Introduction

Android components (activities, fragments, ...) are instantiated by the Android framework which makes it difficult to use dependency injection on them. By doing a little setup, Dagger with provided classes help to simplifies the dependency injection into Android components.

Setup

Dependencies

• Add Dagger dependencies to app.gradle

compile 'com.google.dagger:dagger:2.10'
annotationProcessor 'com.google.dagger:dagger-compiler:2.10'

• In order to use classes from dagger.android package, like DaggerActivity, add the dependencies

compile 'com.google.dagger:dagger-android:2.10'
annotationProcessor 'com.google.dagger:dagger-android-processor:2.10'

• Use support libraries with Dagger 2

compile 'com.google.dagger:dagger-android-support:2.10'

Dagger subcomponent

Subcomponent interface

@Subcomponent
public interface IMainActivitySubcomponent extends AndroidInjector<MainActivity> {

    @Subcomponent.Builder
    abstract class Builder extends AndroidInjector.Builder<MainActivity> {}
}

• Represents a Dagger subcomponent which is able to inherit bindings from any parent subcomponent/component
• Extends AndroidInjector to perform member injection on a concrete Android component type
• Annotate with @Subcomponent

Module to add subcomponent

@Module(subcomponents = {IMainActivitySubcomponent.class})
public abstract class ActivityModule {

    @Binds
    @IntoMap
    @ActivityKey(MainActivity.class)
    abstract AndroidInjector.Factory<? extends Activity> bindYourActivityInjectorFactory(IMainActivitySubcomponent.Builder builder);
}

• Annotate with @Module
• Help the sub components in subcomponent parameter are available
• Binds the builder of the subcomponent to AndroidInjector.Factory

Application

ApplicationComponent interface

@Component(modules = {ActivityModule.class, AndroidInjectionModule.class})
public interface ApplicationComponent {
    void inject(MyApplication application);
}

• Responsible for injecting the Application class
• @Component annotation with the modules parameter to specify which modules are used to create implementation of the component
  • ActivityModule make use of subcomponent to inject activities
  • AndroidInjectionModule is needed to ensure the binding of Android component (Activity, Fragment, ...)