Await
Swift port of C# Await using Cocoa's Run Loop mechanism.
(Useful for unit testing, but needs special care for application use)
Without Await
showProgressHUD("Start!") // displays AFTER download finished (you know why)
let image = downloadLargeImageFromWeb() // OMG downloading from main-thread!!!
showProgressHUD("Finished!") // displays after download finished
showProgressHUD("Start!") // main-thread
// OK let's use dispatch_async then...
dispatch_async(globalQueue) {
image = downloadLargeImageFromWeb() // background-thread
dispatch_async(dispatch_get_main_queue()) {
showProgressHUD("Finished!") // main-thread
// want more nested blocks here?
}
}
With Await
showProgressHUD("Start!")
let image = await { downloadLargeImageFromWeb() }
// because of await's running Run Loop, "Start!" will be displayed immediately here
showProgressHUD("Finished!") // displays after download finished
How to use
await
let image = await { downloadLargeImageFromWeb() }
await calls dispatch_async for given closure & runs Run Loop until finished.
await + until
var shouldStop = false
var container: SomeContainer
let result = await(until: shouldStop) {
container.data = nil
dispatch_async(globalQueue) {
container.data = downloadData()
shouldStop = true
}
return container.data
}
Use until to manually adjust the Run Loop running time.
This is especially useful in combination with async-programming monads like Promise.
Take a look at PromiseKit example:
import PromiseKit
var promise = Promise<NSData> { (fulfiller, rejecter) in ... }
...
let result = await(until: !promise.pending) { promise.value }
// or, let result = await(promise)
await + timeout
let image = await(timeout: 3) { downloadLargeImageFromWeb() } // returns nil if 3 sec has passed
await + finishable closure
You can even use finish() or finish(returnValue) inside closure to manually stop running Run Loop instead of using await + until.
let data = await { finish in
dispatch_async(queue) {
let d = downloadData()
finish(d) // pass result data
}
}
var data: NSData?
await { finish in
dispatch_async(queue) {
let d = downloadData()
data = d
finish() // no return
}
}
For application use
For applicaiton use, you MUST wrap await calls with async as follows:
async {
var image = await { downloadLargeImageFromWeb() }
image = await { filterImage(image) }
}
This is to ensure that await will not block dispatch_get_main_queue()
which often causes UIKit not responding to touches.
Limitation
await + timeout and nested awaits may not finish at proper time inside async due to nested RunLoop running.
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