What does the error “unrecognized selector sent to instance” mean in Xcode

Question

What does it mean \"unrecognized selector sent to instance\" in Xcode?

Answer 1

In my case it means I did not understand (for two days) a very simple requirement of the handler (selector, function): I had left off the ...:(NSNotification*)notification... in my selector (function).

In the end it is just a self.stupidMistake (or programming tired while trying to understand a new thing in iOs/xCode). I read the docs at apple, I read many, many here at stackoverflow and read all kinds of other pages from the search results and just kept overlooking the fact that I had: in the viewDidLoad:

[[NSNotificationCenter defaultCenter] addOberserver:self selector:@selector(myHandler:) name:@"UIApplicationWillResignActiveNotification" object:nil];

in the .h (declaration) and .m (real code) I had invented:

-(void)myHandler { ... }

This generated the unrecognized selector sent to instance (crash and debug output) at runtime (no errors or warnings in xcode). Then I spent almost two whole days trying to figure out the error and the error was:

-(void)myHandler:(NSNotification*)notification { ... }

Hope it helps anyone else stuck - it is a syntax thing (your Selector or Handler or Function or whatever you want to call it) must take a (NSNotification*) 'object' as a parameter whether you use it or not; and xcode (4.2 w/iOs SDK 5.0) does not generate any errors or warnings about this 'mistake'.