Synchronized<T>

A Swift genric wrapper type to help enforce thread-safe usage of stored properties.

This is an idea that started with a post on my blog, and I followed up with a discussion about the Locking protocol.

Current Status

• I've been using it some small projects, and find it useful enough to share.

Example Usage

class C {
    private let criticalCount = Synchronized<Int>(0)

    /// This might be called on any thread, and we want to increment the
    /// `criticalCount`, without introducing a race condition.
    func incrementOnUnknownThread() {
        criticalCount.update { count in
            count += 1
        }
    }
}

Available Locking Strategies

• DispatchSemaphore
• DispatchQueue
• NSLock
• pthread_rwlock_t

Installation

I don't have a Cartfile because it seems silly to add a whole dynamic .framework to your app just to use this. Instead, I'd recommend just grabbing the single Synchronized.swift file, and adding that to your project directly.

I did add a Package.swift file for Swift Package Manager, just for fun. Let's hope Xcode gets better support for that soon!

⚠️ Caveats

• This doesn't fully guarantee thread-safety for reference types, if you hold a reference to the Synchronized resource that escapes the closure, and then access/mutate it. But this at least makes it harder to do that.
• This hasn't been thoroughly tested yet, or used in serious production code, but I do think the idea is sound, so more testing and usage would do it good.

Further Ideas (TODO)

• Add more locking strategies
  • os_unfair_lock
  • pthread_mutex_t directly?
• Refine the naming of the update() and use() methods`
  • Some ideas are:
    • for the use() method: read(), with()
    • for the update() method: write(), mutate()
• Further testing, especially performance testing of the various locking strategies

Similar Projects

I didn't know about this when I wrote this initially, but I noticed something similar to this, in PMHTTP, named QueueConfined. But that one doesn't have the ability to change the locking mechanism (it always uses a serial queue).