Run loop

Middle PRO

The essence

A RunLoop is an event-processing loop bound to a thread: it waits for an event, handles it and goes back to sleep without burning CPU. Each thread can have exactly one run loop, but it is not created automatically: the object appears lazily on the first access to RunLoop.current, and it only spins if you explicitly start it (run()). The exception is the main thread: UIApplicationMain starts its run loop for you, and that loop handles touch events, timers, redrawing and DispatchQueue.main callbacks.

Event sources:

• Input sources — asynchronous events: ports, perform(_:on:), system calls.
• Timer sources — Timer and CADisplayLink, fired on schedule, but only while the run loop is in the right mode.