How to use the CoreAudio API in Swift

Question

I am in the process of migrating my streaming audio engine to swift. i am finding it difficult to use the C Audio API in swift.

I have a problem with AudioFileStreamOpen api where it takes 2 C functions as a parameter. I don't know how to use this API is swift.

AudioFileStreamOpen(self as UnsafePointer<()>, propertyProc, packetProc, kAudioFileMP3Type, audioStreamId)

I have defined the callback method as below for this API. But i am getting the compilation error.

func propertyProc(inClientData: UnsafePointer<()>,inFileStreamId: AudioFileStreamID,inPropertyId: AudioFileStreamPropertyID,ioFlags: UnsafePointer<UInt32>) -> Void {
 ..... 
 }
func packetProc(inClientData: UnsafePointer<()>,inNumberOfBytes: UInt32,inNumberOfPackets: UInt32, ConstUnsafePointer<()>, inPacketDescriptions: UnsafePointer<AudioStreamPacketDescription>) -> Void {
 .....
}

Any help is appreciated to correctly define this C API in swift

Compilation error

Answer 1

You can't (currently) use an API requiring a C callback pointer from pure Swift code. Calling Swift functions or methods using a C function pointer is not supported by the current beta 4 language implementation, according to replies in the Swift forum at devforums.apple.com

UPDATE: The above answer is obsolete as of Swift 2.0

One alternative is to put some small trampoline C callback functions in an Objective C file, which can interoperate with Swift, and have those C functions in turn call a block or closure, which can be in Swift code. Configure the C callbacks with your Swift closures, and then pass those C callbacks to the CoreAudio functions.

• Andrew Ebling: Does this limitation/restriction still apply now, as of Xcode 6.2 beta 3?

Answer 2

I don't know much about Audio API, however, you should replace UnsafePointer by a pointer to an Object. for example:

var clientData : AnyObject?
var listenerProc : AudioFileStream_PropertyListenerProc = AudioFileStream_PropertyListenerProc.convertFromNilLiteral()
var packetsProc : AudioFileStream_PacketsProc = AudioFileStream_PacketsProc.convertFromNilLiteral()
var audioFileTypyeId : AudioFileTypeID = 0
AudioFileStreamOpen(&clientData, listenerProc, packetsProc, audioFileTypyeId, &streamId)

the initialization code for listenerProc, packetsProc or other variables is just to by-pass the compiler error.

To your situation, try to replace 'self as UnsafePointer<>' by '&self'. However 'self' must be something that can be converted to compatible data type.

https://developer.apple.com/library/prerelease/ios/documentation/MusicAudio/Reference/AudioStreamReference/index.html#//apple_ref/c/func/AudioFileStreamOpen