Make an asynchronous function blocking

callBlocking.swift

func callBlocking<T>(
    _ closure: @escaping (_ continuationHandler: @escaping (T) -> Void ) -> Void )
     -> T {
    let dispatchGroup = DispatchGroup()
    dispatchGroup.enter()
    
    var result:T? = nil
    closure { res in
        result = res
        dispatchGroup.leave()
    }
    
    dispatchGroup.wait()
    return result!
}