Objective-C 

is a reflective, object-oriented programming language that adds Smalltalk-stylemessaging to the C programming language.

Today, it is used primarily on Apple's Mac OS X and iOS: two environments based on theOpenStep standard, though not compliant with it.^[1] Objective-C is the primary language used for Apple's Cocoa API, and it was originally the main language on NeXT's NeXTSTEP OS. Generic Objective-C programs that do not use these libraries can also be compiled for any system supported by gcc or Clang.

   Learn Objective-C: Day 1
   Learn Objective-C: Day 2
   Learn Objective-C: Day 3
   Learn Objective-C: Day 14
   Learn Objective-C: Day 5
   Learn Objective-C: Day 6