How can I fail during an init? - swift

Question

I am trying to create this class:
class CaptureVideoDataOutput: AVCaptureVideoDataOutput, AVCaptureVideoDataOutputSampleBufferDelegate {
private let imageCaptureModel: ImageCaptureModel
override init() {
super.init()
}
convenience init?(imageCaptureModel:ImageCaptureModel) {
self.imageCaptureModel = imageCaptureModel
let queueID = "\(Bundle.main.bundleIdentifier!).captureDeviceOutput"
let captureVideoDataOutputQueue = DispatchQueue(label: queueID,
attributes: DispatchQueue.Attributes.concurrent)
let captureVideoDataOutput = CaptureVideoDataOutput()
captureVideoDataOutput.videoSettings = videoSettingsParameter
captureVideoDataOutput.setSampleBufferDelegate(captureVideoDataOutput, queue: captureVideoDataOutputQueue)
guard
imageCaptureModel.captureSession.canAddOutput(captureVideoDataOutput) else {
return nil
}
self = captureVideoDataOutput
}
the idea is the user just using the convenience init and receive a nil if the whole thing fails on the guard or receive the object if it succeeds.
But this code is failing on the first line with
let' property 'imageCaptureModel' may not be initialized directly; use "self.init(...)" or "self = ..." instead
and on the last line with
Cannot assign to value: 'self' is immutable
Any ideas?

Answer 1

Create an additional (private) initializer for CaptureVideoDataOutput that takes ImageCaptureModel and call it in the convenience initializer instead of CaptureVideoDataOutput():
private init(_ model: ImageCaptureModel) {
self.imageCaptureModel = model
super.init()
}
convenience init?(imageCaptureModel:ImageCaptureModel) {
let queueID = "\(Bundle.main.bundleIdentifier!).captureDeviceOutput"
let captureVideoDataOutputQueue = DispatchQueue(label: queueID,
attributes: DispatchQueue.Attributes.concurrent)
self.init(imageCaptureModel)
videoSettings = videoSettingsParameter
setSampleBufferDelegate(captureVideoDataOutput, queue: captureVideoDataOutputQueue)
guard
imageCaptureModel.captureSession.canAddOutput(captureVideoDataOutput) else {
return nil
}
}
Note that also non-convenience initializers may return nil, so I'm not sure why you chose this setup. Consider this:
init?(model: ImageCaptureModel) {
self.imageCaptureModel = model
let queueID = "\(Bundle.main.bundleIdentifier!).captureDeviceOutput"
let captureVideoDataOutputQueue = DispatchQueue(label: queueID,
attributes: DispatchQueue.Attributes.concurrent)
super.init()
videoSettings = videoSettingsParameter
setSampleBufferDelegate(captureVideoDataOutput, queue: captureVideoDataOutputQueue)
guard
imageCaptureModel.captureSession.canAddOutput(captureVideoDataOutput) else {
return nil
}
}