Order of dependency injection when using scopes

问题:

I'm currently trying to figure out Dagger 2. I am trying to set up 4 scopes: App, User, Activity, Fragment. User and Activity components are Subcomponents of App. Fragment is a Component with Activity as its dependency.

Say my UserSettingsActivity needs a Toolbar (provided by ActivityModule), and a UserProfile (provided by UserModule). I won't get a UserProfile until I ask for it from the database, whereas the Toolbar can be provided right away. So the order of injection that takes place is into ActivityComponent first, then into UserComponent. I have 2 @Inject fields, one for Toolbar and one for UserProfile in the activity. I was hoping that dagger will know that the dependencies are coming from different modules, but it seems to complain that UserProfile can't be provided when injected into ActivityComponent. Obviously it can't be provided by ActivityModule, but why is it not making a connection that UserProfile is provided by UserModule?

回答1:

To my best knowledge, Dagger-2 doesn't support "partial injections".

Therefore, when you call myComponent.inject(this), Dagger-2 throws an error if myComponent can't provide all @Inject annotated members of this.

I see two ways to work around this limitation:

1. Remove @Inject annotation from UserProfile, expose UserProfile via public method in UserComponent and inject it manually when UserComponent is ready to be used. Something analogous to this: userProfile = userComponent.getUserProfile()
2. Don't make UserComponent dependent on data fetching. UserComponent could be used to inject Toolbar and some UserProfileProvider at the same time, and you will fetch UserProfile from UserProfileProvider when it is available.

I personally think that second approach is the better choice. DI libraries should be used in order to satisfy objects' dependencies at construction time. In Android we can't construct Activity or Fragment ourselves, therefore we perform DI in onCreate(), onAttach(), onCreateView(), etc., but it does not mean that we should be using DI libraries in order to assist in controlling the flow of applications.

回答2:

Subcomponents work's similar to inheritance(extends), in your case User component and Activity component extending App component but there is no relation between User component and Activity component so when you request User dependency in Activity it will fail.

Subcomponent can't provide any dependency to other Subcomponent.

Instead, you can make Activity component as a subcomponent of User component. This will also give you the flexibility to switch user.