Activity indicator while creating csv file - swift

Question

I am trying to show an activity indicator while creating a csv file, but it does not show. I am guessing I should use dispatch_async somehow, but I cant figure out how to do this in swift 3.
var activityIndicator = UIActivityIndicatorView(activityIndicatorStyle: UIActivityIndicatorViewStyle.gray)
override func viewDidLoad() {
super.viewDidLoad()
// activity indicator
activityIndicator = UIActivityIndicatorView(frame: CGRect(x: 100 ,y: 200,width: 50,height: 50)) as UIActivityIndicatorView
activityIndicator.hidesWhenStopped = true
activityIndicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyle.gray
activityIndicator.center = self.view.center
self.view.addSubview(activityIndicator)
}
func writeToCsv() {
self.activityIndicator.startAnimating() // start the animation
let fileName = "events.csv"
let path = NSURL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(fileName)
var csvText = self.name! + "\n"
csvText += "Date,Start time,End time\n"
// create rest of comma-separated string
for event in self.events! {
let newLine = "\(event.date),\(event.startTime),\(event.endTime)\n"
csvText.append(newLine)
}
// write to csv
do {
try csvText.write(to: path!, atomically: true, encoding: String.Encoding.utf8)
} catch {
print("Failed to create file")
print(error)
}
// create and present view controller with send options
let vc = UIActivityViewController(activityItems: [path as Any], applicationActivities: [])
self.present(vc, animated: true, completion: nil)
self.activityIndicator.stopAnimating() // stop the animation
}

Answer 1

Err, alright bit hard to answer this without a bit more context about your view setup. First of all, make sure your activity indicator is visible without calling the writeCsv method, so you know your view hierarchy is correct. ( I.E. It could be that it is hidden behind some other subview )
Next, in Swift3 Dispatch has been changed to a newer API. I'm not sure whether on OSX they use the raw libdispatch Swift wrapper, but in any case you access it like this:
Background default queue:
DispatchQueue.global(qos: DispatchQoS.QoSClass.default).async { /* code */ }
Main thread:
DispatchQueue.main.async { /* Mainthread code ( UIKit stuff ) */ }
Your own custom queue for CSV generation blocks:
let queue = DispatchQueue(label: "csvgenerator.queue")
queue.async { /* code */ }
Now for your animating / stopAnimation, make sure you call your UIKit related code from the mainthread to prevent weird glitechs and or crashes
Namely:
DispatchQueue.main.async {
self.activityIndicator?.startAnimating()
}
Another good idea might be to use NSOperationQueue instead. It internally uses GCD I believe, but it does integrate very well into iOS and might make some of the dispatching a lot easier to implement. I myself always use GCD instead, but I have never really had long queeu's of work that needed to be done. One of the advantages of NSOperationQueue is that it is a lot more user friendly in terms of cancelling dispatch blocks.
An interesting session video about NSOperationQueue in the WWDC app written by Dave Delong: WWDC Videos 2015
A small minor changes I'd make to your writeCSV method:
guard let path = NSURL(fileURLWithPath: NSTemporaryDirectory()).appendingPathComponent(fileName) else {
// Should throw an error here, or whatever is handy for your app
return
}
Try to avoid forced unwrapping at all stages where possible.
In methods that have this, you can for instance add "throws" to the end of the function definition so you can use try without the do and catch block, while also being able to throw errors in your guard statement so whatever calls writeCsv can catch the error and more easily display it to the user.