Swift casting UnsafeMutablePointer<Void> in C callback

Question

Trying to interface with iOS AudioQueues using swift, and hitting a roadblock when attempting to pass an object defining my state.

My state object is a custom struct defined as:

struct UserData { var active: Bool = false var audioFileId: AudioFileID = nil }

My swift audio input callback:

 let myCallback : @convention(c) (UnsafeMutablePointer<Void>, AudioQueueRef, AudioQueueBufferRef, UnsafePointer<AudioTimeStamp>, UInt32, UnsafePointer<AudioStreamPacketDescription>) -> Void = { (inUserData, inAQ, inBuffer, inStartTime, inNumberPacketDescriptions, inPacketDescs) -> Void in ... }

Lastly, initializing the audio queue:

 AudioQueueNewInput(&audioStreamBasicDescription, myCallback, &userData, nil, nil, 0, &inputQueue)

Where I'm stuck is figuring out how to cast inUserData: UnsafeMutablePointer<Void> to my UserData struct

Or, of course, any better way of handling this.

Thanks!

eik · Accepted Answer · 2015-11-16 16:52:07Z

You can use either

let userData: UserData = UnsafePointer(inUserData).memory

or

let userData = UnsafePointer<UserData>(inUserData).memory

both are equivalent. See also the UnsafeMutablePointer Structure Reference.

To return some modified data you can use

let userDataReference = UnsafeMutablePointer<UserData>(inUserData) var userData = userDataReference.memory userData.… = … /* modifiy local copy */ userDataReference.memory = userData

You could also modify userDataReference.memory directly, but I would advise against it.

sorry to add on..is it possible to modify the property of the struct in my callback? Given an additional property var packetsProcessed: Int64 = 0, and being able to set it in the callback

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