How can I implement a refresh button in my app that displays an activity indicator when pressed?

I have written the following code:

In this app, I am using JSON to retrieve data from a server and I want users to see updated content when they press the refresh button.

Thank you!

// Here is some sample code for implementing a refresh button with an activity indicator

import UIKit
import CoreData

class ViewController: UIViewController, UITableViewDataSource, UITableViewDelegate {

    // URL for fetching JSON data
    var json_data_url = "http://example/json_table_view_images%20(1).json"
    
    // Base URL for retrieving image resources
    var image_base_url = ""

    // Array to store table data fetched from JSON
    var TableData:Array< datastruct > = Array < datastruct >()

    enum ErrorHandler:ErrorType {
        case ErrorFetchingResults
    }

    struct datastruct {
        var imageurl:String?
        var description:String?
        var image:UIImage? = nil

        init(add: NSDictionary) {
            imageurl = add["url"] as? String
            description = add["description"] as? String
        }
    }

    @IBOutlet var tableview: UITableView!

    override func viewDidLoad() {
        super.viewDidLoad()

        // Set data source and delegate for table view
        tableview.dataSource = self
        tableview.delegate = self

        // Fetch data from URL
        get_data_from_url(json_data_url)
    }

    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCellWithIdentifier("cell", forIndexPath: indexPath)

        let data = TableData[indexPath.row]

        cell.textLabel?.text = data.description

        if (data.image == nil) {
            // Set default image and load actual image asynchronously
            cell.imageView?.image = UIImage(named:"image.jpg")
            load_image(image_base_url + data.imageurl!, imageview: cell.imageView!, index: indexPath.row)
        } else {
            // Use cached image if available
            cell.imageView?.image = TableData[indexPath.row].image
        }

        return cell

    }

    func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return TableData.count
    }

    func get_data_from_url(url:String) {

        let url:NSURL = NSURL(string: url)!
        let session = NSURLSession.sharedSession()

        let request = NSMutableURLRequest(URL: url)
        request.HTTPMethod = "GET"
        request.cachePolicy = NSURLRequestCachePolicy.ReloadIgnoringCacheData

        let task = session.dataTaskWithRequest(request) { (let data, let response, let error) in

            guard let _:NSData = data, let _:NSURLResponse = response  where error == nil else {
                print("error")
                return
            }

            dispatch_async(dispatch_get_main_queue(), {
                self.extract_json(data!)
                return
            })
        }

        task.resume()
    }

    func extract_json(jsonData:NSData) {
        let json: AnyObject?
        do {
            json = try NSJSONSerialization.JSONObjectWithData(jsonData, options: [])
        } catch {
            json = nil
            return
        }

        if let list = json as? NSArray {
            for (var i = 0; i < list.count ; i++ ) {
                if let data_block = list[i] as? NSDictionary {
                    TableData.append(datastruct(add: data_block))
                }
            }

            do {
                try read()
            } catch {}

            do_table_refresh()
        }
    }

    func do_table_refresh() {
        dispatch_async(dispatch_get_main_queue(), {
            self.tableview.reloadData()
            return
        })
    }

    func load_image(urlString:String, imageview:UIImageView, index:NSInteger) {

        let url:NSURL = NSURL(string: urlString)!
        let session = NSURLSession.sharedSession()

        let task = session.downloadTaskWithURL(url) { (let location, let response, let error) in

            guard let _:NSURL = location, let _:NSURLResponse = response  where error == nil else {
                print("error")
                return
            }

            let imageData = NSData(contentsOfURL: location!)

            dispatch_async(dispatch_get_main_queue(), {
                self.TableData[index].image = UIImage(data: imageData!)
                self.save(index,image: self.TableData[index].image!)

                imageview.image = self.TableData[index].image
                return
            })

        }

        task.resume()
    }

    func read() throws {

        do {
            let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
            let managedContext = appDelegate.managedObjectContext!
            let fetchRequest = NSFetchRequest(entityName: "Images")

            let fetchedResults = try managedContext.executeFetchRequest(fetchRequest)

            for (var i=0; i < fetchedResults.count; i++) {
                let single_result = fetchedResults[i]
                let index = single_result.valueForKey("index") as! NSInteger
                let img: NSData? = single_result.valueForKey("image") as? NSData

                TableData[index].image = UIImage(data: img!)
            }
        } catch {
            print("error")
            throw ErrorHandler.ErrorFetchingResults
        }
    }

    func save(id:Int,image:UIImage) {
        let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
        let managedContext = appDelegate.managedObjectContext!

        let entity = NSEntityDescription.entityForName("Images", inManagedObjectContext: managedContext)
        let options = NSManagedObject(entity: entity!, insertIntoManagedObjectContext:managedContext)

        let newImageData = UIImageJPEGRepresentation(image,1)

        options.setValue(id, forKey: "index")
        options.setValue(newImageData, forKey: "image")

        do {
            try managedContext.save()
        } catch {
            print("error")
        }
    }
}