A playground demo to demonstrate a common race condition

RaceCondition.swift

import UIKit

public extension DateFormatter {

    static let iso8601withFractionalSeconds: DateFormatter = {
        let formatter = DateFormatter()
        formatter.calendar = Calendar(identifier: .iso8601)
        formatter.locale = Locale(identifier: "en_US_POSIX")
        formatter.timeZone = TimeZone(secondsFromGMT: 0)
        formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSXXXXX"
        return formatter
    }()
}

let naive = DateFormatter.iso8601withFractionalSeconds

print("-naive: yay!, I can access the iso8601withFractionalSeconds and trust that it will be: ", naive, " unless I change it in the scope that I am in of course!")

extension DateFormatter {
    static var yyyyMMDDFormat: DateFormatter {
        let new = DateFormatter.iso8601withFractionalSeconds
        new.dateFormat = "yyyy-MM-dd"
        return new
    }
}

let newFormatter = DateFormatter.yyyyMMDDFormat
print("-other naive: cool I've got a Dateformatter that I can trust with the yyyy-MM-dd format: ", newFormatter)

print("-naive: wth! when I display the date, it should be using \("yyyy-MM-dd'T'HH:mm:ss.SSSXXXXX") but its actually using: yyyy-MM-dd", naive)