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The RSF conception of a [UIType|https://saffron.caret.cam.ac.uk/svn/projects/RSFUtil/trunk/src/uk/org/ponder/rsf/uitype/UIType.java] is a type which "the UI may use to atomically give rise to input". In standard HTTP/HTML (and in most reasonable dialects) a set of types that make sense in this context is {{ {Boolean, String, String[[]} }}, and these are the three UITypes that are supported in RSF. The UIType concept is key to understanding the role of the [component tree|ComponentTree] and in particular the [UIBound|PrimitiveComponents] component and its hierarchy. An RSF bound value is a component holding a an [EL] reference to a part of the [request context|RequestWriteableBean], whose value will be ''flattened'' into one of the UIType types as it passes into the component tree from the model, and reconstituted as it passes back.

The three UIType type correspond to the three concrete descendents of UIBound, being UIBoundBoolean, UIBoundString and UIBoundList. Note that a UIBound component ''never'' contains a {{null}} value as its {{value}} member. An unset value is represented by a special placeholder value, of which there is one per type, per ClassLoader - detecting these placeholder values is done by reference equality for speed. As a convenience method, a UIType placeholder value can be detected by using the API call {{UITypes.isPlaceHolder()}} from PonderUtilCore. This test is sometimes needed in implementations of [

evolvers|Evolver] which might need to anticipate a portion of the RSF [fixup] cycle.