What is the Swift equivalent to Objective-C's "@synchronized"?

I've searched the Swift book, but can't find the Swift version of @synchronized. How do I do mutual exclusion in Swift?

Solution 1:

You can use GCD. It is a little more verbose than @synchronized, but works as a replacement:

let serialQueue = DispatchQueue(label: "com.test.mySerialQueue")
serialQueue.sync {
    // code
}

Solution 2:

I was looking for this myself and came to the conclusion there's no native construct inside of swift for this yet.

I did make up this small helper function based on some of the code I've seen from Matt Bridges and others.

func synced(_ lock: Any, closure: () -> ()) {
    objc_sync_enter(lock)
    closure()
    objc_sync_exit(lock)
}

Usage is pretty straight forward

synced(self) {
    println("This is a synchronized closure")
}

There is one problem I've found with this. Passing in an array as the lock argument seems to cause a very obtuse compiler error at this point. Otherwise though it seems to work as desired.

Bitcast requires both operands to be pointer or neither
  %26 = bitcast i64 %25 to %objc_object*, !dbg !378
LLVM ERROR: Broken function found, compilation aborted!

Solution 3:

I like and use many of the answers here, so I'd choose whichever works best for you. That said, the method I prefer when I need something like objective-c's @synchronized uses the defer statement introduced in swift 2.

{ 
    objc_sync_enter(lock)
    defer { objc_sync_exit(lock) }

    //
    // code of critical section goes here
    //

} // <-- lock released when this block is exited

The nice thing about this method, is that your critical section can exit the containing block in any fashion desired (e.g., return, break, continue, throw), and "the statements within the defer statement are executed no matter how program control is transferred."1