Is there a common run loop for all queues, or does each queue have its own run loop?

Run loop is associated with a thread, not a queue. Each main thread has its own run loop, which is created automatically. Background threads do not have one by default, but it can be created and started if needed. GCD queues do not have their own run loops; they manage task execution based on a thread pool (sometimes using the main thread).

// Getting the current run loop for the current thread
let currentRunLoop = RunLoop.current

// Checking if the current thread is the main thread
if Thread.isMainThread {
    print("Current thread is the main")
    // The main thread always has a run loop
} else {
    print("Current thread is a background")
    // A background thread may need to create and start a run loop explicitly
}

// GCD queues do not have a run loop property
// DispatchQueue.global().sync {
//     Just execute a task
// }

• Main Thread: Has a RunLoop created automatically. It handles UI events, timers, and other asynchronous operations.
• Background Threads: Can have a RunLoop if needed (for example, to manage timers or ports), but it must be initialized and started manually.
• GCD Queues: Do not use RunLoop directly. They enqueue tasks, which are then executed on available threads from the pool.