How do I use the metadataOutputRectOfInterestForRect method and rectOfInterest property to scan a specific area? (QR Code)

I wasn't really able to clarify the issue with metadataOutputRectOfInterestForRect, however, you can directly set the property as well. You need to the have the resolution in width and height of your video, which you can specify in advance. I quickly used the 640*480 setting. As stated in the documentation, these values have to be

"extending from (0,0) in the top left to (1,1) in the bottom right, relative to the device’s natural orientation".

See https://developer.apple.com/documentation/avfoundation/avcaptureoutput/1616304-metadataoutputrectofinterestforr

Below is the code I tried

var x = scanRect.origin.x/480
var y = scanRect.origin.y/640
var width = scanRect.width/480
var height = scanRect.height/640
var scanRectTransformed = CGRectMake(x, y, width, height)
captureMetadataOutput.rectOfInterest = scanRectTransformed

I just tested it on an iOS device and it seems to work.

Edit

At least I've solved the metadataOutputRectOfInterestForRect problem. I believe you have to do this after the camera has been properly set up and is running, as the camera's resolution is not yet available.

First, add a notification observer method within viewDidLoad()

NSNotificationCenter.defaultCenter().addObserver(self, selector: Selector("avCaptureInputPortFormatDescriptionDidChangeNotification:"), name:AVCaptureInputPortFormatDescriptionDidChangeNotification, object: nil)

Then add the following method

func avCaptureInputPortFormatDescriptionDidChangeNotification(notification: NSNotification) {

    captureMetadataOutput.rectOfInterest = videoPreviewLayer.metadataOutputRectOfInterestForRect(scanRect)

}

Here you can then reset the rectOfInterest property. Then, in your code, you can display the AVMetadataObject within the didOutputMetadataObjects function

var rect = videoPreviewLayer.rectForMetadataOutputRectOfInterest(YourAVMetadataObject.bounds)

dispatch_async(dispatch_get_main_queue(),{
     self.qrCodeFrameView.frame = rect
})

I've tried, and the rectangle was always within the specified area.

In iOS 9.3.2 I was able to make metadataoutputRectOfInterestForRect work calling it right after startRunning method of AVCaptureSession:

captureSession.startRunning()
let visibleRect = previewLayer.metadataOutputRectOfInterestForRect(previewLayer.bounds)
captureMetadataOutput.rectOfInterest = visibleRect

Swift 4:

captureSession?.startRunning()
let scanRect = CGRect(x: 0, y: 0, width: 100, height: 100)
let rectOfInterest = layer.metadataOutputRectConverted(fromLayerRect: scanRect)
metaDataOutput.rectOfInterest = rectOfInterest