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 co

Answer 1

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

Answer 2

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

Answer 3

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()

Answer 4

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()

Answer 5

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.

Answer 6

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()