How to pass `self` to a function that takes `UnsafeMutableRawPointer?` in Swift? - swift

Question

I've bought a 3D 4K projector last year, and it uses DLP-Link technology, which requires an 120fps Left/Right alternating video stream for stereoscopy.
I'm writing a player in Swift for Mac using Core Video, and a function require me to pass an UnsafeMutableRawPointer? (void * in Obj-C) to it, and I'm passing the view instance to it using the self keyword.
Here's my code:
//
// MovieView.swift
// Studio Media Player
//
// Created by DannyNiu on 2022-07-03.
//
import Foundation
import Cocoa
import AVFoundation
import CoreVideo
class MovieView : NSView
{
var asset: AVAsset?
var player: AVPlayer?
var item: AVPlayerItem?
var vout: AVPlayerItemVideoOutput?
var cvpb: CVPixelBuffer?
var cii: CIImage?
var cgi: CGImage?
var irect: CGRect?
var vlink: CVDisplayLink?
func setup_displaylink() -> Bool
{
var cvret: CVReturn
cvret = CVDisplayLinkCreateWithActiveCGDisplays(&vlink)
if( vlink == nil ) { return false }
var me: MovieView = self
cvret = CVDisplayLinkSetOutputCallback(
vlink!, vlink_callback, &me)
if( cvret == kCVReturnSuccess ) {
return true
} else { return false }
}
func assign_asset(_ asset: AVAsset)
{
self.asset = asset
item = .init(asset: asset)
player = .init(playerItem: item)
vout = .init()
cvpb = nil
cii = nil
}
func video_render(_ d: CVTimeStamp)
{
let t: CMTime = player!.currentTime()
let cgc: CGContext = (NSGraphicsContext.current?.cgContext)!
let cic: CIContext = (NSGraphicsContext.current?.ciContext)!
if( vout?.hasNewPixelBuffer(forItemTime: t) ?? false )
{
cvpb = vout?.copyPixelBuffer(
forItemTime: t, itemTimeForDisplay: nil)
cii = .init(cvPixelBuffer: cvpb!)
irect = cii!.extent
cgi = cic.createCGImage(cii!, from: irect!)
}
if( irect == nil ) { return }
let orect: CGRect = NSRectToCGRect(bounds)
var vrect: CGRect =
CGRect(origin: orect.origin,
size: CGSize(width: orect.width * 2,
height: irect!.height *
orect.width /
irect!.width))
cgc.setFillColor(gray:0, alpha:1)
cgc.fill(orect)
if( d.videoTime % 2 == 1 )
{
vrect = vrect.offsetBy(dx: -orect.width, dy: 0)
}
cgc.draw(cgi!, in: vrect)
}
}
func vlink_callback(
displayLink: CVDisplayLink,
inNow: UnsafePointer<CVTimeStamp>,
inOutputTime: UnsafePointer<CVTimeStamp>,
flagsIn: CVOptionFlags,
flagsOut: UnsafeMutablePointer<CVOptionFlags>,
arg_mvview: UnsafeMutableRawPointer?
) -> CVReturn
{
let mvview: MovieView = arg_mvview!.load(as: MovieView.self)
mvview.video_render(inNow.pointee)
return kCVReturnSuccess
}
When I debug the program, the mvview.video_render(inNow.pointee) line caused an EXC_BAD_ACCESS trap, with code = 1. I assume this is caused by me not correctly passing self to the display link call-back. So how can I fix this?

Answer 1

Use the Unmanaged class.
in setup_displaylink():
func setup_displaylink() -> Bool
{
var cvret: CVReturn
cvret = CVDisplayLinkCreateWithActiveCGDisplays(&vlink)
if( vlink == nil ) { return false }
var me: UnsafeMutableRawPointer =
Unmanaged.passUnretained(self).toOpaque()
cvret = CVDisplayLinkSetOutputCallback(
vlink!, vlink_callback, &me)
if( cvret == kCVReturnSuccess ) {
return true
} else { return false }
}
and in vlink_callback
func vlink_callback(
displayLink: CVDisplayLink,
inNow: UnsafePointer<CVTimeStamp>,
inOutputTime: UnsafePointer<CVTimeStamp>,
flagsIn: CVOptionFlags,
flagsOut: UnsafeMutablePointer<CVOptionFlags>,
arg_mvview: UnsafeMutableRawPointer?
) -> CVReturn
{
let mvview: MovieView =
Unmanaged.fromOpaque(arg_mvview!).takeUnretainedValue()
mvview.video_render(inNow.pointee)
return kCVReturnSuccess
}
Notice I used the "Unretained" function, because there's no need to add to the reference count of the movie view, as self is being passed around internally. Use "Retained" functions when otherwise appropriate.