iOS development

Cache Formatters for Efficiency
Creating a date formatter is not a cheap operation. If you are likely to use a formatter frequently, it is typically more efficient to cache a single instance than to create and dispose of multiple instances. One approach is to use a static variable.

If you cache date formatters (or any other objects that depend on the user’s current locale), you should subscribe to the NSCurrentLocaleDidChangeNotification notification and update your cached objects when the current locale changes. The code in Listing 3 defines sUserVisibleDateFormatter outside of the method so that other code, not shown, can update it as necessary. In contrast, sRFC3339DateFormatter is defined inside the method because, by design, it is not dependent on the user’s locale settings.

Reference: Date Formatters