Question

I’m getting an error when trying to execute the performSegueWithIdentifier method inside a setCompletionBlock of an ASIHTTPRequest request.

Here’s some code (certain portion were ommitted):

// Instantiate request object
ASIFormDataRequest *request = [ASIHTTPRequest requestWithURL:[NSURL URLWithString:@"http://[...]"]];

// Set request headers
[request addRequestHeader:@"Content-Type" value:@"application/json; charset=utf-8"];

// Format JSON request
NSString *json = [...]

// Set the post data
[request setPostBody:[[NSMutableData alloc] initWithData:[json dataUsingEncoding:NSASCIIStringEncoding]]];

__weak ASIFormDataRequest *_request = request;

// Handle success
[request setCompletionBlock:^{

    // Get the response
    NSDictionary *response = [[_request responseString] JSONValue];

    // Do some stuff with the response...

    // Show the title list
    [self performSegueWithIdentifier:@"ShowTitles" sender:self];

}];

[request startAsynchronous];

The exact error I’m getting is:

bool _WebTryThreadLock(bool), 0x7d70520: Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now...
1   WebThreadLock
2   -[UITextRangeImpl isEmpty]
3   -[UITextRange(UITextSelectionAdditions) _isCaret]
4   -[UITextSelectionView setCaretBlinks:]
5   -[UIKeyboardImpl setCaretBlinks:]
6   -[UIKeyboardImpl setDelegate:force:]
7   -[UIKeyboardImpl setDelegate:]
8   -[UIPeripheralHost(UIKitInternal) _reloadInputViewsForResponder:]
9   -[UIResponder _finishResignFirstResponder]
10  -[UIResponder resignFirstResponder]
11  -[UITextField resignFirstResponder]
12  -[UIView(UITextField) endEditing:]
13  -[UIWindowController _prepareKeyboardForTransition:fromView:]
14  -[UIWindowController transition:fromViewController:toViewController:target:didEndSelector:]
15  -[UIViewController presentViewController:withTransition:completion:]
16  -[UIViewController presentViewController:animated:completion:]
17  -[UIViewController presentModalViewController:animated:]
18  -[UIStoryboardModalSegue perform]
19  -[UIStoryboardSegueTemplate perform:]
20  -[UIViewController performSegueWithIdentifier:sender:]
21  -[BaseLoginViewController viewTitleList]
22  __39-[BaseLoginViewController getTitleList]_block_invoke_0
23  -[ASIHTTPRequest handleStreamComplete]
24  -[ASIHTTPRequest handleNetworkEvent:]
25  _signalEventSync
26  _cfstream_shared_signalEventSync
27  __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__
28  __CFRunLoopDoSources0
29  __CFRunLoopRun
30  CFRunLoopRunSpecific
31  CFRunLoopRun

From that error I’m assuming it has something to do with the fact that I’m trying to go to a new view controller while I’m still using the web thread or something. I’m still new to iOS dev so I’m not too sure. Any help would really be appreciated.

Answer 1

Indeed, you modify the UI from thread other than the UI (main) thread here:

[self performSegueWithIdentifier:@"ShowTitles" sender:self];

You need to execute it on the main thread. You can do it using performSelectorOnMainThread.