Xcode - How to do a proper Date formatter in Swift

Miki
March 2, 2021
158 views
1 vote
2 Answers

I am not quiet sure how to make dateFormatter that is reusable, since I read that dateFormatter is really expensive to do multiple times and that I should reuse it if its possible. Currently my function looks like this:

    func formatDate(date: String) -> String {
    let dateFormatterGet = DateFormatter()

    //this is format I get from my response
    dateFormatterGet.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"
    let dateFormatterSet = DateFormatter()

    //this is format I would like to show on my screen
    dateFormatterSet.dateFormat = "EEE, dd MMM YYYY, HH:mm a"
    let dateObj: Date? = dateFormatterGet.date(from: date)
    return dateFormatterSet.string(from: dateObj!)
}

I wouldn’t be bothered if I would use it on one screen, but I am using on two, yet maybe its not to expensive either, but I want to know for future cases how to do it properly.

This is function from my other screen, only difference is dateFormatterSet, which I use for showing different format on my screen.

    func formatDate(date: String) -> String {
    let dateFormatterGet = DateFormatter()
    dateFormatterGet.dateFormat = "YYYY-MM-dd'T'HH:mm:ss.SSS'Z'"
    let dateFormatterSet = DateFormatter()
    dateFormatterSet.dateFormat = "EEE, dd MMM"
    guard let dateObj = dateFormatterGet.date(from: date) else { return "Start date 
    unknown"}
    return dateFormatterSet.string(from: dateObj)
}

Thanks in advance.

Answers

You don’t need two date formatters to convert one date string to another.

My suggestion is to create one persistent date formatter with a fixed locale

let dateFormatter : DateFormatter = {
    let formatter = DateFormatter()
    formatter.locale = Locale(identifier: "en_US_POSIX")
    return formatter
}()

This could be also an extension

extension DateFormatter {
    static let posixFormatter : DateFormatter = {
        let formatter = DateFormatter()
        formatter.locale = Locale(identifier: "en_US_POSIX")
        return formatter
    }()
}

And secondly a function with parameters for input format with a default value and output format

func formatDate(dateString: String, inFormat: String = "yyyy-MM-dd'T'HH:mm:ss.SSSZ", outFormat: String) -> String? {

    // let dateFormatter = DateFormatter.posixFormatter
   
    //this is format I get from my response
    dateFormatter.dateFormat = inFormat
    guard let date = dateFormatter.date(from: dateString) else { return nil }

    dateFormatter.dateFormat = outFormat
    return dateFormatter.string(from: date)
}

You have to check if a date could be created from the string so the return value is an optional.

Side note:

The Z in an ISO8601 string is the time zone specifier. It must not be wrapped in single quotes. And the year specifier is always lowercase yyyy.

I was reading about how expensive date formatting is and found a good article that proves why they are expensive and how they are expensive. Based on my understanding I created a solution by using a singleton class and an extension of Dateformatter. You can find it below.

Article I read: https://sarunw.com/posts/how-expensive-is-dateformatter/

class MyDateFormatter {
    static let sharedInstance = DateFormatter()
    private init() {
    }
}

extension DateFormatter{
    func getDateFrom(stringDate:String,format dateFormat:String)->Date?{
        let dateFormatter = MyDateFormatter.sharedInstance
        dateFormatter.dateFormat = dateFormat
        let dateObj: Date? = dateFormatter.date(from: stringDate)
        return dateObj
    }
    
    func getStringFrom(date:Date,format dateFormat:String)->String?{
        let dateFormatter = MyDateFormatter.sharedInstance
        dateFormatter.dateFormat = dateFormat
        let dateString: String? = dateFormatter.string(from: date)
        return dateString
    }
}

func formatDate(date: String) -> String {
    let dateObj:Date? = MyDateFormatter.sharedInstance.getDateFrom(stringDate: date, format: "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'")
    let formattedDate:String? = MyDateFormatter.sharedInstance.getStringFrom(date: dateObj!, format: "EEE, dd MMM YYYY, HH:mm a")
    return formattedDate ?? ""
}

formatDate(date: "2011-11-02T02:50:12.208Z")

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Xcode – How to do a proper Date formatter in Swift

Answers