Simple class-level Dependency Injection

ClientClass+Init.swift

//By separating the convenience init into its own file, you can instantiate ClientClass() easily in your project.
//  Your unit tests only need to include the ClientClass.swift source and not this init
extension ClientClass {
    convenience init() {
        self.init(myDependency: MyDependencyConcreteImplementation())
    }
}

ClientClass.swift

final class ClientClass {
    private let myDependency: MyDependencyProtocol
    
    init(myDependency: MyDependencyProtocol) {
        self.myDependency = myDependency
    }
}