GCD Introduction to concurrency and parallelism in iOS

Posted By : Anmol Aggarwal | 31-Aug-2019


Introduction to concurrency and parallelism in iOS



What is GCD?


GCD stands for Grand Central Dispatch. In swift we have very less options when it comes to asynchronous operations, these are that part of the code which we don’t want to execute immediately as in our main execution flow but we rather executed along some other block of code or after completion of certain tasks.


GCD is one such option or we can say it is a “low-level API for managing concurrent operations” used to make your applications run smoothly and provide great responsiveness improving UX for the users using your application. It manages multiple tasks that are needed to be done at same time.


We all have seen code like this:

DispatchQueue.main.async {
       // Perform your async code here




DispatchQueue API works like a company having various low as well high levels of workers, so, that it can manage both heavy and light tasks. We can use DispatchQueue as explained below:


1.Perform network task with UI updates:

  • Global Queue – We use it to perform non-UI related work in the background so that are application doesn’t seem to freeze.
  • Main Queue - We use it to update the UI after task on a concurrent queue is successfully completed.

2. List of DispatchQueue Priority:




3.Perform background Quality of Service (QOS) task:

DispatchQueue.global(qos: .background).async { 
    // Call your background task
    DispatchQueue.main.async { 
       // UI Updates here for task complete.


4.DispatchQueue with delay:

let deadlineTime = DispatchTime.now() + .seconds(1)

DispatchQueue.main.asyncAfter(deadline: deadlineTime) {
    //Perform code here


5. Dispatch Groups:

let dispatchGroup = DispatchGroup()
loadUseractivities {
loaduserComments {
dispatchGroup.notify(queue: .main) {
 print("All the Tasks have been Completed!!")




Finally we are able to say Grand central dispatch is that the powerful API for multitasking with sync and async programming in iOS. Also, GCD is a powerful tool, and while working with asynchronous closures onto different queues barely scratches the surface of what the framework is capable of — it very well may be a great method to begin working with asynchronous operations and concurrency using an extremely lightweight syntax.


About Author

Author Image
Anmol Aggarwal

Anmol is in mobile team, She is hard working developer and dedicated towards her work.

Request for Proposal

Name is required

Comment is required

Sending message..