The callback is a pointer to a C function, and in Swift you can pass
only a global function or a closure (which does not capture any state),
but not an instance method.
So this does work:
CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(),
nil,
{ (_, observer, name, _, _) in
print("received notification: \(name)")
},
"myMessage",
nil,
.DeliverImmediately)
But since the closure cannot capture context, you have no direct reference to self and its properties and instance methods.
For example, you cannot add
self.label.stringValue = "got it"
// error: a C function pointer cannot be formed from a closure that captures context
inside the closure to update the UI when a notification arrived.
There is a solution, but it is a little bit complicated due to
Swift’s strict type system.
Similarly as in Swift 2 – UnsafeMutablePointer<Void> to object, you can convert the pointer to
self to a void pointer, pass that as the observer parameter
to the registration, and convert it back to an object pointer in
the callback.
class YourClass {
func callback(name : String) {
print("received notification: \(name)")
}
func registerObserver() {
// Void pointer to `self`:
let observer = UnsafePointer<Void>(Unmanaged.passUnretained(self).toOpaque())
CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(),
observer,
{ (_, observer, name, _, _) -> Void in
// Extract pointer to `self` from void pointer:
let mySelf = Unmanaged<YourClass>.fromOpaque(
COpaquePointer(observer)).takeUnretainedValue()
// Call instance method:
mySelf.callback(name as String)
},
"myMessage",
nil,
.DeliverImmediately)
}
// ...
}
The closure acts as a “trampoline” to the instance method.
The pointer is an unretained reference, therefore you must ensure
that the observer is removed before the object is deallocated.
Update for Swift 3:
class YourClass {
func callback(_ name : String) {
print("received notification: \(name)")
}
func registerObserver() {
// Void pointer to `self`:
let observer = UnsafeRawPointer(Unmanaged.passUnretained(self).toOpaque())
CFNotificationCenterAddObserver(CFNotificationCenterGetDarwinNotifyCenter(),
observer,
{ (_, observer, name, _, _) -> Void in
if let observer = observer, let name = name {
// Extract pointer to `self` from void pointer:
let mySelf = Unmanaged<YourClass>.fromOpaque(observer).takeUnretainedValue()
// Call instance method:
mySelf.callback(name.rawValue as String)
}
},
"myMessage" as CFString,
nil,
.deliverImmediately)
}
// ...
}
See also How to cast self to UnsafeMutablePointer<Void> type in swift for more information
about the “bridging” between object pointers and C pointers.