A run loop is an abstraction that (among other things) provides a mechanism to handle system input sources (sockets, ports, files, keyboard, mouse, timers, etc).
Each NSThread has its own run loop, which can be accessed via the currentRunLoop method.
In general, you do not need to access the run loop directly, though there are some (networking) components that may allow you to specify which run loop they will use for I/O processing.
A run loop for a given thread will wait until one or more of its input sources has some data or event, then fire the appropriate input handler(s) to process each input source that is “ready.”.
After doing so, it will then return to its loop, processing input from various sources, and “sleeping” if there is no work to do.
That’s a pretty high level description (trying to avoid too many details).
EDIT
An attempt to address the comment. I broke it into pieces.
- it means that i can only access/run to run loop inside the thread
right?
Indeed. NSRunLoop is not thread safe, and should only be accessed from the context of the thread that is running the loop.
- is there any simple example how to add event to run loop?
If you want to monitor a port, you would just add that port to the run loop, and then the run loop would watch that port for activity.
- (void)addPort:(NSPort *)aPort forMode:(NSString *)mode
You can also add a timer explicitly with
- (void)addTimer:(NSTimer *)aTimer forMode:(NSString *)mode
- what means it will then return to its loop?
The run loop will process all ready events each iteration (according to its mode). You will need to look at the documentation to discover about run modes, as that’s a bit beyond the scope of a general answer.
- is run loop inactive when i start the thread?
In most applications, the main run loop will run automatically. However, you are responsible for starting the run loop and responding to incoming events for threads you spin.
- is it possible to add some events to Thread run loop outside the thread?
I am not sure what you mean here. You don’t add events to the run loop. You add input sources and timer sources (from the thread that owns the run loop). The run loop then watches them for activity. You can, of course, provide data input from other threads and processes, but input will be processed by the run loop that is monitoring those sources on the thread that is running the run loop.
- does it mean that sometimes i can use run loop to block thread for a time
Indeed. In fact, a run loop will “stay” in an event handler until that event handler has returned. You can see this in any app simply enough. Install a handler for any IO action (e.g., button press) that sleeps. You will block the main run loop (and the whole UI) until that method completes.
The same applies to any run loop.
I suggest you read the following documentation on run loops:
https://developer.apple.com/documentation/foundation/nsrunloop
and how they are used within threads:
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Multithreading/RunLoopManagement/RunLoopManagement.html#//apple_ref/doc/uid/10000057i-CH16-SW1