Where do Dispatch Group commands go in code? - swift

Question

I am trying to run a function X times in a for in loop but when all the functions have returned I want to run another function.
Currently I have it working by delaying the final function 1 second but I would really like to get Dispatch Group working.
I've been through various online examples and other questions but nothing I try seems to work, The code I have at the moment I know won't work as it is running dispatchGroup.leave() each time the for in functions are sent rather than when they return.
I've tried puting the DispatchGroup code in the function (which is in another file) but I'm stumped, however I think I am close to a solution.
I've also looked at semaphores and using count and incrementing a value each time the loop runs but I keep coming back to DispatchGroups.
My last resort is to ask a question!
ViewController code
#IBAction func removeDeviceBtn(_ sender: Any) {
let dispatchGroup = DispatchGroup()
for owner in arrOwnerList {
dispatchGroup.enter()
self.removeDevice(device: self.device, account: owner as! String, completion: self.completed)
dispatchGroup.leave()
}
dispatchGroup.notify(queue: DispatchQueue.main, execute: {
self.removeDeviceFromServer(device: self.device)
self.sendEmail(to:"gordon#example.co.uk", subject:self.device+" has been removed", text:self.device+" has been removed from the server, please check the sim for bar and termination")
})
Function code in other file as an extension
func completed(isSuccess: Bool) {
}
func removeDevice(device: String, account: String, completion: #escaping (Bool) -> Void) {
let dictHeader : [String:String] = ["username":Username,"password":Password]
let dictArray = [device]
WebHelper.requestPUTAPIRemoveDevice(BaseURL+"rootaccount/removedevices/"+account+"?server=MUIR", header: dictHeader, dictArray: dictArray, controllerView: self, success: { (response) in
if response.count == 0 {
DispatchQueue.main.async {
GlobalConstant.showAlertMessage(withOkButtonAndTitle: GlobalConstant.AppName, andMessage: Messages.ServerError, on: self)
}
}
else {
if response.count != 0 {
let isSuccess = true
completion(isSuccess)
}
else{
DispatchQueue.main.async {
GlobalConstant.showAlertMessage(withOkButtonAndTitle: GlobalConstant.AppName, andMessage: Messages.NoDataFound, on: self)
}
}
}
}) { (error) in
DispatchQueue.main.async {
GlobalConstant.showAlertMessage(withOkButtonAndTitle: GlobalConstant.AppName, andMessage: error?.localizedDescription ?? Messages.ServerError, on: self)
}
}
}
Code from WebHelper file
class func requestPUTAPIRemoveDevice(_ strURL: String,header: Dictionary<String,String>,dictArray: Array<Any>, controllerView viewController: UIViewController, success: #escaping (_ response: [AnyHashable: Any]) -> Void, failure: #escaping (_ error: Error?) -> Void) {
if GlobalConstant.isReachable() {
DispatchQueue.main.async {
LoadingIndicatorView.sharedInstance.showHUD()
}
let loginString = String(format: "%#:%#", header["username"]!, header["password"]!)
let loginData: Data = loginString.data(using: String.Encoding.utf8)!
let base64LoginString = loginData.base64EncodedString(options: NSData.Base64EncodingOptions())
let headers = ["Authorization": "Basic "+base64LoginString, "Referer": "http://www.example.com"]
let postData = try? JSONSerialization.data(withJSONObject: dictArray, options: [])
let request = NSMutableURLRequest(url: NSURL(string: strURL)! as URL,
cachePolicy: .useProtocolCachePolicy,
timeoutInterval: 10.0)
request.httpMethod = "PUT"
request.allHTTPHeaderFields = headers
request.httpBody = postData
let session = URLSession.shared
let dataTask = session.dataTask(with: request as URLRequest, completionHandler: { (data, response, error) -> Void in
if (error != nil) {
DispatchQueue.main.async {
LoadingIndicatorView.sharedInstance.hideHUD()
}
failure(error)
} else {
if let httpResponse = response as? HTTPURLResponse {
print("Server code \(httpResponse.statusCode)")
if httpResponse.statusCode == 200 || httpResponse.statusCode == 208 {
DispatchQueue.main.async {
LoadingIndicatorView.sharedInstance.hideHUD()
}
let jsonResult = try? JSONSerialization.jsonObject(with: data!, options: JSONSerialization.ReadingOptions.mutableContainers)
if (jsonResult is NSDictionary) {
success(jsonResult as! [AnyHashable : Any])
}
else if (jsonResult is NSArray) {
success(["response":jsonResult as! NSArray])
}
else{
success(["response":httpResponse.statusCode])
DispatchQueue.main.async {
}
}
}
else{
DispatchQueue.main.async {
LoadingIndicatorView.sharedInstance.hideHUD()
}
failure(error)
}
}
}
})
dataTask.resume()
}
else {
DispatchQueue.main.async {
LoadingIndicatorView.sharedInstance.hideHUD()
GlobalConstant.showAlertMessage(withOkButtonAndTitle: "", andMessage: "Internet not connected", on: viewController)
}
}
}
The final solution (apart from tidying up the various other issues) was to add success(["response":httpResponse.statusCode]) to the WebHelper file, corrected code above

Answer 1

Put the leave inside the completion handler:
for owner in arrOwnerList {
dispatchGroup.enter()
removeDevice(device: device, account: owner as! String) { [weak self] success in
self?.completed(isSuccess: success)
dispatchGroup.leave()
}
}
Or given that you’re not really doing anything in completed function, I’d just remove that:
for owner in arrOwnerList {
dispatchGroup.enter()
removeDevice(device: device, account: owner as! String) { _ in
dispatchGroup.leave()
}
}
I notice that you have paths of execution in removeDevice that aren’t calling the completion handler. Make sure every path of execution calls the completion handler or else your dispatch group will never get resolved.
func removeDevice(device: String, account: String, completion: #escaping (Bool) -> Void) {
let dictHeader = ["username": Username, "password": Password]
let dictArray = [device]
WebHelper.requestPUTAPIRemoveDevice(BaseURL+"rootaccount/removedevices/"+account+"?server=MUIR", header: dictHeader, dictArray: dictArray, controllerView: self, success: { response in
DispatchQueue.main.async {
if response.count == 0 {
GlobalConstant.showAlertMessage(withOkButtonAndTitle: GlobalConstant.AppName, andMessage: Messages.ServerError, on: self)
completion(false)
} else {
completion(true)
}
}
}, failure: { error in
DispatchQueue.main.async {
GlobalConstant.showAlertMessage(withOkButtonAndTitle: GlobalConstant.AppName, andMessage: error?.localizedDescription ?? Messages.ServerError, on: self)
completion(false)
}
})
}
By the way, I don’t know the name of the “failure” closure, so I assumed it was failure, but adjust as required by your requestPUTAPIRemoveDevice method. We generally avoid the multiple closure pattern in Swift, but if you’re going to do that, I’d avoid the trailing closure syntax. It makes the functional intent of this second closure a bit more explicit.
Or, this all begs the question as to why requestPUTAPIRemoveDevice is initiating UI updates at all. I’d probably put that in the view controller method. So requestPUTAPIRemoveDevice should just pass back enough information so the removeDeviceBtn routines knows what error to present. And this idea of presenting a separate error message for each failure is probably suspect, too. (E.g. if you have lost internet connection and are trying to remove a dozen devices, do you really want to show a dozen separate error messages?) But this is beyond the scope of this question.