iOS UIDatePicker在UIAlertAction上的样式问题

为了方便操作，在UIAlertAction上直接添加了UIDatePicker控件，这样弹出日期控件直接选择，方便快捷, 在升级了手机系统（iOS14）后，发现样式变了！

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这是正常的样子：

@objc func startAction(sender: UITapGestureRecognizer) {
        let message = "\n\n\n\n\n\n"
        let alert = UIAlertController(title: "开始时间", message: message, preferredStyle: UIAlertController.Style.actionSheet)
        alert.isModalInPopover = true
        let datePicker = UIDatePicker(frame: CGRect(x: 10, y: 20, width: screenWidth() - 40, height: 160))
        datePicker.datePickerMode = .date
        datePicker.addTarget(self, action: #selector(startDatePickerValueChanged(datePicker:)), for: .valueChanged)
        alert.view.addSubview(datePicker)
        let okAction = UIAlertAction(title: "确定", style: .default, handler: {
            (alert:UIAlertAction!) -> Void in
             //TODO:

            }
        })
        alert.addAction(okAction)
        let cancelAction = UIAlertAction(title: "取消", style: .destructive, handler: nil)
        alert.addAction(cancelAction)
        self.parent?.present(alert, animated: true, completion: nil)
}

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这是升级了手机系统（iOS14）后的样子：

截屏2020-09-29 下午6.35.54.png

很显然这在新的系统上的变了样子，通过在查找及测试，最后发现确实是UIDatePickerStyle在不同的系统(iOS 13.4以上)下造成的

@available(iOS 13.4, *)
public enum UIDatePickerStyle : Int {
    /// Automatically pick the best style available for the current platform & mode.
    case automatic = 0

    /// Use the wheels (UIPickerView) style. Editing occurs inline.
    case wheels = 1

    /// Use a compact style for the date picker. Editing occurs in an overlay.
    case compact = 2

    /// Use a style for the date picker that allows editing in place.
    @available(iOS 14.0, *)
    case inline = 3
}

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End：给UIDatePicker添加UIDatePickerStyle属性值就

---
if #available(iOS 13.4, *) {
      datePicker.preferredDatePickerStyle = .wheels
  }
---

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关于UIDatePicker的其他样式和属性，自己可以试试，比如：

datePicker.preferredDatePickerStyle = .inline

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datePicker.preferredDatePickerStyle = . compact

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