EducaSwift

Async vs Sync

All the queues we've seen so far have been executed with the keyword async. However, there's an option to execute them with sync. The difference is that async tasks will not run until sync tasks have finished their execution.

Let's see an example using two queues with async.

DispatchQueue.global(qos: .background).async {
    for i in 0..<5 {
        print("👾 \(i)")
    }
}

DispatchQueue.global(qos: .userInteractive).async {
    for i in 0..<5 {
        print("🤖 \(i)")
    }
}

As we mentioned earlier, .userInteractive has higher priority than .background.

🤖 0
👾 0
👾 1
🤖 1
🤖 2
🤖 3
🤖 4
👾 2
👾 3
👾 4

If we execute our background queue with sync, we will see that this queue finishes execution first.

DispatchQueue.global(qos: .background).sync {
    for i in 0..<5 {
        print("👾 \(i)")
    }
}

DispatchQueue.global(qos: .userInteractive).async {
    for i in 0..<5 {
        print("🤖 \(i)")
    }
}

👾 0
👾 1
👾 2
👾 3
👾 4
🤖 0
🤖 1
🤖 2
🤖 3
🤖 4

Conclusion

Unless it's for something specific, we will generally use the async method.