What is the role and lifecycle of StateHelper?

Question

I can see that EL attributes are stored in the StateHelper and String literals in the getAttributes() map of a component. What is the lifecycle and why the need for two containers?

Also at what points in the lifecycle are EL ValueExpression evaluated? (Every time the getStateHelper.eval() is called?)

Answer 1

1. StateHelper provides a convenience for component developers (who subclass UIComponent directly and not a standard component) to maintain the state of a component across requests. It plays an integral role in JSF view state saving and the underlying contract provides more than just a means to stash component attributes.

   By subclassing StateHelper, a component developer can more conveniently key into the saveState and restoreState methods of StateHolder, methods which are supposed to guarantee that a component (and child components) have their state correctly persisted (per the configured state saving mode) of the JSF Impl.

   Contrast that with getAttributes() map which is basically a stash of component variables, and is mostly relevant during the lifecycle of a single request. It offers no other services than to keep stuff.

2. ValueExpression is evaluated at least 2x in a component's lifetime : RESTORE_VIEW and UPDATE_MODEL_VALUES phases, for obvious reasons: The first to give appropriate rendering of components and the second, to commit validated and converted values to the backing bean