Dateのextensionに依存性注入してみる

DateExtension.swift

extension Date {
    struct Extension {
        let date: Date

        struct Dependencies {
            let calendar: Calendar
            let locale: Locale
            let timeZone: TimeZone
        }

        let dateFormatter: DateFormatter

        init(date: Date, dependencies: Dependencies? = nil) {
            self.date = date

            self.dateFormatter = {
                let formatter = DateFormatter()
                formatter.calendar = dependencies?.calendar ?? Calendar.current
                formatter.locale = dependencies?.locale ?? Locale.current
                formatter.timeZone = dependencies?.timeZone ?? TimeZone.current
                return formatter
            }()
        }

        func string(dateStyle: DateFormatter.Style = .none,
                    timeStyle: DateFormatter.Style = .none) -> String
        {
            dateFormatter.dateStyle = dateStyle
            dateFormatter.timeStyle = timeStyle

            return dateFormatter.string(from: date)
        }
    }

    var ex: Extension {
        return Extension(date: self)
    }

    // 依存性注入
    func ex(calendar: Calendar,
            locale: Locale,
            timeZone: TimeZone) -> Extension
    {
        let dependencies = Extension.Dependencies(
            calendar: calendar, locale: locale, timeZone: timeZone)

        return Extension(date: self, dependencies: dependencies)
    }
}