# Internals
Unlike the classic OOP UI, the `Views` you pass to rui are immutable. Mutable state is stored in the `Context`. The state is all keyed by `ViewId`s.
A `ViewId` is the unique identifier for a view (a u64 internally), formed by hashing a traversal down the view tree.
Methods on the `View` trait are the typical stuff you might see in an OOP API: event processing, rendering, layout. Whenever possible, rui tries to implement views in terms of other views, rather than implementing the methods directly. See `examples/custom_modifier.rs` to add modifiers to a view defined through composition.
The entire UI is laid out and redrawn whenever a `State` changes (though multiple changes to `State` in a single event cycle are coalesced). Redrawing only parts of the window and caching layout information is future work.