[objective-c] How to resolve 'unrecognized selector sent to instance'?

In the AppDelegate, I'm alloc'ing an instance defined in a static library. This instance has an NSString property set a "copy". When I access the string property on this instance, the app crashes with 'unrecognized selector sent to instance'. Xcode provides a code hint for the property, which means it is known in the calling app. The particular class is compiled into the static library target. What am I missing?

Adding some code.

//static library 
//ClassA.h
@interface ClassA : NSObject {
...
NSString *downloadUrl;
}
@property(nonatomic, copy) NSString *downloadUrl;

//ClassA.m
@synthesize downloadUrl;

In the calling app's appDelegate.

//app delegate header file
@interface myApp : NSObject <UIApplicationDelegate> {
ClassA *classA;
}
@property (nonatomic, retain) ClassA *classA;

//app delegate .m file
@synthesize classA;

- (void)applicationDidFinishLaunching:(UIApplication *)application {
classA = [[ClassA alloc] init];
//exception occurs here.  downloadUrl is of type NSCFNumber
classA.downloadUrl = @"http://www.abc.com/";
...}

Other classes in the app will get a reference to the delegate and call classA.downloadUrl.

Mine was something simple/stupid. Newbie mistake, for anyone that has converted their NSManagedObject to a normal NSObject.

I had:

@dynamic order_id;

when i should have had:

@synthesize order_id;

You should note that this is not necessarily the best design pattern. From the looks of it, you are essentially using your App Delegate to store what amounts to a global variable.

Matt Gallagher covered the issue of globals well in his Cocoa with Love article at http://cocoawithlove.com/2008/11/singletons-appdelegates-and-top-level.html. In all likelyhood, your ClassA should be a singleton rather than a global in the AppDelegate, although its possible you intent ClassA to be more general purpose and not simply a singleton. In that case, you'd probably be better off with either a class method to return a pre-configured instance of Class A, something like:

+ (ClassA*) applicationClassA
{
    static ClassA* appClassA = nil;
    if ( !appClassA ) {
        appClassA = [[ClassA alloc] init];
        appClassA.downloadURL = @"http://www.abc.com/";
    }
    return appClassA;
}

Or alternatively (since that would add application specific stuff to what is possibly a general purpose class), create a new class whose sole purpose is to contain that class method.

The point is that application globals do not need to be part of the AppDelegate. Just because the AppDelegate is a known singleton, does not mean every other app global should be mixed in with it even if they have nothing conceptually to do with handling the NSApplication delegate methods.

Very weird, but. You have to declare the class for your application instance as myApplication: UIApplication instead of myApplication: NSObject . It seems that the UIApplicationDelegate protocol doesn't implement the +registerForSystemEvents message. Crazy Apple APIs, again.

For me, what caused this error was that I accidentally had the same message being sent twice to the same class member. When I right clicked on the button in the gui, I could see the method name twice, and I just deleted one. Newbie mistake in my case for sure, but wanted to get it out there for other newbies to consider.

In the code you posted, you're sending the setDownloadURL: setter to ClassA — that is, the class itself. You want to set the property of an instance.

Set flag -ObjC in Other linker Flag in your Project setting... (Not in the static library project but the project you that is using static library...) And make sure that in Project setting Configuration is set to All Configuration

How are you importing ClassA into your AppDelegate Class? Did you include the .h file in the main project? I had this problem for a while because I didn't copy the header file into the main project as well as the normal #include "ClassA.h."

Copying, or creating the .h solved it for me.

A lot of people have given some very technical answers for this and similar questions, but I think it's simpler than that. Sometimes if you're not paying attention a selector that you don't intend to use can be attached to something in the interface. You might be getting this error because the selector's there but you haven't written any code for it.

The easiest way to double-check that this is not the case is to control-click the item so you can see all of the selectors that are associated with it. If there's anything in there that you don't want to be, get rid of it! Hope this helps...