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...
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