blocks|key|4615701|text|不调用回调的原因是因为RunLoop没有在Playground中运行(或者在REPL模式下运行)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4615702|让回调运行的一种有点古怪但有效的方法是使用一个标志，然后在runloop上手动迭代：|4615703|//+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)
}|code-block|syntax|javascript|4615704|此模式经常用于需要测试异步回调的单元测试中，例如：Pattern+for+unit+testing+async+queue+that+calls+main+queue+on+completion|offset|length|4615705|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/7817605/pattern-for-unit-testing-async-queue-that-calls-main-queue-on-completion^0|0|0|0|P|20|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@$K|Y|L|Z|1|10]]|A|$]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]]]

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:

<pre><code>// 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)
}
</code></pre>

This pattern has often been used in Unit Tests which need to test async callbacks, for example: <a href="https://stackoverflow.com/questions/7817605/pattern-for-unit-testing-async-queue-that-calls-main-queue-on-completion">Pattern for unit testing async queue that calls main queue on completion</a>

blocks|key|4615750|text|虽然您可以手动运行运行循环(或者，对于不需要运行循环的异步代码，可以使用其他等待方法，如分派信号量)，但我们在游乐场中提供的等待异步工作的“内置”方法是导入XCPlayground框架并设置XCPlaygroundPage.currentPage.needsIndefiniteExecution+=+true。如果设置了此属性，当顶级游乐场源代码完成时，我们将继续旋转主运行循环，以便异步代码有机会运行，而不是停止游乐场。我们最终将在超时后终止游乐场，该超时默认为30秒，但如果您打开助手编辑器并显示时间线助手，则可以对其进行配置；超时在右下角。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4615751|例如，在Swift+3中(使用URLSession而不是NSURLConnection)：|4615752|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|code-block|syntax|javascript|4615753|或者在Swift+2中：|4615754|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|4615755|entityMap^0|26|C|2N|1O|0|F|A|S|F|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@$9|Y|A|Z|B|C]|$9|10|A|11|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|13|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|R|$]]

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 <code>XCPlayground</code> framework and set <code>XCPlaygroundPage.currentPage.needsIndefiniteExecution = true</code>. 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 <code>URLSession</code> instead of <code>NSURLConnection</code>):

<pre><code>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
</code></pre>

Or in Swift 2:

<pre><code>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
</code></pre>

blocks|key|109547|text|NSURLConnection.sendAsynchronousRequest(...)++++
NSRunLoop.currentRunLoop().run()|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|109548|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>NSURLConnection.sendAsynchronousRequest(...) 
NSRunLoop.currentRunLoop().run()
</code></pre>

blocks|key|4615823|text|从XCode+7.1开始，不推荐使用XCPSetExecutionShouldContinueIndefinitely()。现在这样做的正确方法是首先请求无限期执行作为当前页面的一个属性：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4615824|import+XCPlayground

XCPlaygroundPage.currentPage.needsIndefiniteExecution+=+true|code-block|syntax|javascript|4615825|…然后指示执行何时完成，如下所示：|4615826|XCPlaygroundPage.currentPage.finishExecution()|4615827|例如：|4615828|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()|4615829|entityMap^0|I|17|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|10|8|@]|D|@]|E|$]]|$1|Q|3|R|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|S|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|T|$]]

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

<pre><code>import XCPlayground

XCPlaygroundPage.currentPage.needsIndefiniteExecution = true
</code></pre>

…then indicate when execution has finished with:

<pre><code>XCPlaygroundPage.currentPage.finishExecution()
</code></pre>

For example:

<pre><code>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()
</code></pre>

blocks|key|4615850|text|此API在Xcode8中再次更改，并被移至PlaygroundSupport|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4615851|import+PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution+=+true|code-block|syntax|javascript|4615852|Session+213+at+WWDC+2016中提到了这一变化。|4615853|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.apple.com/videos/play/wwdc2016/213/^0|L|H|0|0|0|O|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|M|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

This API changed again in Xcode 8 and it was moved to the <code>PlaygroundSupport</code>:

<pre><code>import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true
</code></pre>

This change was mentioned in <a href="https://developer.apple.com/videos/play/wwdc2016/213/">Session 213 at WWDC 2016</a>.

blocks|key|4615879|text|针对XCode8、Swift3和iOS+10的新API包括：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4615880|//+import+the+module
import+PlaygroundSupport
//+write+this+at+the+beginning
PlaygroundPage.current.needsIndefiniteExecution+=+true
//+To+finish+execution
PlaygroundPage.current.finishExecution()|code-block|syntax|javascript|4615881|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

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

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

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:

<pre><code>// 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)
 }
}
</code></pre>

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

How do I run Asynchronous callbacks in Playground

许多Cocoa和CocoaTouch方法在Objective-C中将完成回调实现为块，在Swift中将闭包实现为闭包。但是，当在Playground中尝试这些时，从不调用完成。例如：// Playground - noun: a place where people can playimport Cocoaimport...

问如何在Playground中运行异步回调
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