Swift - Editing UITextView from inside callback crashes app - swift

Question

I have the following code that makes a HTTP Request, then outputs the result to a UITextView:
#IBOutlet var echoLog: UITextView!
#IBAction func sendEcho(sender: AnyObject) {
let callback = { (textString: String) -> Void in
self.echoLog.text = textString // App crashes here :(
}
HTTPRequest("http://localhost/echo", ["echo": "Echo!"], callback)
}
But when I call sendEcho the app crashes with this error:
2014-07-28 20:58:22.218 AppName[10463:144343] *** Assertion failure in void _UIPerformResizeOfTextViewForTextContainer(NSLayoutManager *, UIView<NSTextContainerView> *, NSTextContainer *, NSUInteger)(), /SourceCache/UIFoundation_Sim/UIFoundation-364/UIFoundation/TextSystem/NSLayoutManager_Private.m:1547
2014-07-28 20:58:22.231 AppName[10463:144343] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Only run on the main thread!'
*** First throw call stack:
(
0 CoreFoundation 0x00496ca6 __exceptionPreprocess + 182
1 libobjc.A.dylib 0x01df08bf objc_exception_throw + 44
2 CoreFoundation 0x00496b3a +[NSException raise:format:arguments:] + 138
3 Foundation 0x00904d2e -[NSAssertionHandler handleFailureInFunction:file:lineNumber:description:] + 102
4 UIFoundation 0x03344072 -[NSLayoutManager(NSPrivate) _resizeTextViewForTextContainer:] + 418
5 UIFoundation 0x03343d6e -[NSLayoutManager(NSPrivate) _recalculateUsageForTextContainerAtIndex:] + 2017
6 UIFoundation 0x0337da43 -[NSLayoutManager textStorage:edited:range:changeInLength:invalidatedRange:] + 871
7 UIFoundation 0x0337db53 -[NSLayoutManager processEditingForTextStorage:edited:range:changeInLength:invalidatedRange:] + 85
8 UIFoundation 0x033a7a7b -[NSTextStorage _notifyEdited:range:changeInLength:invalidatedRange:] + 153
9 UIFoundation 0x033a759a -[NSTextStorage processEditing] + 458
10 UIFoundation 0x033a7124 -[NSTextStorage endEditing] + 80
11 UIFoundation 0x033a71af -[NSTextStorage coordinateEditing:] + 67
12 UIKit 0x014938ae -[UITextView setAttributedText:] + 250
13 UIKit 0x01499745 -[UITextView setText:] + 149
14 AppName 0x00036216 _TFFC8AppName19FirstViewController8sendEchoFS0_FPSs9AnyObject_T_U_FOS_9JSONValueT_ + 1350
15 AppName 0x0003525a _TPA__TFFC8AppName19FirstViewController8sendEchoFS0_FPSs9AnyObject_T_U_FOS_9JSONValueT_ + 106
16 AppName 0x0001de86 _TFF8AppName11HTTPRequestFTSSGVSs10DictionarySSPSs9AnyObject__FOS_9JSONValueT__T_U_FTGSQCSo6NSData_GSQCSo13NSURLResponse_GSQCSo7NSError__T_ + 1350
17 AppName 0x0001d0a4 _TPA__TFF8AppName11HTTPRequestFTSSGVSs10DictionarySSPSs9AnyObject__FOS_9JSONValueT__T_U_FTGSQCSo6NSData_GSQCSo13NSURLResponse_GSQCSo7NSError__T_ + 100
18 AppName 0x0001e289 _TTRXFo_oGSQCSo6NSData_oGSQCSo13NSURLResponse_oGSQCSo7NSError__dT__XFo_iTGSQS__GSQS0__GSQS1____iT__ + 41
19 AppName 0x0001d15a _TPA__TTRXFo_oGSQCSo6NSData_oGSQCSo13NSURLResponse_oGSQCSo7NSError__dT__XFo_iTGSQS__GSQS0__GSQS1____iT__ + 90
20 AppName 0x0001e2d7 _TTRXFo_iTGSQCSo6NSData_GSQCSo13NSURLResponse_GSQCSo7NSError___iT__XFo_oGSQS__oGSQS0__oGSQS1___dT__ + 55
21 AppName 0x0001d224 _TPA__TTRXFo_iTGSQCSo6NSData_GSQCSo13NSURLResponse_GSQCSo7NSError___iT__XFo_oGSQS__oGSQS0__oGSQS1___dT__ + 100
22 AppName 0x0001e36e _TTRXFo_oGSQCSo6NSData_oGSQCSo13NSURLResponse_oGSQCSo7NSError__dT__XFdCb_dGSQS__dGSQS0__dGSQS1___dT__ + 142
23 CFNetwork 0x02c36158 __49-[__NSCFLocalSessionTask _task_onqueue_didFinish]_block_invoke + 181
24 Foundation 0x0092da35 __NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 12
25 Foundation 0x00854635 -[NSBlockOperation main] + 99
26 Foundation 0x00833a97 -[__NSOperationInternal _start:] + 700
27 Foundation 0x008337c9 -[NSOperation start] + 83
28 Foundation 0x00833613 __NSOQSchedule_f + 237
29 libdispatch.dylib 0x022ef3ff _dispatch_client_callout + 14
30 libdispatch.dylib 0x022d8578 _dispatch_queue_drain + 1424
31 libdispatch.dylib 0x022d7f90 _dispatch_queue_invoke + 142
32 libdispatch.dylib 0x022d9e06 _dispatch_root_queue_drain + 312
33 libdispatch.dylib 0x022dae67 _dispatch_worker_thread2 + 45
34 libsystem_pthread.dylib 0x0263fdab _pthread_wqthread + 336
35 libsystem_pthread.dylib 0x02643cce start_wqthread + 30
)
libc++abi.dylib: terminating with uncaught exception of type NSException
Thanks in advance!

Answer 1

It looks like your problem is that you are not on the mainThread, by the second line in the trace.
You are probably still running code in the background, due to the nature of HTTPRequest asynchronous handling of data... that's fine. You just need to get to the main thread, after all is said and done.
Here's how to probably fix it...
#IBAction func sendEcho(sender: AnyObject)
{
let callback = {(textString: String) in
dispatch_sync(dispatch_get_main_queue())
{
self.echoLog.text = textString //Yay!
}
}
HTTPRequest("http://localhost/echo", ["echo": "Echo!"], callback)
}
Reminder time UI updates should only be performed on the mainThread. Simply because doing so provides high priority for this thing to occur. Usually when you try to do UI updates on a background thread, you will see a delay between when the code is executed and when you see results. In this case, an Assertion failure (I like that). If you do not know if you are on the main thread, you can check it out by doing NSThread.isMainThread() which returns a bool.
Warning If you are already on the main thread while calling dispatch_sync(dispatch_get_main_queue()) {...} your app will freeze up, so make sure you know what thread you are on...

Answer 2

Apple will only let you modify UI components on the main event loop. It appears that the HTTPRequest is running callback on a background thread so you will have to explicitly run the change on the main thread:
let callback = { (textString: String) -> Void in
dispatch_async(dispatch_get_main_queue(), {
self.echoLog.text = textString
})
}