Calling a method from another class WITHOUT creating a new instance - objective-c

Question

This is a common topic but, in my case there is one thing I don't understand that I can't find explained in the other asked questions.
Here is the gist of what I'm trying to do:
User clicks a button and something like this is called:
#implementation FirstClass
-(void)clickedButton
{
[SecondClass changeText];
}
And then in SecondClass is:
#implementation SecondClass
- (void)changeText {
[myLabel setText:#"text"];
}
So when the user clicks the button, the text property in myLabel in SecondClass changes to "text".
The only problem I have with this is calling [SecondClass changeText] on the existing instance of SecondClass. Since I'm not initializing the CCNodes programmatically (they are all automatically loaded upon running the app), I don't know where or how SecondClass is initialized. I'm using SpriteBuilder to build this project.
Any help would be appreciated. Thanks!

Answer 1

So, you have two instaces -- one with a button, and one with a label. I'm assuming they are both descendants of NSViewController or otherwise manage underlying views.
The problem is, you found no way to address second instance containing label from the method of first instance.
You need to define a property in first instance's class:
#property(weak) SecondClass *secondInstance;
And then in button clicked method:
-(void)clickedButton
{
[self.secondInstance changeText];
}
There is one issue left: who is responsible to set first instance's property that we defined? This depends on who did create both of them, probably just app delegate or enclosing controller, you know that better.
UPD: If both of the controllers are created by AppDelegate:
#import "FirstClass.h"
#import "SecondClass.h"
#interface AppDelegate ()
// case A - manual
#property(strong) FirstClass *firstInstance;
#property(strong) SecondClass *secondInstance;
// case B - declared in xib
//#property(weak) IBOutlet FirstClass *firstInstance;
//#property(weak) IBOutlet SecondClass *secondInstance;
#end
#implementation AppDelegate
...
- (void)applicationDidFinishLaunching:(NSNotification *)notification
{
// Create them
self.firstInstance = [[FirstClass alloc] init...];
self.secondInstance = [[SecondClass alloc] init...];
// Or maybe they are declared in MainMenu.xib, then you do not create them
// by hand, but must have outlets for both. See case B above.
// Connect them
self.firstInstance.secondInstance = self.secondInstance;
...
}
Note that class is not the same as an object (instance). Class is a named collection of methods, mostly for the instance. In Objective-C, class is not just a name, but an object too, so you can call a method on it (i.e. send an message to the class object). But here we always talk about objects (instances), so forget about classes – we hold objects via strong properties or weak outlets, depending on how they were created, and operate on objects, never on classes.

Answer 2

In objective C, the methods are either instance methods or class methods. As the name suggests, the instance methods require an instance of the class to work, whereas the class methods can be used with just the name of the class. What you need here is a class method. Just change the following line in your code:
#implementation SecondClass
- (id)changeText {
to
#implementation SecondClass
+ (id)changeText {
This will change the method from an instance method to a class method.