override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    self.view.endEditing(true)
}

private func validateInputField() {
        self.confirmButton.isEnabled = !(self.titleTextField.text?.isEmpty ?? true) && !(self.dateTextField.text?.isEmpty ?? true) && !self.contentsTextView.text.isEmpty
    }

let x = something ?? 10 // 여기서 something은 Optional Int 타입

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if let wrietDiaryViewController = segue.destination as? WriteDiaryViewController {
        wrietDiaryViewController.delegate = self
    }
}

// MARK: - userDefaults에서 DiaryList를 가져오는 메서드
private func loadDiaryList() {
    let userDefaults = UserDefaults.standard
    if let data = userDefaults.value(forKey: "diary") as? Data {
        let realData = try? PropertyListDecoder().decode([Diary].self, from: data)
        self.diaryList = realData!
        self.diaryList = self.diaryList.sorted(by: { // 코드
            $0.date > $1.date
            // $0.date.compare($1.date) == .orderedDescending   이렇게도 작성할 수 있다.
        })
    }
}

private var diaryList = [Diary]() {
    didSet {
        saveDiaryList()
    }
}

override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
        super.traitCollectionDidChange(previousTraitCollection)
        if traitCollection.hasDifferentColorAppearance(comparedTo: previousTraitCollection) {
            print(previousTraitCollection?.userInterfaceStyle)
            print(UITraitCollection.current.userInterfaceStyle.rawValue)
            // enum이라 케이스로 나온다. 케이스별 분기처리
        }
    }

extension DiaryViewController: UICollectionViewDelegate {
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        ...
        DiaryDetailViewController.deleteDelegate = self
        DiaryDetailViewController.starDelegate = self
        ... self.navigationController?.pushViewController(DiaryDetailViewController, animated: true)
    }
}