StoryboardからUIViewControllerを生成する。 UIViewControllerひとつに複数のStoryboardがある場合を想定。

StoryboardLoader.swift

public struct Storyboard {
    public init(_ name: String, bundle: Bundle? = nil) {
        self.name = name
        self.bundle = bundle
    }
    public var name: String
    public var bundle: Bundle?

    public func instantiate<VCType>(_ vcType: VCType.Type = VCType.self, identifier: String? = nil) -> VCType? {
        if let identifier = identifier {
            return UIStoryboard(name: name, bundle: bundle).instantiateViewController(withIdentifier: identifier) as? VCType
        } else {
            return UIStoryboard(name: name, bundle: bundle).instantiateInitialViewController() as? VCType
        }
    }
}

public struct StoryboardLoader<VCType: UIViewController> {
    let storyboard: Storyboard
    let identifier: String?

    public init(_ storyboard: Storyboard, identifier: String? = nil) {
        self.storyboard = storyboard
        self.identifier = identifier
    }

    // Storyboard構造体の定義を横着するとき用
    public init(storyboardName: String, identifier: String? = nil, bundle: Bundle? = nil) {
        self.storyboard = Storyboard(storyboardName, bundle: bundle ?? Bundle(for: VCType.self))
        self.identifier = identifier
    }

    public func instantiate() -> VCType {
        return storyboard.instantiate(VCType.self, identifier: identifier)!
    }
}