Swift 2.0 (Xcode 7.0)

There is a bug in Swift 2.0 that messes with chaining, so use this fork by @johncoates

Swift 2.1 (Xcode 7.1)

Async

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

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

Install

CocoaPods

pod 'Async', :git => 'https://github.com/duemunk/Async.git'

Benefits

Less verbose code
Less code indentation

Things you can do

Supports the modern queue classes:

Async.main {}
Async.userInteractive {}
Async.userInitiated {}
Async.utility {}
Async.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
}

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()

How does it work

The way it work is by using the new notification API for GCD introduced in OS X 10.10 and iOS 8. Each chaining block is called when the previous queue has finished.

let previousBlock = {}
let chainingBlock = {}
let dispatchQueueForChainingBlock = ...

// Use the GCD API to extend the blocks
let _previousBlock = dispatch_block_create(DISPATCH_BLOCK_INHERIT_QOS_CLASS, previousBlock)
let _chainingBlock = dispatch_block_create(DISPATCH_BLOCK_INHERIT_QOS_CLASS, chainingBlock)

// Use the GCD API to call back when finishing the "previous" block
dispatch_block_notify(_previousBlock, dispatchQueueForChainingBlock, _chainingBlock)

The syntax part of the chaining works by having class methods on the Async object e.g. Async.main {} which returns a struct. The struct has matching methods e.g. theStruct.main {}.

Known bugs

Modern GCD queues don't work as expected in the iOS Simulator. See issues 13, 22.

Known improvements

The dispatch_block_t can't be extended. Workaround used: Wrap dispatch_block_t in a struct that takes the block as a property.

Bonus stuff

There is also a wrapper for dispatch_apply() for quick parallelisation of a for loop.

Apply.background(100) { i in
	// Do stuff e.g. println(i)
}

Note that this function returns after the block has been run all 100 times i.e. it is not asynchronous. For asynchronous behaviour, wrap it in a an Async block like Async.main{ Apply.background(100) { ... } }.

Legacy support

For support of iOS 7 and OS X 10.9 check out Async.legacy. Joseph Lord works hard to have as high feature parity with Async as possible.

License

The MIT License (MIT)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

vartan/Async