concepts.md

Core Concepts

Factory

A Factory is a stateless object encapsulating initialization details of some object by exposing a protocol or superclass return value. Example:

struct ViewControllerFactory {
    func home() -> UIViewController {
        return LoginViewController()
    }
}

Dependency Container

A Dependency Container is an object capable of registering declarations and functions (bound to some key) that can later be resolved by some other contexts. It's like a dictionary of closures. Registering closures can be configured so that they are always executed for every resolution (thus creating a new instance of returned object every time), or having a "singleton" behavior. For "singletons", registered closures can create the object upon first resolution (.singleton) or directly upon registration (.eagerSingleton).

Example:

container.register(for: "someKey") { return "Hello"}

...

let string = container.resolve("someKey") //Hello

Dependency containers are powerful but also dangerous, since are likeky to lead to dependency cycles.

Example of dependency cycle (this will lead to a runtime crash):

container.register(for: "a") { SomeObject(container.resolve("b") }
container.register(for: "b") { SomeObject(container.resolve("c") }
container.register(for: "c") { SomeObject(container.resolve("a") }