AddOn Example

objects = [[
<objects>
  <object>
    objectID=idInput
    objectType=Input
  </object>
  <object>
    objectID=messageInput
    objectType=Input
  </object>
  <object>
    objectID=messageSender
    objectType=Button
    defaultWidth=14
    defaultHeight=3
    text=Send!
  </object>
</objects>
]]

local args = { ... }
local callType
local objectID
local systemInfo

-- Gets called when an object with type "Custom"
-- needs to be displayed.
function showObject()

end

-- Gets called when a button or a custom object
-- with canClick attribute gets clicked.
function callFunction()
  if (objectID == "messageSender") then
    id = tonumber(systemInfo["userInputs"]["idInput"])
    message = systemInfo["userInputs"]["messageInput"]
    if (id == nil or message == nil) then
      return
    end

    rednet.open("left")
    rednet.send(id, message)
  end
end

function main()
  if (#args == 3) then
    callType = args[1]
    objectID = args[2]
    systemInfo = textutils.unserialize(args[3])

    if (callType == "Show") then
      showObject()
    elseif (callType == "Click") then
      callFunction()
    end
  end
end

main()