Swift utility function to create and reuse a thread-local object

cachedThreadLocalObjectWithKey.swift

import Foundation

/// Return a thread-local object, creating it if it has not already been created
///
/// :param: create closure that will be invoked to create the object
/// :returns: object of type T
public func cachedThreadLocalObjectWithKey<T: AnyObject>(key: String, create: () -> T) -> T {
    if let threadDictionary = NSThread.currentThread().threadDictionary {
        if let cachedObject = threadDictionary[key] as T? {
            return cachedObject
        }
        else {
            let newObject = create()
            threadDictionary[key] = newObject
            return newObject
        }
    }
    else {
        assert(false, "threadDictionary should never be nil")

        // If we don't have a thread-local dictionary for some reason,
        // then just call create() on every call
        return create()
    }
}


// Example

/// Get a thread-local date formatter object
func getThreadLocalRFC3339DateFormatter() -> NSDateFormatter {
    return cachedThreadLocalObjectWithKey("net.kristopherjohnson.example.MyThreadLocalDateFormatter") {
        println("This block will only be executed once")
        let enUSPOSIXLocale = NSLocale(localeIdentifier: "en_US_POSIX")
        let rfc3339DateFormatter = NSDateFormatter()
        rfc3339DateFormatter.locale = enUSPOSIXLocale
        rfc3339DateFormatter.dateFormat = "yyyy'-'MM'-'dd'T'HH':'mm':'ss'Z'"
        rfc3339DateFormatter.timeZone = NSTimeZone(forSecondsFromGMT: 0)
        return rfc3339DateFormatter
    }
}

let x1 = getThreadLocalRFC3339DateFormatter()
let x2 = getThreadLocalRFC3339DateFormatter()
let x3 = getThreadLocalRFC3339DateFormatter()