监控UIDatePicker滚动事件/date值的变化

由于UIDatePicker没有回调事件，所以不能通过回调获得date值的实时变化。
对于一些需要实时监控其date值的场景，可以通过以下方法实现实时监控：

override init(frame: CGRect) {
        super.init(frame: frame)
    ...
       let datePicker = UIDatePicker(frame: pickerFrame)
      datePicker.addTarget(self, action: #selector(datePickerValueChange(_ :)), for: UIControlEvents.valueChanged)
}

@objc func datePickerValueChange(_ picker: UIDatePicker) {
        debugPrint("datePickerValueChange........")
        if let dateStr = JKTools.stringFromDate(picker.date, format: "yyyy-MM-dd") {
            date_label.text = dateStr
        }
    }

需要注意的是，不能通过KVO来监控date值的变化。以下方式不能得到响应：

override init(frame: CGRect) {
        super.init(frame: frame)
    ...
        datePicker.addObserver(self, forKeyPath: "date", options: .new, context: &dpContext)
}

 deinit {
       datePicker.removeObserver(self, forKeyPath: "date", context: &dpContext)
}

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        debugPrint("observeValue........")//没有执行
    }