privateなAPIをテストする方法を考えてみた ref: http://qiita.com/takasek/items/d1f11394b6df2f3f3cc0

file1.swift

public protocol PrivatePublicatable {
    associatedtype Publicater
    var `private`: Publicater { get }
}
public final class Private<Base> {
    public let base: Base
    public init(_ base: Base) { self.base = base }
}
public extension PrivatePublicatable {
    var `private`: Private<Self> { return Private(self) }
}

Fuga.swift

/// Fugaをprivateに保つために作るTest用ラッパー。Fugaと同じファイル内に書く
final class FugaTester {
    private let base: Fuga
    init(args: (x: Int, y: Int)) {
        base = Fuga(x: args.x, y: args.y)
    }
    var x: Int { return base.x }
    var y: Int { return base.y }
    func doSomething(with string: String) -> String {
        return base.doSomething(with: string)
    }
}

FugaTests.swift

let tester = FugaTester(args: (x: 1, y: 2))
tester.x // 1
tester.y // 2
tester.doSomething(with: "ふが") // "ふが 1 2"

Hoge.swift

extension Hoge: PrivatePublicatable {}
public extension Private where Base == Hoge {
    var x: Int { return base.x }
    func doSomething(with string: String) -> String {
        return base.doSomething(with: string)
    }
}

HogeTests.swift

let hoge = Hoge(x: 1)
hoge.private.x // 1
hoge.private.doSomething(with: "ほげ") // ほげ 1