAndroidInjector
    abstract MainActivity contributeMainActivity();

    @ContributesAndroidInjector
    abstract SecondActivity contributeSecondActivity();
}

• App
  1. implements HasActivityInjector
  2. 注入DispatchingAndroidInjector<Activity> 并返回
  3. onCreate中实现注入逻辑

public class App extends Application implements HasActivityInjector {
    
    @Inject
    DispatchingAndroidInjector<Activity> androidInjector;
    
    @Override
    public void onCreate() {
        super.onCreate();
        DaggerAppComponent.builder().appModule(new AppModule()).build().inject(this);
    }
    
    @Override
    public AndroidInjector<Activity> activityInjector() {
        return androidInjector;
    }
}

• BaseActivity
  在super前调用 AndroidInjection.inject(this);

public class BaseActivity extends AppCompatActivity {
    
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        AndroidInjection.inject(this);
        super.onCreate(savedInstanceState);
    }
}

• 继承自BaseActivity的相应Activity

public class SecondActivity extends BaseActivity {
    private static final String TAG = "SecondActivity";
    @Inject
    RetrofitClient client;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);
        Log.e(TAG, client.getName() );
        Log.e(TAG, client.okHttp.getName() );
        Log.e(TAG, String.valueOf(client.hashCode()));
    }
}

注意

• 如果AppBindModule中是这种传统的方式的话, AppComponent中需要分别获得相应的Builder

@Module
public abstract class AppBindModule {
    
    @Binds
    @IntoMap
    @ActivityKey(MainActivity.class)
    abstract AndroidInjector.Factory<? extends Activity> bindAndroidInjector( MainActivityComponent.Builder builder);


    @Binds
    @IntoMap
    @ActivityKey(SecondActivity.class)
    abstract AndroidInjector.Factory<? extends Activity> bindAndroidInjector2(SecondComponent.Builder builder);

}
----------------------------------------------------------------------------------------------------------
@Singleton
@Component(modules = {AppModule.class, AndroidInjectionModule.class, AppBindModule.class})
public interface AppComponent {
    void inject(App app);  
    
    MainActivityComponent.Builder getMainActivityComponent();
    SecondComponent.Builder getSecondComponent();
}