iOS Swift中如何判断Inline样式UIDatePicker切换至年月视图？

一、判断是否处于年月选择状态 #

UIDatePicker没有直接提供状态回调，得靠子视图检测或KVO实现：

1. KVO监听子视图变化（最可靠） #

Inline模式下，日历视图和年月滚轮是不同的子视图。当用户切换到年月选择时，picker会将子视图替换为UIPickerView，我们可以监听这个变化：

class YourViewController: UIViewController {
    @IBOutlet weak var datePicker: UIDatePicker!
    @IBOutlet weak var applyButton: UIButton!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        datePicker.preferredDatePickerStyle = .inline
        // 给picker添加KVO监听subviews变化
        datePicker.addObserver(self, forKeyPath: "subviews", options: [.new, .old], context: nil)
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        guard keyPath == "subviews", let picker = object as? UIDatePicker else { return }
        checkYearMonthMode(picker: picker)
    }
    
    private func checkYearMonthMode(picker: UIDatePicker) {
        // 子视图中存在UIPickerView，就说明处于年月选择状态
        let isInYearMonthMode = picker.subviews.contains { $0 is UIPickerView }
        applyButton.isEnabled = !isInYearMonthMode
    }
    
    deinit {
        datePicker.removeObserver(self, forKeyPath: "subviews")
    }
}

2. 结合日期变化回调辅助验证 #

担心KVO漏触发？可以再加个日期变化的监听，双重验证状态：

// 在viewDidLoad中添加
datePicker.addTarget(self, action: #selector(dateChanged(_:)), for: .valueChanged)

@objc private func dateChanged(_ picker: UIDatePicker) {
    checkYearMonthMode(picker: picker)
}

二、强制切回日历视图 #

要主动退出年月选择状态？直接给picker设置当前选中的日期即可，系统会自动切换回日历视图：

private func switchBackToCalendar() {
    let currentDate = datePicker.date
    datePicker.setDate(currentDate, animated: true)
}

比如用户点击Cancel按钮时，调用这个方法就能回到日历界面。

三、监听年月切换按钮的点击（有兼容性风险） #

如果非要监听左上角的年月按钮点击，可以遍历picker的子视图找到按钮，但注意：苹果可能会修改UI层级，这个方法不是100%稳定：

private func setupYearMonthButtonListener() {
    func findYearMonthButton(in view: UIView) -> UIButton? {
        for subview in view.subviews {
            if let button = subview as? UIButton {
                // 年月按钮的标题格式为"XXXX年XX月"，靠此判断
                if button.titleLabel?.text?.contains("年") == true {
                    return button
                }
            }
            if let foundButton = findYearMonthButton(in: subview) {
                return foundButton
            }
        }
        return nil
    }
    
    if let yearMonthButton = findYearMonthButton(in: datePicker) {
        yearMonthButton.addTarget(self, action: #selector(yearMonthButtonTapped), for: .touchUpInside)
    }
}

@objc private func yearMonthButtonTapped() {
    // 延迟检查状态，因为视图切换需要时间
    DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) { [weak self] in
        self?.checkYearMonthMode(picker: self?.datePicker ?? UIDatePicker())
    }
}

内容的提问来源于stack exchange，提问作者Muhammad Awais Jamil