Warning: UIKit should not be called from a secondary thread

Question

In my iPhone app I need to connect to a web server as this can take some time I'm using threads like this:

[NSThread detachNewThreadSelector:@selector(sendStuff) toTarget:self withObject:nil];

- (void)sendStuff {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    //Need to get the string from the textField to send to server
    NSString *myString = self.textField.text;

    //Do some stuff here, connect to web server etc..

    [pool release];
}

On the row where I use self.textField I get a warning in console saying: void _WebThreadLockFromAnyThread(bool), 0x5d306b0: Obtaining the web lock from a thread other than the main thread or the web thread. UIKit should not be called from a secondary thread.

How can I use the textField without getting this error?

To help you we would need to know, why you are detaching a thread (What takes so long to do), which UI information you need and which UI information you want to update. — tonklon, Jul 08 '10 at 08:32
Hi! I updated the question. I need to connect to web server and I need to use the text from the textField to send to server. — Martin, Jul 08 '10 at 09:25
You should accept Graham Lee or tonklon's answer. You should pull the search text before you spawn the thread and pass it as an argument to the selector. — Christopher Pickslay, Sep 26 '11 at 23:08

score 12 · Answer 1 · answered Jul 08 '10 at 08:31

It depends a little bit on what you want to do with the textField. If reading the value is the only thing, you can do:

[NSThread detachNewThreadSelector:@selector(sendStuff) toTarget:self withObject:self.textField.text];

- (void)sendStuff:(NSString*)myString {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    //Do someting with myString
    [pool release];
}

If you want to change a value on the textField you could:

[self.textField performSelectorOnMainThread:@selector(setText:) withObject:@"new Text"];

score 11 · Answer 2 · edited Feb 15 '18 at 15:03

11

Perform any selectors that handle UI updates on the main thread. You can do this with the NSObject method -performSelectorOnMainThread:withObject:waitUntilDone:

edited Feb 15 '18 at 15:03

Xavier Lowmiller

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24

answered Jul 08 '10 at 08:24

Alex Reynolds

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score 3 · Answer 3 · answered Jul 08 '10 at 08:25

3

Why not:

[NSThread detachNewThreadSelector: @selector(sendStuff:) toTarget: self withObject: self.textField.text];

?

answered Jul 08 '10 at 08:25

score 0 · Answer 4 · edited Feb 18 '14 at 09:19

0

[self performSelectorOnMainThread:@selector(myMethod) withObject:nil waitUntilDone:NO];

will work

edited Feb 18 '14 at 09:19

kleopatra

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answered Feb 18 '14 at 09:14

user3269480

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1

score 0 · Answer 5 · answered Jul 08 '10 at 08:21

This is indeed unsafe behaviour. The MainThread is the only one that should interface the UI. Have your thread return for instance a string to the mainthread and have a method there update the UI. You can do for instance do this by passing a selector to the other thread method, and then have the other thread call the selector on the mainthread.

Warning: UIKit should not be called from a secondary thread

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