using AsyncOperation

Operation.swift

import AsyncOperation
        
      let op1 = AsyncBlockOperation() { operation in
            operation.state = .executing
            
            let urlString = URL(string: "http://jsonplaceholder.typicode.com/users/1")
            if let url = urlString {
                let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
                    if error != nil {
                        print(error)
                    } else {
                        if let usableData = data {
                            print("op1") //JSONSerialization
                            operation.state = .finished
                            
                        }
                    }
                }
                task.resume()
            }
        }
        let op2 = AsyncBlockOperation() { operation in
            operation.state = .executing
            
            let urlString = URL(string: "http://jsonplaceholder.typicode.com/users/2")
            if let url = urlString {
                let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
                    if error != nil {
                        print(error)
                    } else {
                        if let usableData = data {
                            print("op2") //JSONSerialization
                            operation.state = .finished
                            
                        }
                    }
                }
                task.resume()
            }
        }
        let op3 = AsyncBlockOperation() { operation in
            operation.state = .executing
            
            let urlString = URL(string: "http://jsonplaceholder.typicode.com/users/3")
            if let url = urlString {
                let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
                    if error != nil {
                        print(error)
                    } else {
                        if let usableData = data {
                            print("op3") //JSONSerialization
                            operation.state = .finished
                        }
                    }
                }
                task.resume()
            }
            
        }

        let queue = OperationQueue()
        queue.maxConcurrentOperationCount = 1
        queue.addOperations([op1,op2,op3], waitUntilFinished: true)