Alamofire http json request block ui

Question

I've been creating a function which retrieve objects from a JSON script. I've chosen for this to use alamofire for async request and swiftyJSON for easy parsing. However i seem to have a problem with it blocking the UI? How come it does that when it is async request? Do i need to run it on a separate thread or what could the explanation be?

Basically what i mean by blocking UI is that it does not react on other buttons before the below function is finished executing.

func getRecent() {


    var url = "http://URL/recent.php?lastid=\(lastObjectIndex)&limit=\(numberOfRecordsPerAPICall)"


    isApiCalling = true
    request(.GET, url, parameters: nil)
        .response { (request, response, data, error) in
            if error == nil {

        let data: AnyObject = data!
        let jsonArray = JSON(data: data as! NSData)
        if jsonArray.count < self.numberOfRecordsPerAPICall {
            self.recentCount = 0
            self.tableVIew.tableFooterView = nil
        } else {
            self.recentCount = jsonArray.count
            self.tableVIew.tableFooterView = self.footerView
        }
        for (key: String, subJson: JSON) in jsonArray {
            // Create an object and parse your JSON one by one to append it to your array
            var httpUrl = subJson["image_url"].stringValue
            let url = NSURL(string: httpUrl)
            let data = NSData(contentsOfURL: url!)

            if UIImage(data: data!) != nil {

            // Create an object and parse your JSON one by one to append it to your array
            var newNewsObject = News(id: subJson["id"].intValue, title: subJson["title"].stringValue, link: subJson["url"].stringValue, imageLink: UIImage(data: data!)!, summary: subJson["news_text"].stringValue, date: self.getDate(subJson["date"].stringValue))







            self.recentArray.append(newNewsObject)

            }
        }

        self.lastObjectIndex = self.lastObjectIndex + self.numberOfRecordsPerAPICall
        self.isApiCalling = false
        self.tableVIew.reloadData()
        self.refreshControl?.endRefreshing()

        }
        }

}

Answer 1

The response closure is executed on the main thread. If you are doing your JSON parsing there (and you have a large amount of data) it will block the main thread for a while.

In that case, you should use dispatch_async for the JSON parsing and only when you are completed update the main thread.

Just do your parsing like this

func getRecent() {

var url = "http://URL/recent.php?lastid=\(lastObjectIndex)&limit=\(numberOfRecordsPerAPICall)"


isApiCalling = true
request(.GET, url, parameters: nil)
    .response { (request, response, data, error) in
        if error == nil {

    let priority = DISPATCH_QUEUE_PRIORITY_DEFAULT
    dispatch_async(dispatch_get_global_queue(priority, 0)) {

        // Parse stuff here

        let data: AnyObject = data!
        let jsonArray = JSON(data: data as! NSData)
        if jsonArray.count < self.numberOfRecordsPerAPICall {
            self.recentCount = 0
            self.tableVIew.tableFooterView = nil
        } else {
            self.recentCount = jsonArray.count
            self.tableVIew.tableFooterView = self.footerView
        }
        for (key: String, subJson: JSON) in jsonArray {
            // Create an object and parse your JSON one by one to append it to your array
            var httpUrl = subJson["image_url"].stringValue
            let url = NSURL(string: httpUrl)
            let data = NSData(contentsOfURL: url!)

            if UIImage(data: data!) != nil {

            // Create an object and parse your JSON one by one to append it to your array
            var newNewsObject = News(id: subJson["id"].intValue, title: subJson["title"].stringValue, link: subJson["url"].stringValue, imageLink: UIImage(data: data!)!, summary: subJson["news_text"].stringValue, date: self.getDate(subJson["date"].stringValue))

            self.recentArray.append(newNewsObject)
            }
        }

        dispatch_async(dispatch_get_main_queue()) {

            // Update your UI here

            self.lastObjectIndex = self.lastObjectIndex + self.numberOfRecordsPerAPICall
            self.isApiCalling = false
            self.tableVIew.reloadData()
            self.refreshControl?.endRefreshing()
        }
    }
    }
    }
}

Answer 2

Swift5 An update to Stefan Salatic's answer, if you are parsing a large amount of json data from the Alamofire response it is better you use a global dispatch Queue and if for any reason you need to update the UI in the main thread switch to DispatchQueue.main.async.

So a sample code will look like this.

AF.request(UrlGetLayers, method: .post, parameters: parameters, headers: headers)
            .responseJSON { response in
             DispatchQueue.global(qos: .background).async {
                
                //parse your json response here
                
                //oops... we need to update the main thread again after parsing json
                DispatchQueue.main.async {
                    
                }
                
             }

        }