How to resolve unrecognized selector sent to instance

Question

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  lt UIApplicationDelegate gt    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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

1  Is the synthesize within  implementation block   2  Should you refer to self classA     ClassA alloc  init   and self classA downloadUrl          instead of plain classA   3  In your myApp m file you need to import ClassA h  when it s missing it will default to a number  or pointer   in C variables default to int if not found by compiler     import  ClassA h

User · Answer

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

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

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