Create WeakResolver

[skorulis-ap]

Background

It is expected that parts of the dependency graph will directly hold onto a Resolver as they need to resolve services after initialisation. (e.g. pushing a new view controller). This creates the potential that if the object holding the Resolver leaks then so does the Resolver. While this can be detected, it is difficult to pinpoint the cause of the issue without the aid of debug tooling.

Solution

This PR adds the concept of a WeakResolver which holds a weak reference to the underlying Container which works as a Resolver until the Container has been deallocated at which point any attempt to resolve a service will cause a crash.

[brianpartridge]

What are these properties used for?

[bradfol]

I think this is to allow for specific use cases to access the underlying container if they are willing to take responsibility of deallocating correctly. Example would be some short-lived service on log out. I don't think it will likely be needed but it is good to have in case.

[bradfol]

Adding some documentation to both properties to make explicit the responsibility of the consumer to correctly manage life cycle would be good

[skorulis-ap]

isAvailable: Can I do the thing?
optionalResolver: Do the thing if possible
strongResolver: The thing has to happen, I'm aware of the risks (now removed)

[bradfol]

Looks great!

I think we should consider removing the strongResolver property however