Swift: Multithreading

Wednesday, March 25, 2015

Overview

Multithreading is a big part of Programming and you should know it well. You can find more information about multithreading here.

Content

Multithreading in iOS is done with “queues”. Functions and closures are placed in a queue and then pulled off the queue to execute on an associated thread.

There is a special queue called the main queue. All UI interections MUST happen on this queue only. You should put all non-UI activity that is time consuming on a queue other than the main queue so that you have a responsive UI (should also consider an ActivityIndicator/spinning wheel thingy).

There are 2 ways to do multithreading.

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let queue = NSOperationQueue()
queue.addOperationWithBlock {
    // do something in the background

    let mainQ: NSOperationQueue = NSOperationQueue.mainQueue()
    mainQ.addOperationWithBlock {
        // when done, update your UI and/or model on the main queue
    }
}

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let queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
dispatch_async(queue) {
    // do something in the background

    let mainQ: dispatch_queue_t = dispatch_get_main_queue()
    dispatch_async(mainQ) {
        // when done, update your UI and/or model on the main queue
    }
}

When using the second option, you can use different priorities.

  1. QOS_CLASS_USER_INTERACTIVE: quick and high priority
  2. QOS_CLASS_USER_INITIATED: high priority, might take time
  3. QOS_CLASS_UTILITY: long running
  4. QOS_CLASS_BACKGROUND: user not concerned with this
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let qos = Int(QOS_CLASS_USER_INTERACTIVE.value)
let queue = dispatch_get_global_queue(qos, 0)

Javascript's setTimeout

You can also do a setTimeout from javascript. As in, you can dispatch a function call after a certain interval. I really recommend creating a helped method for this one because it doesn’t look very nice.

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let delayInSeconds = 25.0
let delay = Int64(delayInSeconds * Double(NSEC_PER_MSEC))
let dispatchTime = dispatch_time(DISPATCH_TIME_NOW, delay)
dispatch_after(dispatchTime, dispatch_get_main_queue()) {
    // do something on the main queue 25 seconds from now
}

Synchronization

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func synchronized(lock: AnyObject, closure: () -> ()) {
    objc_sync_enter(lock)
    closure()
    objc_sync_exit(lock)
}

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

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