Swift 3: Execute multiple threads and wait for finalize

Question

I'm trying to send my API multiple photos iterating an image array. I want to send each image in a different thread but when the fist thread finish it doesn't wait for the rest of the threads and only one picture is sended.

This is my code:

// create the concurrent queue
    let asyncQueue = DispatchQueue(label: "asyncQueue", attributes: .concurrent)

    // perform the task asynchronously
    for image in self.arrayImages{
        asyncQueue.async() {
            let strBase64:String = image.base64EncodedString(options: .lineLength64Characters)
            self.sendImage(image: strBase64)
        }

    }

I've recently started programming in Swift and I don't know how to synchronize threads. Could you help me, please?

The code to do my requests is:

// Función para mandar a la API la petición de añadir una imagen a la idea
func sendImage(image:String){
    let data = NSMutableDictionary()

    let id:Int = (idea?.id)!
    let pasoAct = idea?.pasoActual
    data["id_idea"] = id
    data["id_paso"] = pasoAct
    data["contenido"] = image

    let jsonData = try! JSONSerialization.data(withJSONObject: data, options: JSONSerialization.WritingOptions(rawValue: 0))
    let request = NSMutableURLRequest(url: NSURL(string:"http://192.168.2.101:3001/api/imagen") as! URL)
    request.httpMethod = "POST"
    request.addValue("application/json", forHTTPHeaderField: "Content-Type")
    request.addValue("application/json", forHTTPHeaderField: "Accept")
    request.httpBody = jsonData

    URLSession.shared.dataTask(with: request as URLRequest, completionHandler: self.responseImage).resume()
}
// Función de control de la respuesta del servidor tras enviar una imagen
func responseImage(data: Data?, response: URLResponse?, error: Error?) {

    if (error != nil) {
        print("No se ha podido contactar con el servidor")
    }
    else{
        // Connected, check response, GET status codes.
        let statusCode = (response as! HTTPURLResponse).statusCode

        switch statusCode{
            case 200:
                print("Imagen subida")

            default:
                print("Error en la petición al servidor")
        }
    }
}

The first image is uploaded correctly (response returns status code=200) but next images don't

This is the code I've modified following your advice:

func sendImage(image:String, completion: @escaping () -> Void) {

    let data = NSMutableDictionary()

    let id:Int = (idea?.id)!
    let pasoAct = idea?.pasoActual
    data["id_idea"] = id
    data["id_paso"] = pasoAct
    data["contenido"] = image

    let jsonData = try! JSONSerialization.data(withJSONObject: data, options: JSONSerialization.WritingOptions(rawValue: 0))
    let request = NSMutableURLRequest(url: NSURL(string:"http://192.168.2.101:3001/api/imagen") as! URL)
    request.httpMethod = "POST"
    request.addValue("application/json", forHTTPHeaderField: "Content-Type")
    request.addValue("application/json", forHTTPHeaderField: "Accept")
    request.httpBody = jsonData

    URLSession.shared.dataTask(with: request as URLRequest) { (data, response, error) in
        self.responseImage(data: data, response: response, error: error)

        // When responseImage is complete, call the completion handler
        completion()
    }
}


// And the following code is called when I try to send the array of images
let group = DispatchGroup()
    let asyncQueue = DispatchQueue(label: "asyncQueue", attributes: .concurrent)
    for image in self.arrayImages {

        asyncQueue.async {
            group.enter()

            let strBase64:String = image.base64EncodedString(options: .lineLength64Characters)
            self.sendImage(image: strBase64) {
                group.leave()
            }
        }
    }

    group.wait()

Code Different · Accepted Answer · 2017-03-08 13:36:20Z

2

In your case, it may be better to use concurrentPerform:

DispatchQueue.concurrentPerform(iterations: self.arrayImages.count) { i in
    let strBase64 = self.arrayImages[i].base64EncodedString(options: .lineLength64Characters)
    self.sendImage(image: strBase64)
}

print("Done") // This line will only be executed after self.send() is
              // called on all images. However, this does not mean that
              // the server has received all the images.

edited Mar 8, 2017 at 13:36

answered Mar 8, 2017 at 13:22

Code Different

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1 Comment

Daij-Djan Over a year ago

agreed. in case all the asyncs basically execute the same code, I'd also recommend this

Ashley Mills · Accepted Answer · 2017-03-08 17:53:14Z

2

You can use a DispatchGroup…

let group = DispatchGroup()

for image in self.arrayImages {

    let workItem = DispatchWorkItem(qos: .default, flags: []) {
        let strBase64:String = image.base64EncodedString(options: .lineLength64Characters)
        self.sendImage(image: strBase64)
    }
    // perform the task asynchronously
    group.notify(queue: asyncQueue, work: workItem)
}

group.wait()

// Done!!

EDIT: Now you've updated your question, I can see you're making another async call. You can handle that as follows:

func sendImage(image:String, completion: @escaping () -> Void) {

        // Your code here

        URLSession.shared.dataTask(with: URL()) { (data, response, error) in
            self.responseImage(data: data, response: response, error: error)

            // When responseImage is complete, call the completion handler
            completion()
         }

}

let group = DispatchGroup()

for image in self.arrayImages {

    asyncQueue.async {
        group.enter()

        let strBase64:String = image.base64EncodedString(options: .lineLength64Characters)
        self.sendImage(image: strBase64) {
            group.leave()
        }
    }
}

group.wait()

// Done!!

A simple rule I like to follow, is to always add a completion handler as a parameter to every async func that I write.

edited Mar 8, 2017 at 17:53

answered Mar 8, 2017 at 13:25

Ashley Mills

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4 Comments

Daij-Djan Over a year ago

while in this case id use DispatchQueue.concurrentPerform this answer has value in showing a general approach

Álvaro Fernández Over a year ago

didn't work for me. The first pic is uploaded an then the response to my API fails

Álvaro Fernández Over a year ago

I send my images as a 64Bytes string to my API, and I need to start a thread for each request because my API throws a "request entity too large" error

Álvaro Fernández Over a year ago

I've followed your advice but it doesn't seem to work for me. I've only changed a line because I need to send a request instead an URL. I'm going to edit my question so you can watch it

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