A container hierarchy is a relationship of containers to share registrations of dependency injections. Service types registered to a parent container can be resolved in its child containers too. Use init(parent: Container) to instantiate a child container with its parent passed.
let parentContainer = Container()
parentContainer.register(AnimalType.self) { _ in Cat() }
let childContainer = Container(parent: parentContainer)
let cat = childContainer.resolve(AnimalType.self)
print(cat != nil) // prints "true"In contrast, service types registered to a child container are not resolved in its parent container.
let parentContainer = Container()
let childContainer = Container(parent: parentContainer)
childContainer.register(AnimalType.self) { _ in Cat() }
let cat = parentContainer.resolve(AnimalType.self)
print(cat == nil) // prints "true"