Andrew Harris Andrew Harris - 2 years ago 127
Swift Question

Why does the iPhone seem to freeze when using a while loop with swift?

I am trying to take a picture every 2 seconds by using a while loop. but when I try this the screen freezes.

This is the function that takes the photo:

func didPressTakePhoto(){

if let videoConnection = stillImageOutput?.connectionWithMediaType(AVMediaTypeVideo){
videoConnection.videoOrientation = AVCaptureVideoOrientation.Portrait
stillImageOutput?.captureStillImageAsynchronouslyFromConnection(videoConnection, completionHandler: {
(sampleBuffer, error) in

if sampleBuffer != nil {

let imageData = AVCaptureStillImageOutput.jpegStillImageNSDataRepresentation(sampleBuffer)
let dataProvider = CGDataProviderCreateWithCFData(imageData)
let cgImageRef = CGImageCreateWithJPEGDataProvider(dataProvider, nil, true, .RenderingIntentDefault)

let image = UIImage(CGImage: cgImageRef!, scale: 1.0, orientation: UIImageOrientation.Right)

UIImageWriteToSavedPhotosAlbum(image, nil, nil, nil)

//Adds every image taken to an array each time the while loop loops which will then be used to create a timelapse.




To take the picture I have a button which will use this function in a while loop when a variable called count is equal to 0, but when the end button is pressed, this variable is equal to 1, so the while loop ends.

This is what the startPictureButton action looks like:

@IBAction func TakeScreanshotClick(sender: AnyObject) {

TipsView.hidden = true
XBtnTips.hidden = true

self.takePictureBtn.hidden = true

self.stopBtn.hidden = false

controls.hidden = true
ExitBtn.hidden = true

PressedLbl.text = "Started"

while count == 0{


pressed = pressed + 1
PressedLbl.text = "\(pressed)"




But when I run this and start the timelapse the screen looks frozen.

Does anyone know how to stop the freeze from happening - but also to add each image taken to an array - so that I can turn that into a video?

Answer Source

The problem is that the method that processes clicks on the button (TakeScreanshotClick method) is run on the UI thread. So, if this method never exits, the UI thread gets stuck in it, and the UI freezes.

In order to avoid it, you can run your loop on the background thread (read about NSOperation and NSOperationQueue). Occasionally you might need to dispatch something from the background thread to the UI thread (for instance, commands for UI updates).

UPDATE: Apple has a really great documentation (best of what I've seen so far). Have a look at this: Apple Concurrency Programming Guide.

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