Любые, какие тебе будут нужны, помимо дефолтных таймеров и прочего списка:
Input Source: this will be the asynchronous source of the events. There are different types of input sources:
◦ Port-based: The events come asynchronously from ports. Examples of port-based input sources are, Mach-ports, sockets, message ports, and file descriptors.
◦ Timer: For this input source the events are fired with a timer object.
◦ Perform selector: This input source allows us to call a selector on any thread we have access to.
◦ Custom: As the name implies, we have to implement all of the signalling for this input source.