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.

3 files changed, 57 insertions, 14 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
diff --git a/indra/newview/scripts/lua/require/util.lua b/indra/newview/scripts/lua/require/util.lua
index bfbfc8637c..a000359226 100644
--- a/indra/newview/scripts/lua/require/util.lua
+++ b/indra/newview/scripts/lua/require/util.lua
@@ -15,16 +15,9 @@ local util = {}
 -- util.classctor(MyClass, MyClass.construct)
 -- return MyClass
 function util.classctor(class, ctor)
-    -- get the metatable for the passed class
-    local mt = getmetatable(class)
-    if mt == nil then
-        -- if it doesn't already have a metatable, then create one
-        mt = {}
-        setmetatable(class, mt)
-    end
-    -- now that class has a metatable, set its __call method to the specified
-    -- constructor method (class.new if not specified)
-    mt.__call = ctor or class.new
+    -- set class's __call metamethod to the specified constructor function
+    -- (class.new if not specified)
+    util.setmetamethods{class, __call=(ctor or class.new)}
 end
 
 -- check if array-like table contains certain value
@@ -66,4 +59,37 @@ function util.equal(t1, t2)
     return util.empty(temp)
 end
 
+-- Find or create the metatable for a specified table (a new empty table if
+-- omitted), and to that metatable assign the specified keys.
+-- Setting multiple keys at once is more efficient than a function to set only
+-- one at a time, e.g. setametamethod().
+-- t = util.setmetamethods{__index=readfunc, __len=lenfunc}
+-- returns a new table with specified metamethods __index, __len
+-- util.setmetamethods{t, __call=action}
+-- finds or creates the metatable for existing table t and sets __call
+-- util.setmetamethods{table=t, __call=action}
+-- same as util.setmetamethods{t, __call=action}
+function util.setmetamethods(specs)
+    -- first determine the target table
+    assert(not (specs.table and specs[1]),
+           "Pass setmetamethods table either as positional or table=, not both")
+    local t = specs.table or specs[1] or {}
+    -- remove both ways of specifying table, leaving only the metamethods
+    specs.table = nil
+    specs[1] = nil
+    local mt = getmetatable(t)
+    if not mt then
+        -- t doesn't already have a metatable: just set specs
+        setmetatable(t, specs)
+    else
+        -- t already has a metatable: copy specs into it
+        local key, value
+        for key, value in pairs(specs) do
+            mt[key] = value
+        end
+    end
+    -- having set or enriched t's metatable, return t
+    return t
+end
+
 return util
diff --git a/indra/newview/scripts/lua/test_popup.lua b/indra/newview/scripts/lua/test_popup.lua
index e48f89c3a7..156c09c78f 100644
--- a/indra/newview/scripts/lua/test_popup.lua
+++ b/indra/newview/scripts/lua/test_popup.lua
@@ -1,4 +1,5 @@
-popup = require 'popup'
+UI = require 'UI'
+popup = UI.popup
 
 response = popup:alert('This just has a Close button')
 response = popup:alertOK(string.format('You said "%s", is that OK?', next(response)))