WoWee/docs/widget-system.md

The widget system

The addon API used to answer without doing anything. CreateFrame returned a table, events dispatched to it, and CreateTexture handed back an object whose every method was a no-op — so an addon could be written, loaded and run without putting a pixel on the screen.

There is now a real retained widget tree behind it. The same tree is what FrameXML targets, because FrameXML is only Lua and XML over a widget system, so building it once serves both goals: addons that draw, and a route to running the original interface rather than imitating it.

Shape

Piece	Where	Notes
Widget tree, anchors, draw order, hit testing	src/ui/widget_tree.cpp	No Vulkan or ImGui, so the layout rules are testable without a device
Drawing, texture cache, backdrops, status bars	src/ui/widget_renderer.cpp	Reads Interface\ art through the existing asset path
XML reader	src/ui/xml_parser.cpp	Enough for FrameXML: CDATA, comments, both quote styles
XML to Lua	src/ui/framexml_emitter.cpp	Emits the calls a script would make
Lua bindings	src/addons/lua_engine.cpp	Frames and regions are Lua tables carrying a __wid handle

Coordinates follow WoW throughout — origin bottom-left, y upward — and flip once at the point of drawing, so every anchor rule reads the way Blizzard documents it rather than mirrored.

Anchors are constraints, not positions. An anchor says "this fraction of my rect sits at that point", so one anchor plus a size places a frame and two opposing anchors give the size as well. That is what SetAllPoints relies on, and how most of FrameXML sizes its backgrounds without ever stating a size.

Why XML becomes Lua

The alternative was to build widgets from C++ while walking the XML, which would have meant a second implementation of everything CreateFrame already does — parenting, naming, templates, script binding — kept in step with the first by hand. Emitting Lua means XML frames and hand-written frames travel one path, a template declared in XML is usable from a script without translation, and the emitter's output is a string a test can read without a Lua state.

Environment switches

Both are off by default. Both exist because the work they enable is not finished.

WOWEE_LUA_API_FALLBACK=1

Unknown globals answer with a no-op instead of erroring, and every name asked for is logged once and listed at shutdown.

This is how a large body of Lua gets brought up: rather than guessing which of the missing functions matter, run it and collect the ones it actually reaches.

It has a real cost. Code that checks whether a function exists before using it — which addons do constantly — sees everything as present and takes branches meant for a different client. Names in SCREAMING_SNAKE_CASE are treated as constants and still come back nil, because handing a function to something expecting a number turns a missing value into a confusing type error further away.

WOWEE_LOAD_FRAMEXML=1

Loads the original interface from Interface/FrameXML/FrameXML.toc, in the order that manifest states, before any addon. It turns the fallback above on by itself, because FrameXML cannot get through its own load without one.

Every file that fails is listed together at the end of the load, with the reason carried up from whichever include or referenced script actually broke, and each error carries the Lua call stack that reached it.

All 139 files in the manifest now load, in around 380ms.

FrameXML: 13 Lua files and 126 XML files loaded, 0 failed in 377ms

That is the whole original interface built against this client's widget tree. What remains is behaviour rather than loading: frames exist, are laid out and are named the way FrameXML expects, but the API behind them mostly answers with what the absence of a feature looks like.

Working out what is still missing

tools/framexml_api_gap.py <path to Interface/FrameXML>

reports 1,142 names FrameXML calls that this client does not define, out of 4,217 it calls in total and 2,724 it defines itself as it loads. That ranking counts static call sites, though, and most are never reached.

The measurement that matters is a run with the fallback off:

WOWEE_LUA_API_FALLBACK=0 WOWEE_LOAD_FRAMEXML=1 ./wowee

With the fallback on, a missing name answers and the gap is invisible. With it off, the log names every one FrameXML actually reached — which is how the list that mattered was found, rather than by guessing from the ranking.

Two tools check the front half of the pipeline, and neither has been the constraint for some time: tools/framexml_compile_check.cpp asks Lua whether every generated file compiles (140/140), and the emitter has unit tests in tests/test_framexml.cpp covering the XML features that were silently absent — template inheritance, parentKey, id, <ScrollChild>, button art, handler argument names, and $parent through unnamed frames.

Known gaps

• Type is drawn from the game's own faces — FRIZQT, MORPHEUS, SKURRI, ARIALN and FRIENDS — at the size and colour FrameXML's 42 font objects specify. Each face is built into the atlas at one size and scaled, so a heading is the right face rather than the right rasterisation. Outlines are drawn by offsetting copies of the glyphs, which is what the effect amounts to at these sizes.
• EditBox takes text, keeps a caret and fires OnTextChanged, OnEnterPressed and the focus handlers. It has no selection, no clipboard and no scrolling past its own width. Slider drags and reports its value; Cooldown sweeps.
• The texture cache never evicts, and cannot yet: uploadImGuiTexture has no counterpart, so releasing one would mean tracking its image and memory and destroying them only once the GPU is done. Interface\ art is small, bounded and reused, so this grows to a few hundred entries and stops; a session that loaded art from many addons would keep growing.
• The widget method set in lua_engine.cpp is enumerated rather than derived. A method outside it answers nil instead of doing nothing, which for an addon is an error rather than a shrug. Every such name is recorded once as widget:Name, so the gap shows up in the shutdown report rather than as a mystery; adding it to the set is a one-line fix.