It sounds like A needs a Foo specifically crafted for it, using something
that only it knows about.
Why not use assisted injection?
interface IFooFactory {
IFoo createFoo(C myC);
}
class Foo implements IFoo {
@AssistedInject
Foo(@Assisted C myC) {
this.myC = myC;
}
}
class A {
@Inject
A(IFooFactory fooFactory) {
this.foo = fooFactory.createFoo(this.getC());
}
}
Then in your module:
install(new FactoryModuleBuilder().implement(IFoo.class,
Foo.class).build(IFooFactory.class));
Would that solve your issue?
-Jared
On Thu, Feb 14, 2013 at 1:26 PM, Dale King <[email protected]> wrote:
> For simplicity lets say I have class A and that has an injection point of
> interface IFoo. I want to inject into this an implementation of IFoo called
> Foo. Class Foo's only constructor takes an instance of class C.
>
> Class C is abstract, there are no public implementations of it. The only
> way to get an instance of class C is to invoke a method on an instance of A
> (i.e. myA.getC()).
>
> If you want real world names for these, this is on Android using Roboguice
> and A is an Activity or Fragment and C is LoaderManager which is abstract
> and the can only be retrieved by calling getLoaderManager() on the Activity
> of Fragment to get the LoaderManager specificly for that Activity or
> Fragment..
>
> And note that there is not just a single instance of A, There will be
> instances of A created and destroyed as the user uses the app.
>
> I considered getting rid of the constructor parameter in Foo and adding a
> setC method that A has to explicitly invoke itself, but that requires
> exposing setC in IFoo interface which makes no sense as C is an
> implementation detail of Foo and has no business being exposed in the
> interface IFoo. The point of DI is that I should not have to include the
> implementation details of how to initialize class Foo and only depends on
> IFoo.
>
> I have done the custom injection thing for injecting the target class name
> for logger, but in this case I need the actual instance being injected into
> to to use in the provider. I can probably do it with CustomInjection but it
> sure seems the wrong way to go, but probably what I will be forced into. To
> me the whole custom injection thing seems like I am rewriting much of the
> logic
>
> I have gathered from the web that the designers of Guice have a
> philosophical bias that considers a binding knowing anything about the
> target to be horrible design and would actually design the it to make this
> hard if not impossible. But I'm not sure I see a good way around this.
>
> I have a concept of how this and other related things could be designed
> easily into the external API, but I'm not sure if they would go for this.
>
> The idea is to have another provider interface that would provide the
> context when getting an instance.
>
> public interface ContextualProvider<T, Context>
> {
> T get(InjectionPoint point, Context target);
> }
>
> Which would let me implement this as:
>
>
> bind(IFoo.class).inTarget(Matchers.subclassesOf(A.class)).toProvider(new
> ContextualProvider<IFoo, A>()
> {
> IFoo get(InjectionPoint point, A a)
> {
> return new Foo(a.getC());
> }
> }
>
> Might have to be reworked slightly for type safety, but hopefully you get
> the idea.
>
> Something like this would also simplify the logger example shown for
> custom injection which is way too much work just to get the name of the
> target class.
>
> Are there any other thoughts on how to get around this?
>
> This idea of getting access to the InjectionPoint would also let you
> easily query for other annotations being applied to the Injection and
> getting their values to alter the object that is provided.
>
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