DependencyInjectionWithValueTypes.swift

import Foundation
import PlaygroundSupport
import UIKit

enum PassingStructInInit {

    struct A {
        let b: B
    }

    struct B {
        let doX: () -> Int
        let doY: () -> String

        static var standard: B {
            B(doX: { 42 }, doY: { "Standard" })
        }

        static var mock: B {
            B(doX: { 0 }, doY: { "Mock" })
        }
    }

    struct ProductionUsage {
        init() {
            let b = B.standard
            let a = A(b: b)
        }
    }

    struct TestUsage {
        init() {
            let b = B.mock
            let a = A(b: b)
        }
    }

}

enum PassingClosureInInit {

    struct A {
        let doX: () -> Int
    }

    struct B {
        let doX: () -> Int
        let doY: () -> String

        static var standard: B {
            B(doX: { 42 }, doY: { "Standard" })
        }

        static var mock: B {
            B(doX: { 0 }, doY: { "Mock" })
        }
    }

    struct ProductionUsage {
        init() {
            let b = B.standard
            let a = A(doX: b.doX)
        }
    }

    struct TestUsage {
        init() {
            let b = B.mock
            let a = A(doX: b.doX)
        }
    }

}