mocks.swift

protocol SomeProtocol {
  func foo() -> Int?
}

struct Live: SomeProtocol {
  func foo() -> Int? {
    // do some work to return an integer
  }
}

struct Mock: SomeProtocol {
  let value: Int?

  func foo() -> Int? {
    self.value
  }
}

// versus:

struct Something {
  var foo: () -> Int?
}

let live = Something(foo: { /* do something to return an int */ })
let happyMock = Something(foo: { 42 })
let unhappyMock = Something(foo: { nil })