Unrecognized selector sent to instance (multiple view controllers) - objective-c

Question

There are two view controllers that I access using an NSTabView: ViewController1 and ViewController2. In my AppDelegate, I have a variable that I wish to share between the two:
AppDelegate.h:
#interface AppDelegate : NSObject <NSApplicationDelegate>
{
NSMutableString *myString;
}
#property (assign) NSMutableString *myString;
ViewController.h:
-(void)doStuff
{
AppDelegate *del = (AppDelegate *)[[NSApplication sharedApplication] delegate];
[del.myString setString:#"This is a test"];
}
This is the error I get when doStuff() is called:
2014-06-10 16:29:09.240 MyApp[32297:303] -[ViewController2 myString]:
unrecognized selector sent to instance 0x6100001a7700
2014-06-10 16:29:09.240 MyApp[32297:303] An uncaught exception was raised
2014-06-10 16:29:09.240 MyApp[32297:303] -[ViewController2 myString]:
unrecognized selector sent to instance 0x6100001a7700
2014-06-10 16:29:09.241 MyApp[32297:303] (
0 CoreFoundation 0x00007fff8d52d25c __exceptionPreprocess + 172
1 libobjc.A.dylib 0x00007fff93f16e75 objc_exception_throw + 43
2 CoreFoundation 0x00007fff8d53012d -[NSObject(NSObject)
doesNotRecognizeSelector:] + 205
3 CoreFoundation 0x00007fff8d48b322 ___forwarding___ + 1010
4 CoreFoundation 0x00007fff8d48aea8 _CF_forwarding_prep_0 + 120
5 Recorder 0x0000000100006b55 -[ViewController1 doStuff:] + 549
Why is there an error being thrown in the other view controller?

Answer 1

Because at some point you have reset the app delegate so that it is a ViewController2. That happened in code you have not shown, so I don't know how and when you did that. But the point is, your message myString is never arriving at the AppDelegate. It is arriving at the ViewController2.
In other words, you are saying
(AppDelegate *)[[NSApplication sharedApplication] delegate]
but in fact that object has somehow been repointed at the ViewController2. Thus, even though you are casting to an AppDelegate, it isn't an AppDelegate at all. The compiler allows you to say myString to this object because you said (falsely) that it is an AppDelegate, but the reality causes the crash at runtime when the myString message arrives at the ViewController2 object.
Look for code where you say setDelegate: or .delegate = ... to work out when you repointed the app delegate.

Answer 2

A few things:
1) The error "unrecognized selector" means you're trying to call some function ("doStuff" or "setMyString") on an object that doesn't contain a public method with that name. It crashes because it doesn't know how to respond. Most often you'll see this when you forget to hook up a storyboard properly, have a typo in a method call, or if you send a message to the wrong object.
2) You're trying to access myString on AppDelegate but it's actually being called on ViewController2. The error is because ViewController2 doesn't have a #property called myString. As the response above stated, doublecheck how you're actually assigning the appDelegate.
3) You're declaring both an instance variable myString AND the #property myString. This is redundant and really outdated (< iOS4) practice. Instead, remove the {NSMutableString *myString} portion in curly braces & only use #property (nonatomic, strong) NSMutableString *myString;
4) Unless you specifically created a setString method in your AppDelegate.m, the default way to access an #property named "myString" would probably be [del setMyString] or del.myString = ....
Calling [del.myString setString] is also a redundant mixture of dot syntax & brackets. It may throw a separate error after you fix the first one. Let the #property handle creating getter/setter methods.
Call [del setMyString:#"whatever value"]; or alternatively del.myString = #"whatever value";
5) In Obj-C this sort of crash is often avoided by first asking whether or not the object will respond:
if ([someObject respondsToSelector:#selector(doStuff:)] {//then do something
}
6) This sort of error is one good reason why Apple's new language Swift has moved away from the pattern above. Swift discourages asking about selectors & instead requires more specific/optional data types so you're less likely to run into this problem.