Mickael Bellboys

For an app, performance is very important and as an iPhone user, no one wants to wait to see a response after taking an action, if it happens then it can cost you a negative rating for your app.

We can optimize the performance of an app in so many ways, concurrency is one of them

What is Concurrency?

If more than one task is happening at the same time then it’s called concurrency. Since 2011 all iPhone supports multiple tasks to execute at the same time. We can run multiple tasks at the same time by adding some code.

this article is all about improving the performance of the app, we can run code concurrently mainly by two ways in iOS -

Let’s deep dive into it -

Grand Central Dispatch(GCD)

Dispatch, also known as Grand Central Dispatch (GCD), contains language features, runtime libraries, and system enhancements that provide systemic, comprehensive improvements to the support for concurrent code execution on multicore hardware in macOS, iOS, watchOS, and tvOS.The BSD subsystem, Core Foundation, and Cocoa APIs have all been extended to use these enhancements to help both the system and your application to run faster, more efficiently, and with improved responsiveness. Consider how difficult it is for a single application to use multiple cores effectively, let alone to do it on different computers with different numbers of computing cores or in an environment with multiple applications competing for those cores. GCD, operating at the system level, can better accommodate the needs of all running applications, matching them to the available system resources in a balanced fashion. — Apple documentation.

Serial and concurrent queues

Serial queues only have a single thread associated with them and thus allows only a single task to be executed at any given point of time.

A concurrent queue can utilize as many threads as the system has resources for. A thread will be created and released as necessary in the concurrent queue.

Synchronous and asynchronous tasks

If we have some tasks pending in queue either we can complete it synchronously or asynchronously.

When a task is running synchronously, the current queue will be blocked until the task is not completed before moving to another task.

On the other hand, asynchronous task on the current queue will start, but return control to the current queue immediately. This way the current queue will not be blocked and we can perform other tasks on the current queue.

We can finish the task faster by running it concurrently instead of serial.

Dispatch Queues

when you create a queue, the OS will assign one or more threads to the queue. If an existing thread is available, they can be reused; if not, then the OS will create them accordingly. Creating a dispatch queue -

That’s it, a serial queue has been created. :)

when the app starts by default a main dispatch queue will be created for you. it’s a serial queue and its mainly responsible for UI. You can access it like -

You should not execute any synchronous task here unless it is not related to UI. Otherwise, it will freeze your UI until the execution of the synchronous task.

You can create a concurrent queue by passing attribute as .concurrent. Like this -

Concurrent queues are divided into six different global concurrent queues categories, depending on the Quality of service (QoS).

Quality of service

You can prioritize your task using QoS. Higher priority tasks should perform faster compared to the lower priority task because it takes more system resources. If you want a concurrent queue and don’t want to manage it on your own then use pre-defined global queues.

Here are different Quality of services -

.default and .unspecified are rarely used Qos.

You can create your own concurrent dispatch queue with Qos like this -

You can add a task to queues like this -

Always update UI inside main queue. If you want to use the sync method then use it carefully here otherwise it can cause you a deadlock situation. Never call sync method from main thread it will block main thread and cause you deadlock.

When you are implementing concurrency in your app, you should be careful otherwise you can face concurrency problems like Race conditions, deadlock, or priority inversion.

Sometimes you need to execute multiple tasks at the same time but it doesn’t matter which task is finishing first, you just want to know when all tasks are finish. For this scenario, we can use DispatchGroup. Here is a basic implementation of DispatchGroup.

In some cases, you want to specify how many threads have access to a shared resource in this situation we can use a semaphore.

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