Introduction
                        
                            Grand Central Dispatch (GCD)   In iOS, Apple provides two ways to do multitasking: The Grand Central Dispatch (GCD) and NSOperationQueue frameworks.We will discuss here about GCD.   GCD is a lightweight way to represent units of work that are going to be executed concurrently You don’t schedule these units of work; the system takes care of scheduling for you. Adding dependency among blocks can be a headache. Canceling or suspending a block creates extra work for you as a developer!