ios - Has NSURLSession dataTaskWithRequest completion block always run on Main Thread? -

it seems me changes ios8.4 maybe. completion block nsurlsession datataskwithrequest running on main thread rather thread executing resume. new and/or correct behavior?

there many postings this users want completion code on main thread, find there (breaking code). here isolated sample:

func application(application: uiapplication, didfinishlaunchingwithoptions launchoptions: [nsobject: anyobject]?) -> bool {     // override point customization after application launch.     dispatch_async(dispatch_get_global_queue(qos_class_background, 0)) {         self.threadtest()     }     return true }  func threadtest() {     println("threadtest on main thread: \(nsthread.currentthread().ismainthread)")     let urlstring = "http://www.apple.com/contact/"     let url = nsurl(string:urlstring)     var request = nsmutableurlrequest(url: url!)     var configuration = nsurlsessionconfiguration.defaultsessionconfiguration()     var session = nsurlsession(configuration: configuration, delegate:nil, delegatequeue:nsoperationqueue.mainqueue())      var task = session.datataskwithrequest(request) {         (data: nsdata!, response: nsurlresponse!, error: nserror!) -> void in          if error != nil {             print("error: \(error.localizeddescription)")         }         else {             if let result = nsstring(data: data, encoding:nsasciistringencoding)                    {                 let partial = result.substringtoindex(10)                 println("retrieved page: \(partial)...")                 println("nsurlsession.datatashwithrequest completion block on main thread: \(nsthread.currentthread().ismainthread)")             }         }     }     task.resume() } 

with results:

threadtest on main thread: false retrieved page: <!doctype ... nsurlsession.datatashwithrequest completion block on main thread: true

i figured out answer. thread of completion handler setup in init of nsurlsession.

from documentation:

init(configuration configuration: nsurlsessionconfiguration, delegate delegate: nsurlsessiondelegate?, delegatequeue queue: nsoperationqueue?)

queue - queue scheduling delegate calls , completion handlers. if nil, session creates serial operation queue performing delegate method calls , completion handler calls.

my code setup completion on main thread:

var session = nsurlsession(configuration: configuration, delegate:nil, delegatequeue:nsoperationqueue.mainqueue())