Add property in a category Collected from http://inchoo.net/mobile-development/iphone-development/how-to-add-a-property-via-class-category/

PropertyInCategoryFile.h

// 

// category_file.h
@interface SomeClass (Private)
@property (nonatomic, assign) id newProperty;
@end
NSString * const kNewPropertyKey = @"kNewPropertyKey";
@implementation SomeClass (Private)
@dynamic newProperty;
- (void)setNewProperty:(id)aObject
{
    objc_setAssociatedObject(self, kNewPropertyKey, aObject, OBJC_ASSOCIATION_ASSIGN);
}
- (id)newProperty
{
    return objc_getAssociatedObject(self, kNewPropertyKey);
}
@end

PropertyInCategoryFile.m

// category_file.m
@interface MyClass : NSObject
@property (retain, readonly) float value;
@end
// Private extension, typically hidden in the main implementation file.
@interface MyClass ()
@property (retain, readwrite) float value;
@end
@interface MyClass : NSObject
- (float)value;
@end
@interface MyClass () {
    float value;
}
- (void)setValue:(float)newValue;
@end
@implementation MyClass
- (float)value {
    return value;
}
- (void)setValue:(float)newValue {
    value = newValue;
}
@end