How do I run Asynchronous callbacks in Playground

Question

Many Cocoa and CocoaTouch methods have completion callbacks implemented as blocks in Objective-C and Closures in Swift. However, when trying these out in Playground, the completion is never called. For example:

// Playground - noun: a place where people can play

import Cocoa
import XCPlayground

let url = NSURL(string: "http://stackoverflow.com")
let request = NSURLRequest(URL: url)

NSURLConnection.sendAsynchronousRequest(request, queue:NSOperationQueue.currentQueue() {
response, maybeData, error in

    // This block never gets called?
    if let data = maybeData {
        let contents = NSString(data:data, encoding:NSUTF8StringEncoding)
        println(contents)
    } else {
        println(error.localizedDescription)
    }
}

I can see the console output in my Playground timeline, but the println in my completion block are never called...

score 196 · Accepted Answer · edited Dec 21 '17 at 09:26

While you can run a run loop manually (or, for asynchronous code that doesn't require a run loop, use other waiting methods like dispatch semaphores), the "built-in" way we provide in playgrounds to wait for asynchronous work is to import the XCPlayground framework and set XCPlaygroundPage.currentPage.needsIndefiniteExecution = true. If this property has been set, when your top level playground source finishes, instead of stopping the playground there we will continue to spin the main run loop, so asynchronous code has a chance to run. We will eventually terminate the playground after a timeout which defaults to 30 seconds, but which can be configured if you open the assistant editor and show the timeline assistant; the timeout is in the lower-right.

For example, in Swift 3 (using URLSession instead of NSURLConnection):

import UIKit
import PlaygroundSupport

let url = URL(string: "http://stackoverflow.com")!

URLSession.shared.dataTask(with: url) { data, response, error in
    guard let data = data, error == nil else {
        print(error ?? "Unknown error")
        return
    }

    let contents = String(data: data, encoding: .utf8)
    print(contents!)
}.resume()

PlaygroundPage.current.needsIndefiniteExecution = true

Or in Swift 2:

import UIKit
import XCPlayground

let url = NSURL(string: "http://stackoverflow.com")
let request = NSURLRequest(URL: url!)

NSURLConnection.sendAsynchronousRequest(request, queue: NSOperationQueue.currentQueue()) { response, maybeData, error in
    if let data = maybeData {
        let contents = NSString(data:data, encoding:NSUTF8StringEncoding)
        println(contents)
    } else {
        println(error.localizedDescription)
    }
}

XCPlaygroundPage.currentPage.needsIndefiniteExecution = true

For all it's worth, This is covered in WWDC 2014 §408: Swift Playgrounds, second half — Chris Conover, Jul 01 '14 at 15:43
Worth noting that from DP4 the `XCPlayground` framework is now available for iOS Playgrounds as well. — ikuramedia, Jul 27 '14 at 04:23
Updated method : `XCPlaygroundPage.currentPage.needsIndefiniteExecution = true` — R Menke, Nov 21 '15 at 04:24
Updated method: `import PlaygroundSupport` and `PlaygroundPage.current.needsIndefiniteExecution = true` — SimplGy, Jun 29 '16 at 22:57

score 54 · Answer 2 · answered Jun 16 '16 at 16:24

54

This API changed again in Xcode 8 and it was moved to the PlaygroundSupport:

import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

This change was mentioned in Session 213 at WWDC 2016.

answered Jun 16 '16 at 16:24

BalestraPatrick

9,944
4
30
43

3

Don't forget to call `PlaygroundPage.current.finishExecution()`. – Glenn Posadas Dec 06 '17 at 07:13

score 37 · Answer 3 · answered Nov 05 '15 at 03:39

As of XCode 7.1, XCPSetExecutionShouldContinueIndefinitely() is deprecated. The correct way to do this now is to first request indefinite execution as a property of the current page:

import XCPlayground

XCPlaygroundPage.currentPage.needsIndefiniteExecution = true

…then indicate when execution has finished with:

XCPlaygroundPage.currentPage.finishExecution()

For example:

import Foundation
import XCPlayground

XCPlaygroundPage.currentPage.needsIndefiniteExecution = true

NSURLSession.sharedSession().dataTaskWithURL(NSURL(string: "http://stackoverflow.com")!) {
    result in
    print("Got result: \(result)")
    XCPlaygroundPage.currentPage.finishExecution()
}.resume()

score 16 · Answer 4 · edited May 23 '17 at 12:03

The reason the callbacks are not called is because the RunLoop isn't running in Playground (or in REPL mode for that matter).

A somewhat janky, but effective, way to make the callbacks operate is with a flag and then manually iterating on the runloop:

// Playground - noun: a place where people can play

import Cocoa
import XCPlayground

let url = NSURL(string: "http://stackoverflow.com")
let request = NSURLRequest(URL: url)

var waiting = true

NSURLConnection.sendAsynchronousRequest(request, queue:NSOperationQueue.currentQueue() {
response, maybeData, error in
    waiting = false
    if let data = maybeData {
        let contents = NSString(data:data, encoding:NSUTF8StringEncoding)
        println(contents)
    } else {
        println(error.localizedDescription)
    }
}

while(waiting) {
    NSRunLoop.currentRunLoop().runMode(NSDefaultRunLoopMode, beforeDate: NSDate())
    usleep(10)
}

This pattern has often been used in Unit Tests which need to test async callbacks, for example: Pattern for unit testing async queue that calls main queue on completion

score 12 · Answer 5 · answered Nov 25 '16 at 10:58

12

The new APIs as for XCode8, Swift3 and iOS 10 are,

// import the module
import PlaygroundSupport
// write this at the beginning
PlaygroundPage.current.needsIndefiniteExecution = true
// To finish execution
PlaygroundPage.current.finishExecution()

answered Nov 25 '16 at 10:58

bradd123

131
1
3

p-sun · Answer 6 · 2017-09-27T04:41:19.837

Swift 4, Xcode 9.0

import Foundation
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

let url = URL(string: "https://jsonplaceholder.typicode.com/posts/1")!

let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
    guard error == nil else {
        print(error?.localizedDescription ?? "")
        return
    }

    if let data = data, let contents = String(data: data, encoding: String.Encoding.utf8) {
        print(contents)
    }
}
task.resume()

Lloyd Briggs · Answer 7 · 2017-01-10T22:20:54.253

Swift 3, xcode 8, iOS 10

Notes:

Tell the compiler that the playground file requires "indefinite execution"

Manually terminate execution via a call to PlaygroundSupport.current.completeExecution() within your completion handler.

You may run into problems with the cache directory and to resolve this you will need to manually re-instantiate the UICache.shared singleton.

Example:

import UIKit
import Foundation
import PlaygroundSupport

// resolve path errors
URLCache.shared = URLCache(memoryCapacity: 0, diskCapacity: 0, diskPath: nil)

// identify that the current page requires "indefinite execution"
PlaygroundPage.current.needsIndefiniteExecution = true

// encapsulate execution completion
func completeExecution() {
    PlaygroundPage.current.finishExecution()
}

let url = URL(string: "http://i.imgur.com/aWkpX3W.png")

let task = URLSession.shared.dataTask(with: url!) { (data, response, error) in
    var image = UIImage(data: data!)

    // complete execution
    completeExecution()
}

task.resume()

score -3 · Answer 8 · answered Jun 20 '15 at 23:27

-3

NSURLConnection.sendAsynchronousRequest(...)    
NSRunLoop.currentRunLoop().run()

answered Jun 20 '15 at 23:27

Tony Pan

55
5

How do I run Asynchronous callbacks in Playground

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