Setting `device.activeVideoMinFrameDuration` and `device.activeVideoMaxFrameDuration` doesn't do anything - ios

Question

I'm making a customized camera app, and I'd like to get the camera input at 60fps (I don't care much about quality) and show it in a UIView.
What I have so far is the following:
struct CustomCameraView: UIViewRepresentable {
func makeUIView(context: Context) -> some UIView {
let previewView = UIImageView()
func configureCameraForHighestFrameRate(device: AVCaptureDevice) {
do {
try device.lockForConfiguration()
let fr = CMTimeMake(value: 1, timescale: 60)
device.activeVideoMinFrameDuration = fr
device.activeVideoMaxFrameDuration = fr
device.unlockForConfiguration()
} catch {
print("error setting frame rate")
}
}
func setupLivePreview() {
videoPreviewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
videoPreviewLayer.videoGravity = .resizeAspect
videoPreviewLayer.connection?.videoOrientation = .portrait
previewView.layer.addSublayer(videoPreviewLayer)
captureSession.commitConfiguration()
DispatchQueue.global(qos: .userInitiated).async {
captureSession.startRunning()
DispatchQueue.main.async {
videoPreviewLayer.frame = previewView.bounds
}
}
}
var captureSession: AVCaptureSession!
var videoPreviewLayer: AVCaptureVideoPreviewLayer!
captureSession = AVCaptureSession()
captureSession.beginConfiguration()
guard let backCamera = AVCaptureDevice.default(for: AVMediaType.video) else {
print("Unable to access back camera!")
return UIImageView()
}
configureCameraForHighestFrameRate(device: backCamera)
do {
let input = try AVCaptureDeviceInput(device: backCamera)
print("max frame duration on input",
input.device.activeVideoMaxFrameDuration)
if captureSession.canAddInput(input) {
captureSession.addInput(input)
setupLivePreview()
}
}
catch let error {
print("Error Unable to initialize back camera: \(error.localizedDescription)")
}
return previewView
}
func updateUIView(_ uiView: UIViewType, context: Context) {}
}
Pretty simple, I access the camera, set its activeVideoMinFrameDuration and activeVideoMaxFrameDurations and I use a AVCaptureVideoPreviewLayer to replace the UIImageView's CALayer.
I've tried changing the timescale of CMTime to 30, 20, 10, even 1 but it doesn't have any effect. What should I do?
I don't get any errors, and here's my system info:
iPhone 11 Pro
XCode Version 12.0.1 (12A7300)
iOS 14.0.1

Answer 1

Turns out Apple's documentation is wrong:
Directly configuring a capture device’s activeFormat property changes the capture session’s preset to inputPriority.
This is not true, I have to set the capture session's preset to .inputPriority for the captured device's configuration to take place. So adding this line solved the problem:
captureSession.sessionPreset = .inputPriority
I've made a post at the Apple Developer Forum, hopefully they'll take a look at it.