This post is based on a short talk I gave about using GCD with Swift 3.

We have been pretty bad with using the full capabilities of GCD - using only the main serial queue and the global background queue. Hopefully this will help you learn from our lesson.

What is GCD?

Grand Central Dispatch was developed by Apple in order to simplify work with systems with multi-core processors and released with Mac OS X Snow Leopard and iOS 4.

GCD’s task parallelism is based on the thread pool pattern. Multiple threads are always available — waiting for tasks to be executed concurrently.

Since we don’t need to keep creating and destroying threads we get great performance and low execution latency.

Dispatch Queues

Dispatch queues manages tasks in FIFO order. Tasks run in the order in which they are added to the queue — the first task in the queue will be the first to start.

There are two types of dispatch queues:

S erial Queues

Execute one task at a time

Can be used to synchronize access to a specific resource

Concurrent Queues

Execute one or more tasks concurrently

Tasks are completed according to complexity and not by the order in the queue

Synchronous vs. Asynchronous

When using dispatch queues you can choose to execute your code synchronously or asynchronously.

With synchronous execution the program waits for the work to finish before returning.

With asynchronous execution the program returns immediately without waiting for the work to complete.

Thread Safety

Dispatch queues are thread safe. They can be accessed from different threads simultaneously without locking. Developers can use dispatch queues to make their own code thread safe.

Types of Dispatch Queues

Main Queue

When your app launches, the system automatically creates a serial queue and binds it to the application’s main thread. All UI tasks have to be run on the main thread.

Global Queues

There are four global concurrent queues which are provided and shared by the system.

Quality of Service attribute indicates the priority of the tasks in the queue:

User-interactive: update the UI or other small tasks that should occur instantly. Tasks in this queue are the highest priority tasks and they will run on the main thread.

User-initiated: tasks in this queue are tasks that should run immediately -like opening documents or react to user actions. Tasks should complete in a few seconds or less. Will be mapped to High priority queue.

Utility: this queue is for longer tasks that should complete immediately (think tasks with a loading bar such as downloading or importing). Tasks should complete in seconds or minutes. Will be mapped to Low priority queue.

Background: tasks that takes minutes to hours to complete — indexing, syncing, etc… This queue is energy optimized and any disk I/O actions are throttled. Will be mapped to Background priority queue.

Custom Queues

Serial queues must be created and managed by the developer. You can also create custom concurrent queues but it is encouraged to use one of the global queues instead.

Code examples

Working with system queues

Working with custom queues/

Performing background tasks and then updating the UI

Beware of deadlocks with serial queues

Beware of Synchronously running on the main thread from a synchronous background thread

Where to go from here?