An example addon showing a simple frame in XML used in Lua

After.lua

print("After the XML is loaded", FrameTestFrame)

local seconds = 10
local total = 0
FrameTestFrame:SetScript("OnUpdate", function(self, elapsed)
  total = total + elapsed
	if total > seconds then
		self:SetScript("OnUpdate", nil)
	end
	local sides = 100 + (100 * (total / seconds))
	self:SetSize(sides, sides)
end)

Before.lua

print("Before the XML is loaded", FrameTestFrame)

FrameTest.toc

## Interface: 50300
## Name: FrameTest

Before.lua
FrameTest.xml
After.lua

FrameTest.xml

<Ui xmlns="http://www.blizzard.com/wow/ui/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.blizzard.com/wow/ui/
..\FrameXML\UI.xsd">
    <Frame name="FrameTestFrame">
  	<Size>
			<AbsDimension x="25" y="25"/>
		</Size>
		<Anchors>
			<Anchor point="CENTER"/>
		</Anchors>
		<Backdrop bgFile="Interface\DialogFrame\UI-DialogBox-Background" edgeFile="Interface\DialogFrame\UI-DialogBox-Border" tile="true">
			<BackgroundInsets>
				<AbsInset left="11" right="12" top="12" bottom="11"/>
			</BackgroundInsets>
			<TileSize>
				<AbsValue val="32"/>
			</TileSize>
			<EdgeSize>
				<AbsValue val="32"/>
			</EdgeSize>
		</Backdrop>
    </Frame>
</Ui>