NLGUI (nel/src/gui, headers nel/include/nel/gui, namespace NLGUI) is NeL's data-driven widget library: the XML-described, database-reactive, Lua-scriptable interface system originally built for the Ryzom client and extracted into the engine so any NeL application can use it. The client keeps its game-specific widgets and handlers on top; the library alone is a complete windowing toolkit.

Documentation for authoring interfaces (XML, layout, widgets, database, scripting) lives under Interface. This section covers the library itself.

• Embedding NeL GUI in an application: the contract a host application must fulfill
• GUI widget sample: nl_sample_gui, the standalone showcase (native + WebGL)
• Feature flags: conditional interface XML per product flavor

The library is a set of cooperating singletons around one element tree:

• CWidgetManager owns the master groups (window roots), window ordering and activation, input dispatch, capture and keyboard focus, the mouse pointer, tooltips, modals, procedures, and the frame pump (checkCoords, drawViews, clock-tick events). Holds the parser instance.
• CInterfaceParser (IParser) parses the merged XML document into elements (pipeline details); extensible with IParserModules for application-specific elements; owns templates, defines, styles, procedures, options blocks and the Lua bootstrap (initLUA).
• CViewRenderer batches every visible view into per-layer quad streams over the texture atlas; owns screen metrics and the "figur" digit textures. Interfaces render as very few draw calls.
• CDBManager: the interface database tree; leaf/branch observers feed CInterfaceLink triggers, flushed once per frame.
• CAHManager: the action handler registry and dispatcher.
• CLuaManager + CLuaIHM: the shared Lua state and the reflection bridge that exposes widgets as Lua objects.

CInterfaceElement            id, active, position/size machinery, reflection root
  └ CViewBase                drawable leaf (views)
      └ CCtrlBase            + input handling, tooltips (controls)
          └ CInterfaceGroup  + children (groups, windows)

Groups are controls are views: one factory serves all three XML tags, and each tag merely enforces a dynamic cast. Elements register with NLMISC_REGISTER_OBJECT(CViewBase, Class, std::string, "typename"); the parser instantiates by type= attribute. A <group> with an unknown type degrades to a plain interface_group; unknown views/ctrls fail.

Reflection: classes export properties through REFLECT_* macro tables; the same property set serves Lua field access, <link target=...> writes, getprop(...) reads and the set handler. One registry, four consumers.

Input: driver events → CEventListener (adapter subscribing mouse/keyboard on the event server) → CWidgetManager::handle* → hit-testing through window order → captured/focused element → widget handleEvent → action handlers. Nothing in the library moves pointer button state into the pointer view; the embedder mirrors it (see embedding).

Update/draw per frame: sendClockTickEvent (drives repeat buttons, animations, onclock_tick) → checkCoords/updateCoords (invalidated subtrees re-measure; db-view widgets poll their leaves here) → observer flush (triggered CInterfaceLinks evaluate) → drawViews (renderer batches by window then render layer, applies global color and per-window alphas, draws the pointer last).

The XML type names the library registers, by family. Attributes for each are documented in the linked authoring pages.

The Ryzom client registers more (sheet, in-scene interfaces, …) into the same factory; those are client code, not library.

Options-block classes (<options type=...>): layer, container_move_opt, container_insertion_opt, list; see skinning.

This library is the Ryzom client's interface code, lifted out of the client tree (where it lived under interface_v3) and librarified into NeL after a decade of production use. The extraction kept the data formats intact: Ryzom's interface XML parses unmodified. A community CEGUI integration is also available as a separate NeL plugin.