This is a small utility module that helps in the creation of variables that can be checked for state changes in update functions. Code is worth a thousand words, so:

local obv = require 'obvar'

function love.load()
  var = obv(true)
end

function love.update(dt)
  if var+false then print('var changed to false') end
  if var-true then print('var changed from true') end
end

function love.keypressed(key)
  if key == ' ' then var:set(false) end
end

Then the actions inside the conditions on update will only happen on the frame var is set to false or is changed from being true. This means that when the space key is pressed both things will happen, and so 'var changed to false' and 'var changed from true' will be printed once on that frame.

The module file should be dropped on your project and required like so:

obv = require 'obvar'

Then, at the END of your main update function, update:

function love.update(dt)
  -- your stuff here

  obv:update()
end

var = obv(true)

Just call obv and pass in the value you want this variable to have.

print(var.v)

The variable's value is on the v attribute.

var:set(39)

This assigns the value 39 to the attribute v and also assigns the previous value (the one that was set before 39) to the attribute prev_v. This is what allows variables created with this module to be checked for state changes. If you do var.v = 39 then things should still work but not for this state change.

-- var.v is 38
var.v = 39 -- v is now 39 and prev_v is the previous prev_v, not 38
if var+39 then print('enter 39') end -- prints 'enter 39' if the state was changed from something else to 39, succeeds
if var-38 then print('exit 38') end -- prints 'exit 38' if the state was change from 38 to something else, fails because prev_v is not set properly
obv:update() -- now prev_v is set to 39 and the step where prev_v = 38 never happened

In comparison with assigning appropriately:

-- var.v is 38
var:set(39) -- v is now 39 and prev_v is 38
if var+39 then print('enter 39') end -- prints 'enter 39' if the state was changed from something else to 39, succeeds
if var-38 then print('exit 38') end -- prints 'exit 38' if the state was change from 38 to something else, succeeds
obv:update() -- now prev_v is set to 39

Use + to check for enter state changes and - for exit state changes:

function love.update(dt)
  if var+true then print('var changed to true') end
  if var-true then print('var changed from true') end
end

Alternatively you can just use enter or exit directly:

function love.update(dt)
  if var:enter(true) then print('var changed to true') end
  if var:exit(true) then print('var changed from true') end
end

Sometimes you want to change some state after a previous state check but also make sure that that previous state check gets triggered. For instance, say in your game your entities are update in the following order: Enemies -> Player -> obv:update(). And then say that in the Player class, after doing something, you have to change the state of some obvar on an enemy:

enemy.o_attacker:set(self)

Then in the enemy class, you wanna do something if some state change from the player happened:

if self.o_attacker-nil then
  local player = self.o_attacker.v
  player:dealDamage(10)
end

In this case it's just checking to see if o_attacker was set to something and exited its nil state. In any case, given the order of updates is Enemies -> Player -> obv:update(), this check will fail because the obv:update() call after the player will set o_attacker.prev_v to its current value, meaning that on the next frame, when the check in the enemy class is performed, o_attacker will have the current value pointing to the player as well as the previous one, so the check will fail. To prevent stuff like this from happening an additional call exists:

enemy.o_attacker:setd(self)

setd will make it so that state changes are only performed on the first next state check, meaning that in this example the o_attacker variable will only actually start holding a reference to the player when the enemy class checks if self.o_attacker-nil on the next frame.

You can do whatever you want with this. See the LICENSE.