Swift3.0
public extension DispatchQueue {
private static var _onceTracker = [String]()
public class func once(file: String = #file, function: String = #function, line: Int = #line, block:(Void)->Void) {
let token = file + ":" + function + ":" + String(line)
once(token: token, block: block)
}
/**
Executes a block of code, associated with a unique token, only once. The code is thread safe and will
only execute the code once even in the presence of multithreaded calls.
- parameter token: A unique reverse DNS style name such as com.vectorform.<name> or a GUID
- parameter block: Block to execute once
*/
public class func once(token: String, block:(Void)->Void) {
objc_sync_enter(self)
defer { objc_sync_exit(self) }
if _onceTracker.contains(token) {
return
}
_onceTracker.append(token)
block()
}
}