Convert callback/closure to Swift from Objective C - objective-c

Question

So I am trying to implement the Superpowered library in Swift, and am getting stuck around initialisation with a callback. How would I convert that line:
__unsafe_unretained Superpowered *self = (__bridge Superpowered *)clientdata;
into Swift?
Here is the simplified Objective C implementation:
#implementation Superpowered {
SuperpoweredIOSAudioIO *audioIO;
SuperpoweredBandpassFilterbank *filters;
unsigned int samplerate;
}
static bool audioProcessing(void *clientdata, float **buffers, unsigned int inputChannels, unsigned int outputChannels, unsigned int numberOfSamples, unsigned int samplerate, uint64_t hostTime) {
__unsafe_unretained Superpowered *self = (__bridge Superpowered *)clientdata;
if (samplerate != self->samplerate) {
self->samplerate = samplerate;
};
// Update position.
self->lastNumberOfSamples = numberOfSamples;
return false;
}
- (id)init {
self = [super init];
if (!self) return nil;
samplerate = 44100;
audioIO = [[SuperpoweredIOSAudioIO alloc] initWithDelegate:(id<SuperpoweredIOSAudioIODelegate>)self preferredBufferSize:12 preferredMinimumSamplerate:44100 audioSessionCategory:AVAudioSessionCategoryRecord channels:2 audioProcessingCallback:audioProcessing clientdata:(__bridge void *)self];
[audioIO start];
return self;
}
And here is the start of my Swift version:
func bridge<T : AnyObject>(obj : T) -> UnsafeMutableRawPointer {
return UnsafeMutableRawPointer(Unmanaged.passUnretained(obj).toOpaque())
// return unsafeAddressOf(obj) // ***
}
func bridge<T : AnyObject>(ptr : UnsafeMutableRawPointer) -> T {
return Unmanaged<T>.fromOpaque(ptr).takeUnretainedValue()
// return unsafeBitCast(ptr, T.self) // ***
}
open class EchoesEngine: NSObject, CLLocationManagerDelegate, SuperpoweredIOSAudioIODelegate {
public var audioIO:SuperpoweredIOSAudioIO
static var lastNumberOfSamples:UInt32!
static var samplerate:UInt32!
override init() {
super.init()
audioIO = SuperpoweredIOSAudioIO.init(delegate: self, preferredBufferSize: 12, preferredMinimumSamplerate: 44100, audioSessionCategory: AVAudioSessionCategoryPlayAndRecord, channels: 2, audioProcessingCallback: EchoesEngine.audioProcessingCallback, clientdata: bridge(obj: self))
…
}
#objc static let audioProcessingCallback : #convention(c) (UnsafeMutableRawPointer?, UnsafeMutablePointer<UnsafeMutablePointer<Float>?>?, UInt32, UInt32, UInt32, UInt32, UInt64) -> Bool = {
(clientdata, buffers, inputChannels, outputChannels, numberOfSamples, _samplerate, hostTime) in
/*
let unsafePointer = Unmanaged<EchoesEngine>.fromOpaque(clientdata!).takeUnretainedValue()
let pointer = AutoreleasingUnsafeMutablePointer<EchoesEngine>(unsafePointer)
*/
self = bridge(ptr: clientdata!)
if samplerate != _samplerate {
samplerate = _samplerate
}
lastNumberOfSamples = numberOfSamples
return false
}

Answer 1

So I discovered that in this context the self I was trying to cast to is actually a new variable providing an __unsafe_unretained reference to self, as passed to the initialiser for SuperpoweredIOSAudioIO.
This answer provided me with almost everything I needed. Swift 3.3 changes a couple of things, and this is what I ended up with:
func bridge<T : AnyObject>(obj : T) -> UnsafeMutableRawPointer {
return UnsafeMutableRawPointer(Unmanaged.passUnretained(obj).toOpaque())
// return unsafeAddressOf(obj) // ***
}
func bridge<T : AnyObject>(ptr : UnsafeMutableRawPointer?) -> T {
return Unmanaged<T>.fromOpaque(ptr!).takeUnretainedValue()
// return unsafeBitCast(ptr, T.self) // ***
}