Disclaimer: I'm still learning Swift so forgive me if I haven't understood certain concepts/capabilities/limitations of Swift.
With the Swinject framework, if you wanted to bind a protocol to a class - it seems you have to return the class instance in a closure such as:
container.register(Animal.self) { _ in Cat() }
Is is possible to be able to instead pass in two types to the register() method and have the framework instantiate the class for you? It would need to recursively see if that class had any initializer dependencies of course (Inversion of Control).
This is possible in the PHP world as you have the concept of reflection, which allows you to get the class types of the dependencies, allowing you instantiate them on the fly. I wonder if Swift has this capability?
It would be much nicer to write this:
container.register(Animal.self, Cat.self)
This would also allow you to resolve any class from the container and have it's dependencies resolved also (without manually registering the class):
container.resolve(NotRegisteredClass.self)
Note: This only makes sense for classes that do not take scalar types as a dependency (as they need to be explicitly given of course).
The second case - resolving a type without the explicit registration - is currently not possible because of Swift's very limited support for the reflection.
However, there is a SwinjectAutoregistration extension which will enable you to write something very close to your first example:
container.autoregister(Animal.self, initializer: Cat.init)