Referencing `self` from within a VTCompressionOutputCallback

Question

I'm currently trying to use VideoToolbox to encode video data from an AVCaptureVideoDataOutput, but I'm having an issue referencing self from within the VTCompressionOutputCallback.

My code is as follows:

...

var sessionRef: VTCompressionSession?

let outputCallback: VTCompressionOutputCallback = { _, _, status, _, sampleBuffer in
    guard status == noErr, let sampleBuffer = sampleBuffer else {
        return
    }

    debugPrint("[INFO]: outputCallback: sampleBuffer: \(sampleBuffer)")
}

let sessionErr = VTCompressionSessionCreate(allocator: nil,
                                            width: width,
                                            height: height,
                                            codecType: kCMVideoCodecType_H264,
                                            encoderSpecification: nil,
                                            imageBufferAttributes: nil,
                                            compressedDataAllocator: nil,
                                            outputCallback: outputCallback,
                                            refcon: nil,
                                            compressionSessionOut: UnsafeMutablePointer(&sessionRef))

...

That works fine, and the print outputs as expected, but as soon as I try and add a reference to self within the VTCompressionOutputCallback, it get a compiler error stating

A C function pointer cannot be formed from a closure that captures context

How can I use self from within the callback?

Thanks in advance for the help.

Answer 1

I figured out a solution.

The VTCompressionSessionCreate call has a parameter for outputCallbackRefCon which gets passed along to the VTCompressionOutputCallback.

By wrapping self in an UnsafeMutableRawPointer like so

let unmanagedSelf = UnsafeMutableRawPointer(Unmanaged.passUnretained(self).toOpaque())

I was able to pass that value into the VTCompressionSessionCreate under the refcon parameter. Inside of the callback I was then able to do pull that value back out using

let scopedSelf = Unmanaged<ViewController>.fromOpaque(unmanagedSelf).takeUnretainedValue()