Having trouble with class extension in Objective C, mac OS. Getting error 'NSInvalidArgumentException', no visible #interface declares the selector - objective-c

Question

I'm working on exercise 3 on page 76 in Apple's developer pdf in the class categories and extensions section, "Programming with Objective C", found here: (https://developer.apple.com/library/mac/documentation/cocoa/conceptual/ProgrammingWithObjectiveC/Introduction/Introduction.html)
my XYZPerson header looks like this:
#import <Foundation/Foundation.h>
#interface XYZPerson : NSObject
#property (readonly) NSNumber *height;
#property (readonly) NSNumber *weight;
-(NSNumber *)measureHeight;
-(NSNumber *)measureWeight;
#end
My implementation file looks like this:
#import "XYZPerson.h"
#property (readwrite) NSNumber *height;
#property (readwrite) NSNumber *weight;
#end
/////////////////////
#implementation XYZPerson
-(NSNumber *)measureHeight{
if(!_height){
_height = [[NSNumber alloc] init];
}
return _height;
}
-(NSNumber *)measureWeight{
if(!_weight){
_weight = [[NSNumber alloc] init];
}
return _weight;
}
#end
And then in my main file I have:
#import <Foundation/Foundation.h>
#import "XYZPerson.h"
int main(int argc, const char * argv[])
{
#autoreleasepool {
XYZPerson *aPerson = [[XYZPerson alloc] init];
[aPerson setHeight: [NSNumber numberWithInt:72] ];
[aPerson setWeight: [NSNumber numberWithInt:190] ];
NSLog(#"%# %#",[aPerson measureHeight],[aPerson measureWeight]);
}
return 0;
}
There might be more than one error unrelated to the issue I brought up, I'm a huge novice at Objective C right now. The exact compiler error I am getting is on the line that says,
[aPerson setHeight: [NSNumber numberWithInt:72] ];
The compiler error reads, "ARC Semantic Issue. No visible #interface for 'XYZperson' declares the selector 'setWeight:'.

Answer 1

Oh doh.
You will not be able to call that method from OUTSIDE of your class implementation.
Overriding properties in class extensions in the .m file doesn't expose that to the world.
At least not to the compiler.
That's the whole point.
It's for when you want properties that are readonly to the world outside of that object, but internally to the object you still want to be able to conveniently do things.

Answer 2

You declared the property read-only in the header file, so to everything outside of the class, the properly is read-only, and thus a setHeight: method doesn't exist.

Answer 3

You have made he properties height and weight read only. Then in the main function you are using setWeight and setHeight.
You can't do that as that would be writing the weights but you explicitly set them to read only.

Answer 4

Place your properties in .m with class extension:
#interface XYZPerson ()
#property (readwrite) NSNumber *height;
#property (readwrite) NSNumber *weight;
#end
See Apple docs about class extension:
https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/CustomizingExistingClasses/CustomizingExistingClasses.html