Is there a way to utilize Swift commands within a low-level C method call? - swift

Question

I am maintaining a set of data obtained from a AVCaptureSynchronizedData. One of the methods I use modifies the CVPixelBuffers obtained from the AVCaptureSynchronizedData. While modifying. the CVPixelBuffer, I create a copy of the CVPixelBuffer via
let status = CVPixelBufferCreateWithBytes(nil, scaleWidth, scaleHeight,
pixelFormat, destData,
destBytesPerRow, releaseCallback,
nil, nil, &dstPixelBuffer)
The releaseCallBack is referenced as
let releaseCallback: CVPixelBufferReleaseBytesCallback = { _, ptr in
if let ptr = ptr {
free(UnsafeMutableRawPointer(mutating: ptr))
}
}
However, I would like to be able to keep up with how many times this is called. But, if I add something like
var num:Int = 0
let releaseCallback: CVPixelBufferReleaseBytesCallback = { _, ptr in
if let ptr = ptr {
num += 1
free(UnsafeMutableRawPointer(mutating: ptr))
}
}
I get the error
A C function pointer cannot be formed from a closure that captures context
Not sure if it is possible, but it would be incredibly useful to be able to keep count of the number of times a pointer is destroyed

Answer 1

Referencing the instance variable num in the closure captures self and that is not possible in the callback which is a pure C function.
Similarly as in How to use instance method as callback for function which takes only func or literal closure you have to use that a custom pointer (here: releaseRefCon) can be passed to the callback:
let releaseCallback: CVPixelBufferReleaseBytesCallback = { releaseRefCon, ptr in
let mySelf = Unmanaged<YourClass>.fromOpaque(releaseRefCon!).takeUnretainedValue()
if let ptr = ptr {
free(UnsafeMutableRawPointer(mutating: ptr))
mySelf.num += 1
}
}
let releaseRefCon = UnsafeMutableRawPointer(Unmanaged.passUnretained(self).toOpaque())
let status = CVPixelBufferCreateWithBytes(nil, scaleWidth, scaleHeight,
pixelFormat, destData,
destBytesPerRow,
releaseCallback, releaseRefCon,
nil, &dstPixelBuffer)
For the conversions from an instance pointer (here: self) to a void pointer and back to an instance pointer (here: mySelf) see How to cast self to UnsafeMutablePointer<Void> type in swift.