Implementing a c function callback in Swift 2.0?

Question

CFReadStreamSetClient has a C-function callback (CFReadStreamClientCallBack) in it's initializer,

CFReadStreamClientCallback looks like this:

typealias CFReadStreamClientCallBack = (CFReadStream!,
 CFStreamEventType, UnsafeMutablePointer<Void>) -> Void

I have a method that attempts to handle the CFReadStreamClientCallBack C-function callback:

func callback(stream: CFReadStreamRef,
 eventType: CFStreamEventType,
 inClientInfo: UnsafeMutablePointer<Void>) {
 }

but when i attempt to set the callback in CFReadStreamCallback as follows, it doesn't compile.

CFReadStreamSetClient(stream, registeredEvents, callback, clientContextPtr)

I know with Swift 2.0 there's a way to use Swift closures with C-Function callbacks but I cant seem to get it to work. Does anyone know how it can be done in this situation?

Answer 1

You can make a closure that does what you do in the function like this:

CFReadStreamSetClient(stream, registeredEvents, { readStream, event, data -> Void in 
 // Do stuff here.
}, clientContextPtr)

Also note that C function blocks can't have a context so you can't access self, or any other variable from outside the block. If that doesn't work can you include the compiler error in your question?

• 3254523: this works, thank you so much. i was wondering if you may know how to handle this one as well? : AURenderCallbackStruct( inputProc: , inputProcRefCon: &selfPtr )
• Manav Gabhawala: I haven't worked with AudioUnit too much but I'm guessing you got selfPtr by using unsafeAddressOf(self) and that that bit is working. Other than that you need this: AURenderCallbackStruct(inputProc: { inRefCon, ioActionFlags, inTimeStamp, inBusNumber, inNumberFrames, ioData -> OSStatus in // TODO: Stuff that returns an OSStatus }, inputProcRefCon: &selfPtr)

Answer 2

Swift 2.0:

Given a function which takes a function pointer and a context:

void c_func_with_callback(void *context, void (*callback)(void *context))

It's very easy to call it from Swift, but it won't be possible to access the variables of the enclosing class:

c_func_with_callback(nil) { context in
 print("Callback in Swift!") // OK
 self.xxx // error
}

If you need to access variables outside of the block, it's possible to use the context to store a pointer to the class:

class MyClass {
 func hello() {
 print("MyClass.hello()")
 }
 func classMethodAsCallback() {
 let context = unsafeBitCast(self, UnsafeMutablePointer<Void>.self)
 c_func_with_callback(context) { context in
 let caller = unsafeBitCast(context, MyClass.self)
 caller.hello()
 }
 }
}
let myClass = MyClass()
myClass.classMethodAsCallback()

It's even possible to use ANY closure as a callback:

class MyClass {
 typealias Closure = ()->()
 func anySwiftClosureAsCallback(closure: Closure) {
 var c = closure
 // closure won't fit in a pointer, so take it's address first
 let context = withUnsafePointer(&c) {
 //(ptr: UnsafePointer<Closure>) -> UnsafeMutablePointer<Void> in
 ptr in
 return unsafeBitCast(ptr, UnsafeMutablePointer<Void>.self)
 }
 c_func_with_callback(context) { context in
 let ptr = unsafeBitCast(context, UnsafePointer<Closure>.self)
 let closure = ptr.memory // dereference the address
 closure()
 }
 }
}
let myClass = MyClass()
myClass.anySwiftClosureAsCallback {
 print("Swift closure called")
}