lua using member functions in constructor parameters

Question

I'm probably not using the right words in the title since I'm new to lua and it's not exactly OO like I'm used to. So I'll use code to explain myself and what I'm attempting.

I have a class I define by (simplified):

function newButton(params)
  local button, text
  function button:setText(newtext) ... end
  return button
end

I'm trying to create a button that will change it's text once clicked. So I create it as follows(simplified):

local sound = false
local soundButton = Button.newButton{
  text = "Sound off",
  onEvent = function(event)
    if sound then
      sound = false; setText("Sound on")
    else
      sound = true; setText("Sound off")
    end
  end
}

Which is all good and well, it works except it tells me I can't call setText attempt to call global 'setText' <a nil value> I've tried using soundButton:setText("") but that doesn't work either.
Is there a pattern I can follow to achieve what I want?

Answer 1

Personally I would take "onEvent" out, like this:

function soundButton:onEvent(event)
  if sound then       
    sound = false     
    self:setText("Sound on")
  else
    sound = true
    self:setText("Sound off")
  end
end

But if you really want to keep it in, then onEvent must be declared as a function that takes two parameters, the (explicit) self parameter, and the event. Then the call is still self:setText.

For example:

local soundButton = Button.newButton{
  text = "Sound off",
  onEvent = function(self, event)
    if sound then
      sound = false; self:setText("Sound on")
    else
      sound = true; self:setText("Sound off")
    end
  end
}