When should you use a run loop in iOS development?

Run loop should be used when it is necessary to control the behavior of a thread, allowing it to wait for events and execute corresponding code, instead of terminating immediately after completing its task. This is critical for:

1. Main Thread: The main thread's run loop handles interactions with UIKit/AppKit, user events (touches, clicks), timers, network events, and calls to performSelector:withObject:afterDelay:. Without it, the application cannot respond to user actions or update the UI.

2. Secondary Threads: When a background thread needs to perform tasks asynchronously, listen for incoming data (e.g., sockets), process messages from other threads, or use timers. Typical scenarios include:

   • Working with sockets.
   • Performing recurring tasks with a timer.
   • Receiving and processing messages from IPC (Inter-Process Communication).
   • Maintaining thread liveliness to handle future requests without constantly creating and destroying the thread.

An example of using a run loop in a secondary thread could be maintaining a network connection:

// Example in Swift
class NetworkManager {
    private let networkThread = Thread {
        // Code to initialize network session, sockets, etc.

        // Adding an event source (e.g., socket or port) to the run loop
        // CFRunLoopAddSource(CFRunLoopGetCurrent(), source, CFRunLoopMode.defaultMode)

        // Adding a timer if needed
        // CFRunLoopAddTimer(CFRunLoopGetCurrent(), timer, CFRunLoopMode.defaultMode)

        // Starting the run loop. It will block the thread until an event is received.
        // .distantFuture - so the run loop does not exit immediately
        RunLoop.current.run()

        // Cleanup code after the run loop finishes
        print("Network thread finishing")
    }

    func start() {
        // Using `start()` instead of `detachNewThreadSelector`
        // a more modern approach
        networkThread.start()
    }

    // Method to stop the run loop from another thread
    func stop() {
        // Signaling the run loop to terminate
        // CFRunLoopStop(CFRunLoopGetCurrent()) // Or add a special event for handling
        // The run loop usually terminates when it has no sources or by explicit stop call
    }
}

Run loop is not needed for threads that perform a single task and then terminate (e.g., one-time operations in a background thread via DispatchQueue.global().async). In such cases, Grand Central Dispatch (GCD) is sufficient.