local dlg
local timer = Timer{ interval=0.25, ontick=function() dlg:repaint() end }
dlg = Dialog { title="Paint every 1/4 sec", onclose=function() timer:stop() end }

local function paintCanvas(ev)
  local spr = app.sprite
  if not spr then return end
  local gc = ev.context
  local img = Image(spr.width, spr.height)
  img:drawSprite(spr, app.frame, 0, 0)
  gc:drawImage(img, img.bounds, Rectangle(0, 0, gc.width, gc.height))
end

dlg:canvas { id="canvas", width=128, height=128, onpaint=paintCanvas }
dlg:show { wait=false }
timer:start()