Problems with singletons when using component dependencies

Question

I'm having a problem in understanding why the following code doesn't work.

I have following project structure:

@Component(modules = CCModule.class)  public interface CComponent {     XXX getXXX(); } 

where

@Module public class CCModule {      @Provides @Singleton     public XXX provide XXX(){         return new XXX();     } } 

and

@Component(dependencies = CComponent.class, modules = AAModule.class)  public interface AComponent {     YYY getYYY(); } 

where

class YYY {    @Inject    public YYY(XXX xxx) {        ...    } } 

I initialize everything as:

CComponent c_component = Dagger_CComponent.builder().cCModule(new CCModule()).build();  AComponent a_component = Dagger_AComponent.builder()         .cComponent(c_component)         .aAModule(new AAModule())         .build(); 

Once compilation takes place i get the following error:

Error:(11, 1) error: com.test.CComponent (unscoped) may not reference scoped bindings: @Provides @Singleton com.test.XXX com.test.CCModule.provideXXX()

What I'm aiming for is to have one component inherit bindings from other components to have the same references to an objects (singletons).

Answer 1

You should put @Singletonto CComponent class declaration.

@Singleton @Component(modules = CCModule.class)  public interface CComponent {     XXX getXXX(); } 

Explanation is in error message: CComponent is unscoped, @Singleton is a scope. Dagger 2 does not allow unscoped components to use modules with scoped bindings.
However, now you will get the following error:

AComponent (unscoped) cannot depend on scoped components: @Component(dependencies = CComponent.class, modules = AModule.class) 

Unscoped components cannot have scoped dependencies. So you need to make AComponent scoped. To do this create custom AScope annotation.

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

And annotate with it AComponent:

@AScope @Component(dependencies = CComponent.class, modules = AModule.class) public interface AComponent { } 

These are new requirements appeared in latest snapshot release. It was discussed in corresponding issue and may still be changed.