Once you have a reactor, you can register interest in events that happen on specific IOs. In mio, all supported IOs are represented as IoHandles, which wrap the low-level file descriptor concept in each supported platform.
When you register interest, you also provide a Token that mio will send to the handler when an event occurs on the IoHandle. Since the low-level event loop sends events to a single Handler, you use this token to determine which register call was responsible for the event. The description of handlers shows how the Token is used.
To register interest, use event_loop.register():
let (mut reader, mut writer) = io::pipe();
// when an event occurs, the handler will be passed `1`
event_loop.register(&reader, 1u);
You can use this facility to build a higher-level callback API based on functions or closures.
TODO: Unregister interest
The Token API is a thin shim over the APIs provided by the low-level kernel APIs that mio is abstracting.
event_loop.register() allows you to pass any Rust value that implements Token and Copy as the token. Under the hood, the low-level selector APIs can portably support sending back a pointer-sized value (uint) when an event occurs.
You can implement Token on any newtype of uint (which is pointer sized) to more clearly express the intent of the token value, and to add additional convenience methods for use in your handler.
The Copy restriction is necessary because we cannot reliably learn when an IoHandle is closed, so we cannot guarantee that destructors ever get run. Abstractions built on mio could require users to close IoHandles through their API, and associate a destructible object with an IoHandle.