ios - 从日期选择器开始的 Swift CountDown 天数

Question

我一直在努力在 Swift 中进行倒计时，它只显示距离输入为 DatePicker 的某个日期还剩多少天...我有 Swift 的 creo 经验，所以我一直在努力一段时间。我在这里尝试了一些类似的答案，但没有奏效，我看了一个教程，但这是一个正常的倒计时，有月、日、分和秒，这是代码。

import UIKit

class ViewController: UIViewController {

@IBOutlet weak var timeLabel: UILabel!

let formatter = DateFormatter()
let userCleander = Calendar.current;
let requestedComponent : Set<Calendar.Component> = [
    Calendar.Component.month,
    Calendar.Component.day
]

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

    let timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(timePrinter), userInfo: nil, repeats: true)

    timer.fire()
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

func timeCalculator(dateFormat: String, endTime: String, startTime: Date = Date()) -> DateComponents {
    formatter.dateFormat = dateFormat
    let _startTime = startTime
    let _endTime = formatter.date(from: endTime)

    let timeDifference = userCleander.dateComponents(requestedComponent, from: _startTime, to: _endTime!)
    return timeDifference
}

func timePrinter() -> Void {
    let time = timeCalculator(dateFormat: "MM/dd/yyyy a", endTime: "12/25/2018 a")
    timeLabel.text = "\(time.month!) Months \(time.day!) Days"
}

}

Answer 1

几件事:不要使用字符串来比较日期。使用日期对象和日历操作。 (稍后会详细介绍。)

不要每秒运行一次计时器。将当前日期保存为用户默认值。当您的应用启动时，将保存的日期与当前日期进行比较，看看日期是否发生了变化。

运行时，监听 UIApplicationSignificantTimeChange 通知，当收到通知时，检查日期是否已更改。

至于将当前日期与用户选择的日期进行比较，您使用 dateComponents(_:from:to:) 的想法是正确的，但是您应该传入 [.day].

编辑:

像这样的代码可以解决问题:

override func viewDidLoad() {
    super.viewDidLoad()
    //Set up the date picker to pick dates, not dates & times
    datePicker.datePickerMode =  .date
    
    //Force the date picker to use midnight today as it's base date and
    //to pick a date at least 1 day in the future
    guard let today = Calendar.current.date(bySettingHour: 0, minute: 0, second: 0, of: Date()),
        let tomorrow = Calendar.current.date(byAdding: .day, value: 1, to: today)
        else {
            return
    }
    datePicker.minimumDate = tomorrow
    datePicker.date = tomorrow
}

@IBAction func datePickerChanged(_ sender: UIDatePicker) {
    let future = datePicker.date
    //Use midnight today as the starting date
    guard let today = Calendar.current.date(bySettingHour: 0, minute: 0, second: 0, of: Date()) else { return }
    
    //Calculate the number of days between today and the user's chosen day.
    let difference = Calendar.current.dateComponents([.day], from: today, to: future)
    guard let days = difference.day else { return }
    let ess = days > 1 ? "s" : ""
    infoLabel.text = "That date is \(days) day\(ess) away."
}