How do I record a video on iOS without using a pre

The simpler way to record a video on iOS is by setting a AVCaptureSession.sessionPreset.

But that doesn't work for me since I want to control parameters like binning, stabilization (cinematic, standard, or none) and ISO.

I find the format I want and assign it to activeFormat, but when I try to start recording, I get an error:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 
'*** -[AVCaptureMovieFileOutput startRecordingToOutputFileURL:recordingDelegate:] No active/enabled connections'

Here is my initialisation code:

let device = AVCaptureDevice.defaultDevice(
    withDeviceType: .builtInWideAngleCamera,
    mediaType: AVMediaTypeVideo,
    position: .back)!
let session = AVCaptureSession()
session.addInput(try! AVCaptureDeviceInput(device: device))
output = AVCaptureMovieFileOutput()
session.addOutput(output)
device.setFormatWithHighestIso()
session.startRunning()

setFormatWithHighestIso() is defined as:

extension AVCaptureDevice {
  var goodVideoFormats: [AVCaptureDeviceFormat] {
    return (formats as! [AVCaptureDeviceFormat])
      .filter { CMFormatDescriptionGetMediaSubType($0.formatDescription) != 875704422 } // 420f
      .filter { $0.autoFocusSystem == .phaseDetection }
  }

  func setFormatWithHighestIso() {
    let format = goodVideoFormats
      .filter { $0.maxISO > 1759 }
      .filter { $0.height < 1937 }
      .first!

    try! lockForConfiguration()
    defer { unlockForConfiguration() }
    activeFormat = format
    NSLog("\(format)")
  }
}

The last log statement produces:

<AVCaptureDeviceFormat: 0x1702027d0 'vide'/'420f' 2592x1936, { 3- 30 fps}, HRSI:4032x3024, fov:58.986, max zoom:189.00 (upscales @1.56), AF System:2, ISO:22.0-1760.0, SS:0.000005-0.333333, supports wide color>

This is indeed the format I want, so setFormatWithHighestIso() is working as expected. See the Apple reference.

Some other things I tried:

Using 420v instead of 420f, by changing the == 875704422 to !=.
Instead of starting the camera in photo mode, starting it in video mode, and then changing it to video mode by removing the AVCapturePhotoOutput and adding the AVCaptureMovieFileOutput.
Verifying that the AVCaptureConnection is enabled, and it is.
Verifying that the connection is active, but it's not:

let conn = output.connection(withMediaType: AVMediaTypeVideo)! verify(conn.isActive)

I also tried using some other AVCaptureDeviceFormats, and they work:

extension AVCaptureDevice { 
  func setFormatWithCinematicVS() {
    let format = goodVideoFormats
      .filter { $0.isVideoStabilizationModeSupported(.cinematic) }
      .filter { $0.height == 720 }
      .first!

    try! lockForConfiguration()
    defer { unlockForConfiguration() }
    activeFormat = format
  }

  func setFormatWithStandardVS() {
    let format = goodVideoFormats
      .filter { $0.isVideoStabilizationModeSupported(.standard) }
      .filter { $0.height == 540 }
      .first!

    try! lockForConfiguration()
    defer { unlockForConfiguration() }
    activeFormat = format
  }
}

It's only the format with the highest ISO that doesn't work. What's special about this format?

Do I need to manually create an AVCaptureConnection? But there's already a connection; it's just not active.

This is on the iPhone 7 Plus running iOS 10.3.3. How do I record video in a specific format by setting the activeFormat without using a session?

If, instead of assigning to activeFormat, I use a sessionPreset, it does record a video successfully.

There are other questions talking of this error message, but this isn't a dupe of them since I specifically need to capture video without using a preset.

The solution turned out to be configuring the AVCaptureDevice before adding it to a session. Instead of:

session.addInput(try! AVCaptureDeviceInput(device: device))
output = AVCaptureMovieFileOutput()
session.addOutput(output)
device.setFormatWithHighestIso()

You should do:

device.setFormatWithHighestIso()  // Do this first!
session.addInput(try! AVCaptureDeviceInput(device: device))
output = AVCaptureMovieFileOutput()
session.addOutput(output)

When the device is added to the session, an AVCaptureConnection is created and configured in a certain way. If you change the device's resolution later, the configuration no longer matches, so the connection gets deactivated, and the video won't record.