Adding thread safety in Swift 3

One of the pieces that I’ve brushed up against recently, but didn’t understand in any great depth, were techniques for making various sections of code thread-safe. There are some excellent articles out there on it, so if you’re looking and found this, let me provide a few references:

In Jun 2016 Matt Gallagher wrote Mutexes and closure capture in Swift, which was oriented towards how to optimize beyond the “obvious advice”, and a great read for the more curious.

The “usual advice” referenced a StackOverflow question: What is the Swift equivalent to Objective-C’s @synchronized? that as a technique spans swift 2 and 3 – the gist being to leverage the libDispatch project, creating a dispatch queue to handle synchronization and access to shared data structures. I found another question+answer on StackOverflow to be a bit easier to read and understand: Adding items to Swift array across multiple threads causing issues…, and matched a bit more of the technique that you can spot in Swift Package Manager code.

One of the interesting quirks in Swift 3 is that let myQueue = DispatchQueue(label: "my queue") returns a serialized queue, as opposed to the asynchronous queue, which you can get invoking I’m not sure where in the formal documentation that default information appears – the guides I found to Grand Central Dispatch are all still written for C and Objective-C primarily, so the mapping of Swift libraries to those structures wasn’t at all obvious to me. I particularly liked the answer that described this detail, as it was some of the better descriptions that mapped to the Apple developer guides on concurrency (and which desperately needs to be updated to related to Swift IMO).

Circling back to Matt Gallagher’s piece, the overhead of the scope capture in passing through the closure was more than he wanted to see – although it was fine for my needs. None the less, his details are also available in github at CwlUtils, which has a number of other interesting pieces and tidbits in it that I’ll circle back later to look at in depth.


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