A fast, lightweight tweening library for Lua with easing functions and the ability to group tweens together.

The github page for flux is over here.

-- title:  Tween demo
-- author: Al Rado 07.03.2017
-- desc:   simple tween demo
-- script: lua
-- input:  gamepad

-- flux
-- Copyright (c) 2016 rxi
-- This library is free software; you can redistribute it and/or modify it
-- under the terms of the MIT license. See LICENSE for details.

local flux = { _version = "0.1.5" }
flux.__index = flux

flux.tweens = {}
flux.easing = { linear = function(p) return p end }

local easing = {
  quad    = "p * p",
  cubic   = "p * p * p",
  quart   = "p * p * p * p",
  quint   = "p * p * p * p * p",
  expo    = "2 ^ (10 * (p - 1))",
  sine    = "-math.cos(p * (math.pi * .5)) + 1",
  circ    = "-(math.sqrt(1 - (p * p)) - 1)",
  back    = "p * p * (2.7 * p - 1.7)",
  elastic = "-(2^(10 * (p - 1)) * math.sin((p - 1.075) * (math.pi * 2) / .3))"
}

local makefunc = function(str, expr)
  local load = loadstring or load
  return load("return function(p) " .. str:gsub("%$e", expr) .. " end")()
end

for k, v in pairs(easing) do
  flux.easing[k .. "in"] = makefunc("return $e", v)
  flux.easing[k .. "out"] = makefunc([[
    p = 1 - p
    return 1 - ($e)
  ]], v)
  flux.easing[k .. "inout"] = makefunc([[
    p = p * 2
    if p < 1 then
      return .5 * ($e)
    else
      p = 2 - p
      return .5 * (1 - ($e)) + .5
    end
  ]], v)
end

local tween = {}
tween.__index = tween

local function makefsetter(field)
  return function(self, x)
    local mt = getmetatable(x)
    if type(x) ~= "function" and not (mt and mt.__call) then
      error("expected function or callable", 2)
    end
    local old = self[field]
    self[field] = old and function() old() x() end or x
    return self
  end
end

local function makesetter(field, checkfn, errmsg)
  return function(self, x)
    if checkfn and not checkfn(x) then
      error(errmsg:gsub("%$x", tostring(x)), 2)
    end
    self[field] = x
    return self
  end
end

tween.ease  = makesetter("_ease",
                         function(x) return flux.easing[x] end,
                         "bad easing type '$x'")
tween.delay = makesetter("_delay",
                         function(x) return type(x) == "number" end,
                         "bad delay time; expected number")
tween.onstart     = makefsetter("_onstart")
tween.onupdate    = makefsetter("_onupdate")
tween.oncomplete  = makefsetter("_oncomplete")

function tween.new(obj, time, vars)
  local self = setmetatable({}, tween)
  self.obj = obj
  self.rate = time > 0 and 1 / time or 0
  self.progress = time > 0 and 0 or 1
  self._delay = 0
  self._ease = "quadout"
  self.vars = {}
  for k, v in pairs(vars) do
    if type(v) ~= "number" then
      error("bad value for key '" .. k .. "'; expected number")
    end
    self.vars[k] = v
  end
  return self
end

function tween:init()
  for k, v in pairs(self.vars) do
    local x = self.obj[k]
    if type(x) ~= "number" then
      error("bad value on object key '" .. k .. "'; expected number")
    end
    self.vars[k] = { start = x, diff = v - x }
  end
  self.inited = true
end

function tween:after(...)
  local t
  if select("#", ...) == 2 then
    t = tween.new(self.obj, ...)
  else
    t = tween.new(...)
  end
  t.parent = self.parent
  self:oncomplete(function() flux.add(self.parent, t) end)
  return t
end

function tween:stop()
  flux.remove(self.parent, self)
end

function flux.group()
  return setmetatable({}, flux)
end

function flux:to(obj, time, vars)
  return flux.add(self, tween.new(obj, time, vars))
end

function flux:update(deltatime)
  for i = #self, 1, -1 do
    local t = self[i]
    if t._delay > 0 then
      t._delay = t._delay - deltatime
    else
      if not t.inited then
        flux.clear(self, t.obj, t.vars)
        t:init()
      end
      if t._onstart then
        t._onstart()
        t._onstart = nil
      end
      t.progress = t.progress + t.rate * deltatime
      local p = t.progress
      local x = p >= 1 and 1 or flux.easing[t._ease](p)
      for k, v in pairs(t.vars) do
        t.obj[k] = v.start + x * v.diff
      end
      if t._onupdate then t._onupdate() end
      if p >= 1 then
        flux.remove(self, i)
        if t._oncomplete then t._oncomplete() end
      end
    end
  end
end

function flux:clear(obj, vars)
  for t in pairs(self[obj]) do
    if t.inited then
      for k in pairs(vars) do t.vars[k] = nil end
    end
  end
end

function flux:add(tween)
  -- Add to object table, create table if it does not exist
  local obj = tween.obj
  self[obj] = self[obj] or {}
  self[obj][tween] = true
  -- Add to array
  table.insert(self, tween)
  tween.parent = self
  return tween
end

function flux:remove(x)
  if type(x) == "number" then
    -- Remove from object table, destroy table if it is empty
    local obj = self[x].obj
    self[obj][self[x]] = nil
    if not next(self[obj]) then self[obj] = nil end
    -- Remove from array
    self[x] = self[#self]
    return table.remove(self)
  end
  for i, v in ipairs(self) do
    if v == x then
      return flux.remove(self, i)
    end
  end
end

local Tween = {
  to      = function(...) return flux.to(flux.tweens, ...) end,
  update  = function(...) return flux.update(flux.tweens, ...) end,
  remove  = function(...) return flux.remove(flux.tweens, ...) end,
}
setmetatable(Tween, flux)

-------------------------------------------------------------------
-- demo code
TYPES={"elasticin", "elasticout", "elasticinout", "linear", "quadin", "quadout", "quadinout", "cubicin", "cubicout", "cubicinout", "quartin", "quartout", 
"quartinout", "quintin", "quintout", "quintinout", "expoin", "expoout", "expoinout", "sinein", "sineout", "sineinout", 
"circin", "circout", "circinout", "backin", "backout", "backinout"} 

TIME=1500
START_Y=15
FINISH_Y=50
START_COLOR=0
FINISH_COLOR=15
DELAY=500

local lastTime=0
local counter=0
local easeType = TYPES[counter]
local tween=nil

logo={
    x=104,
    y=START_Y,
  color=START_COLOR,
  msg="easing type: ",
  easeType="",
    draw = function () 
    spr(1+(time()%1000)/500,logo.x,logo.y,-1,4) 
    local fullMsg=counter..". "..logo.msg..logo.easeType
    local msgX=(240-print(fullMsg,-100,-100))/2
    print(fullMsg,msgX,136-logo.y,logo.color)
  end
}

function startTween()
  if(tween ~=nil) then tween:stop() end
  counter = counter+1
  counter = counter>#TYPES and 1 or counter
  easeType = TYPES[counter]
  logo.easeType = easeType
  tween=Tween.to(logo,TIME,{y=FINISH_Y,color=FINISH_COLOR}):ease(easeType):onupdate(logo.draw):delay(DELAY)
    :after(logo,TIME,{y=START_Y,color=START_COLOR}):ease(easeType):onupdate(logo.draw):oncomplete(startTween)
end

startTween()

function TIC()
  cls(12)
  poke(0x3FF8,logo.color)
  print("Flux - tweening library",55,4,logo.color)

  Tween.update(time()-lastTime)
  lastTime=time()
end