"As is completely standard in declarative UI, it is done by diffing the old view tree against the new one, in this case calling the rebuild method on the View trait. This method compares the data, updates the associated widget if there are any changes, and also traverses into children. The view tree is retained just long enough to do event propagation and to be diffed against the next iteration of the view tree, at which point it is dropped. At any point in time, there are at most two copies of the view tree in existence."
Retained mode is distinct from immediate mode where it _retains_ what is _drawn on the screen_, only redrawing visual components that have changed. Immediate mode redraws the entire application each time, replacing all pixels it's responsible for even if nothing has changed.
Throwing away some internal state is not relevant to this distinction.
