Question

I declare a class extension interface adding vars to it. Is it possible to access those vars in a category of that class?

Answer 1

Sure – any variable is accessible through the runtime, even if it isn’t visible in the @interface:

SomeClass.h

@interface SomeClass : NSObject {
    int integerIvar;
}

// methods

@end

SomeClass.m

@interface SomeClass() {
   id idVar;
}

@end

@implementation SomeClass

// methods

@end

SomeClass+Category.m

@implementation SomeClass(Category)

-(void) doSomething {
    // notice that we use KVC here, instead of trying to get the ivar ourselves. 
    // This has the advantage of auto-boxing the result, at the cost of some performance.
    // If you'd like to be able to use regex for the query, you should check out this answer:
    // http://stackoverflow.com/a/12047015/427309  
    static NSString *varName = @"idVar"; // change this to the name of the variable you need

    id theIvar = [self valueForKey:varName]; 

    // if you want to set the ivar, then do this:
    [self setValue:theIvar forKey:varName];
}

@end

You can also use KVC to get iVars of classes in UIKit or similar, while being easier to use than pure runtime-hacking.