Syntactic sugar in Swift for asynchronous dispatches in Grand Central Dispatch (GCD)

Async rewritten for iOS7 and OS X 10.9 Compatibility. Unless you are targeting iOS7 or 10.9 I recommend you stick to the full Async for more features and possibly better performance. You will particularly benefit from the availability of the new QoS classes available in iOS8 and OS X 10.10 (Yosemite).

See this article about how it works.

Async sugar looks like this:

Async.background {
	println("This is run on the background queue")
}.main {
	println("This is run on the main queue, after the previous block")
}

Instead of the familiar syntax for GCD:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), {
	println("This is run on the background queue")
	
	dispatch_async(dispatch_get_main_queue(), 0), {
		println("This is run on the main queue, after the previous block")
	})
})

Just drop the AsyncLegacy.swift file into your project then you can make the calls as described in this document.

1. Less verbose code
2. Less code indentation

OS X 10.9+ and iOS 7+

Access different priority queues:

Async.main {}
Async.userInteractive {} // Remapped to DISPATCH_QUEUE_PRIORITY_HIGH
Async.userInitiated {}   // Remapped to DISPATCH_QUEUE_PRIORITY_HIGH
Async.default_ {}        // Remapped to DISPATCH_QUEUE_PRIORITY_DEFAULT 
Async.utility {}         // Remapped to DISPATCH_QUEUE_PRIORITY_LOW
Async.background {}      // Remapped to DISPATCH_QUEUE_PRIORITY_BACKGROUND

Chain as many blocks as you want:

Async.userInitiated {
	// 1
}.main {
	// 2
}.background {
	// 3
}.main {
	// 4
}

Store reference for later chaining:

let backgroundBlock = Async.background {
	println("This is run on the background queue")
}

// Run other code here...

// Chain to reference
backgroundBlock.main {
	println("This is run on the \(qos_class_self().description) (expected \(qos_class_main().description)), after the previous block")
}

Custom queues:

let customQueue = dispatch_queue_create("CustomQueueLabel", DISPATCH_QUEUE_CONCURRENT)
let otherCustomQueue = dispatch_queue_create("OtherCustomQueueLabel", DISPATCH_QUEUE_CONCURRENT)
Async.customQueue(customQueue) {
	println("Custom queue")
}.customQueue(otherCustomQueue) {
	println("Other custom queue")
}

Dispatch block after delay:

let seconds = 0.5
Async.main(after: seconds) {
	println("Is called after 0.5 seconds")
}.background(after: 0.4) {
	println("At least 0.4 seconds after previous block, and 0.9 after Async code is called")
}

Cancel blocks that aren't already dispatched:

// Cancel blocks not yet dispatched
let block1 = Async.background {
	// Heavy work
	for i in 0...1000 {
		println("A \(i)")
	}
}
let block2 = block1.background {
	println("B – shouldn't be reached, since cancelled")
}
Async.main { 
	// Cancel async to allow block1 to begin
	block1.cancel() // First block is _not_ cancelled
	block2.cancel() // Second block _is_ cancelled
}

Blocks chained to cancelled blocks WILL be run.

Wait for block to finish – an ease way to continue on current queue after background task:

let block = Async.background {
	// Do stuff
}

// Do other stuff

block.wait()

It creates a dispatch group for each block and uses that to notify other blocks to run. In places blocks are wrapped in other blocks to explitly enter or leave groups so that following blocks are appropriately signalled.

See this article for more details or just review the code, it is fairly short. Get in touch if you have any questions.

The syntax part of the chaining works by having class methods on the Async object e.g. Async.main {} which returns an Async object. The object has matching methods e.g. theObject.main {}. Objects are used so that you can pass the the object by reference so that you can cancel it if required.

default is a keyword. Workaround used: default_. Could use this trick shown be Erica Sadun, i.e. class func `default`() -> {} but it results in this use Async.`default`{}

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Copyright (c) 2014 Tobias Due Munk Copyright (c) 2014 Joseph Lord (Human Friendly Ltd.)

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