问题
Using XCode-8.2.1, Swift-3.0.2 and iOS-10.2.1,
I am trying to call two different URLSession.shared.dataTasks (the first is a simple URL-request and the second is a POST-request).
Since my first dataTask delivers a result that is needed in the httpBody of the second dataTask, the two URLSession.shared.dataTasks shall run in series, one after the other! (and also the preparative code shall run consecutively).
I tried, so far, using two consecutive serialQueue.sync{} queues. But I had to realize that the code does not perform in the order I would like to.
The print-statement in the log turn out to be as follows:
Hmmmmmm 2
Hmmmmmm 1
Hmmmmmm 3
(instead of 1, 2, 3 as needed)!
How can you get the order 1, 2, 3 ??
(i.e. how can you make sure the httpBody of the second dataTask can be filled with a result coming from the first dataTask ?)
Here is my code: (not executable as URL's were taken out - but you get the point)!
import UIKit
class ViewController: UIViewController {
let serialQueue = DispatchQueue(label: "myResourceQueue")
var stationID: Int = 0
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
self.serialQueue.sync {
let myResourceURL = URL(string: "myQueryString1")
let task = URLSession.shared.dataTask(with: myResourceURL!) { (data, response, error) in
if (error != nil) {
// print(error.debugDescription)
} else {
if let myData = data {
do {
let myJson = try JSONSerialization.jsonObject(with: myData, options: JSONSerialization.ReadingOptions.mutableContainers) as AnyObject
// print(myJson)
// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
print("Hmmmmmm 1")
// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
} catch {
// error
}
}
}
}
task.resume()
}
self.serialQueue.sync {
var request = URLRequest(url: URL(string: "myQueryString2")!)
request.httpMethod = "POST"
request.addValue("API_id", forHTTPHeaderField: "Authorization")
request.addValue("application/xml", forHTTPHeaderField: "Content-Type")
// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
print("Hmmmmmm 2")
// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
let postString: String = "My_XML_POST_Body"
request.httpBody = postString.data(using: .utf8)
let task = URLSession.shared.dataTask(with: request) { data, response, error in
// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
print("Hmmmmmm 3")
// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
}
task.resume()
}
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}
Any help appreciated!
回答1:
I finally found a solution:
Inspired by this answer, I introduced a URLSessionDataDelegate, together with its delegate callback-methods (i.e. didReceive response:, didReceive data: and didCompleteWithError error:.
Important: You need to set up your URLSession with a delegate in order to make the introduced URLSessionDelegate's callback methods work: Use URLSession(configuration: ....) for this like shown here:
let URLSessionConfig = URLSessionConfiguration.default
let session = URLSession(configuration: URLSessionConfig, delegate: self, delegateQueue: OperationQueue.main)
After that, you are good to go, i.e. the log is as expected now:
Hmmmmmm 1
Hmmmmmm 2
Hmmmmmm 3
Here is the final code (again not executable as URL's were taken out - but you get the point)!
import UIKit
class ViewController: UIViewController, URLSessionDataDelegate {
var stationID: Int = 0
let URLSessionConfig = URLSessionConfiguration.default
var session: URLSession?
var task1: URLSessionTask?
var task2: URLSessionTask?
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
self.session = URLSession(configuration: URLSessionConfig, delegate: self, delegateQueue: OperationQueue.main)
// prepare dataTask Nr 1
let myResourceURL = URL(string: "myQueryString1")
self.task1 = session?.dataTask(with: myResourceURL!)
// start dataTask Nr 1 (URL-request)
self.task1?.resume()
}
// Optional: Use this method if you want to get a response-size information
func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive response: URLResponse, completionHandler: @escaping (URLSession.ResponseDisposition) -> Void) {
// print(Int(response.expectedContentLength))
completionHandler(URLSession.ResponseDisposition.allow)
}
func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
if dataTask == self.task1 {
do {
let myJson = try JSONSerialization.jsonObject(with: myData, options: JSONSerialization.ReadingOptions.mutableContainers) as AnyObject
// print(myJson)
// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
print("Hmmmmmm 1")
// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
// prepare dataTask Nr 2
self.task2 = self.session?.dataTask(with: self.prepareMyURLRequest())
} catch {
// error
}
} else if dataTask == self.task2 {
// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
print("Hmmmmmm 3")
// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
} else {
print("unknown dataTask callback")
}
}
func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
if (error != nil) {
// print(error.debugDescription)
} else if task == self.task1 {
// start dataTask Nr 2 (POST URL-request)
self.task2?.resume()
}
}
func prepareMyURLRequest() -> URLRequest {
var request = URLRequest(url: URL(string: "myQueryString2")!)
request.httpMethod = "POST"
request.addValue("API_id", forHTTPHeaderField: "Authorization")
request.addValue("application/xml", forHTTPHeaderField: "Content-Type")
// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
print("Hmmmmmm 2")
// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
let postString: String = "My_XML_POST_Body"
request.httpBody = postString.data(using: .utf8)
return request
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}
回答2:
If task2 needs the results from task1 you should start task2 from the completion block of task1
URLSession.shared.dataTask(with: request) { data, response, error in
// process task1 and setup request2
URLSession.shared.dataTask(with: request2) { data, response, error in
// process task2
}.resume()
}.resume()
Of course, this will get a bit unwieldy with multiple requests, so it might be better to use Promises & Futures. There are several implementations of Promises & Futures for Swift, e.g. Promis.
来源：https://stackoverflow.com/questions/42056365/how-to-sync-serial-queue-for-urlsession-tasks