Defines an object that can observe changes to properties of a page and its components, store the
state of the page between request cycles, and restore a page's state on a subsequent request
cycle.
Concrete implementations of this can store the changes in memory, as client-side cookies, in a
flat file, or in a database.
Concrete implementations of this can store the changes in memory, as client-side cookies, in a flat file, or in a database.