Allow UI to have lazily-loaded submodules.

Equip UI with an __index metamethod. When someone references an unknown
key/field in UI, require() that module and cache it for future reference.

Add util.setmetamethods() as a way to find or create a metatable on a
specified table containing specified metamethods.

Exercise the new functionality by referencing UI.popup in test_popup.lua.

1 files changed, 19 insertions, 3 deletions

diff --git a/indra/newview/scripts/lua/require/UI.lua b/indra/newview/scripts/lua/require/UI.lua
index 464e6547ea..969a2cbded 100644
--- a/indra/newview/scripts/lua/require/UI.lua
+++ b/indra/newview/scripts/lua/require/UI.lua
@@ -1,10 +1,26 @@
 -- Engage the viewer's UI
 
 local leap = require 'leap'
-local Timer = (require 'timers').Timer
 local mapargs = require 'mapargs'
-
-local UI = {}
+local Timer = (require 'timers').Timer
+local util = require 'util'
+
+-- Allow lazily accessing certain other modules on demand, e.g. a reference to
+-- UI.Floater lazily loads the Floater module. Use of UI's __index metamethod
+-- theoretically permits any other module you can require() to appear as a
+-- submodule of UI, but it doesn't make sense to support (e.g.) UI.Queue.
+local submods = { 'Floater', 'popup' }
+local UI = util.setmetamethods{
+    __index=function(t, key)
+        if not table.find(submods, key) then
+            error(`Invalid UI submodule {key}`, 2)
+        end
+        local mod = require(key)
+        -- cache the submodule
+        t[key] = mod
+        return mod
+    end
+}                               
 
 -- ***************************************************************************
 --  registered menu actions