はじめに

SNSなどでよくみる「〇日前」のような表示をSwiftで作成してみようと思います。

実装

extension Date {
    static let dateFormatter: DateFormatter = {
        let dateFormatter = DateFormatter()
        dateFormatter.dateStyle = .short
        dateFormatter.timeStyle = .none
        dateFormatter.locale = Locale(identifier: "ja_JP")
        return dateFormatter
    }()

    var elapsedTimeText: String {
        let components = Calendar.current.dateComponents([.year, .month, .weekOfYear, .day, .hour, .minute, .second], from: self, to: Date())

        if let year = components.year, year > 0 {
            return Date.dateFormatter.string(from: self)
        } else if let month = components.month, month > 0 {
            return "\(month)ヶ月前"
        } else if let week = components.weekOfYear, week > 0 {
            return "\(week)週間前"
        } else if let day = components.day, day > 0 {
            return "\(day)日前"
        } else if let hour = components.hour, hour > 0 {
            return "\(hour)時間前"
        } else if let minute = components.minute, minute > 0 {
            return "\(minute)分前"
        } else if let second = components.second, second > 0 {
            return "\(second)秒前"
        } else {
            return "たった今"
        }
    }
}

テストコード

    func testElapsedTimeText() {
        let now = Date()

        // 1年以上前の場合
        let oneYearAgo = Calendar.current.date(byAdding: .year, value: -1, to: now)!
        XCTAssertEqual(oneYearAgo.elapsedTimeText, Date.dateFormatter.string(from: oneYearAgo))

        // 11ヶ月前の場合
        let elevenMonthsAgo = Calendar.current.date(byAdding: .month, value: -11, to: now)!
        XCTAssertEqual(elevenMonthsAgo.elapsedTimeText, "11ヶ月前")

        // 1ヶ月前の場合
        let oneMonthAgo = Calendar.current.date(byAdding: .month, value: -1, to: now)!
        XCTAssertEqual(oneMonthAgo.elapsedTimeText, "1ヶ月前")

        // 2週間前の場合
        let twoWeeksAgo = Calendar.current.date(byAdding: .day, value: -14, to: now)!
        XCTAssertEqual(twoWeeksAgo.elapsedTimeText, "2週間前")

        // 1日前の場合
        let oneDayAgo = Calendar.current.date(byAdding: .day, value: -1, to: now)!
        XCTAssertEqual(oneDayAgo.elapsedTimeText, "1日前")

        // 2時間前の場合
        let twoHoursAgo = Calendar.current.date(byAdding: .hour, value: -2, to: now)!
        XCTAssertEqual(twoHoursAgo.elapsedTimeText, "2時間前")

        // 30分前の場合
        let thirtyMinutesAgo = Calendar.current.date(byAdding: .minute, value: -30, to: now)!
        XCTAssertEqual(thirtyMinutesAgo.elapsedTimeText, "30分前")

        // 10秒前の場合
        let tenSecondsAgo = Calendar.current.date(byAdding: .second, value: -10, to: now)!
        XCTAssertEqual(tenSecondsAgo.elapsedTimeText, "10秒前")

        // たった今の場合
        XCTAssertEqual(now.elapsedTimeText, "たった今")
    }

おわり

Dateの扱いは難しいです。
もっとこうしたほうがいいとかありましたらコメントください。。

参考記事