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

Question

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
}

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!

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

You can sandwich statements between objc_sync_enter(obj: AnyObject?) and objc_sync_exit(obj: AnyObject?). The @synchronized keyword is using those methods under the covers. i.e.

objc_sync_enter(self)
... synchronized code ...
objc_sync_exit(self)

Analog of the @synchronized directive from Objective-C can have an arbitrary return type and nice rethrows behaviour in Swift.

// Swift 3
func synchronized<T>(_ lock: AnyObject, _ body: () throws -> T) rethrows -> T {
    objc_sync_enter(lock)
    defer { objc_sync_exit(lock) }
    return try body()
}

The use of the defer statement lets directly return a value without introducing a temporary variable.

---

In Swift 2 add the @noescape attribute to the closure to allow more optimisations:

// Swift 2
func synchronized<T>(lock: AnyObject, @noescape _ body: () throws -> T) rethrows -> T {
    objc_sync_enter(lock)
    defer { objc_sync_exit(lock) }
    return try body()
}

Based on the answers from GNewc [1] (where I like arbitrary return type) and Tod Cunningham [2] (where I like defer).