GUI

local button = {
  button_defaults = {
    __index = {
      color_bg  = colors.orange;
      color_cl  = colors.blue;
      color_txt = colors.black;

      height = 3;
      padding = 2;
      isClicked = false;
    };
  };

    mt = {
    __call = function(self)
      for index, btn in pairs(self.buttons) do
        local color = btn.isClicked and btn.color_cl or btn.color_bg
        term.setBackgroundColor(color)
        term.setTextColor(btn.color_txt)
        for yPos = btn.y, btn.bounds.y2 do
          term.setCursorPos(btn.x ,yPos)
          term.write(string.rep(" ",btn.width))
        end
        local text = btn.isClicked and btn.clickText or btn.text
        term.setCursorPos(btn.x + (btn.width/2-#btn.text/2),btn.y + (btn.height/2))
    end;

    __newindex = function(t,k,v)
      assert(type(value)=="table","Requires a table")
      assert(value.x,"Requires initial x")
      assert(value.y,"Requires initial y")
      assert(value.text,"Requires text value")
      setmetatable(value,t.button_defaults)
      value.width = #value.text + (value.padding * 2)
      value.bounds = {
        x1 = value.x;
        y1 = value.y;
        x2 = value.x + value.width - 1;
        y2 = value.y + value.height - 1;
      }

      rawset(t.button, key, value)
    end;
  };

  checkClick = function(self,x,y)
    for index, btn in pairs(self.buttons) do
      if x>=btn.x and x<=btn.bounds.x2 and y>=btn.y and y<=btn.bounds.y2 then
        btn.isClicked = true
        if btn.onClick then
          btn:onClick()
        end
        return index
      end
    end
  end;
  buttons = {};
}
setmetatable(button,button.mt)

button[1] = {
  x = 1;
  y = 1;
  text = "This is a button";
  clickText = "You just clicked me!"
}

button[2] = {
  x = 5;
  y = 5;
  height = 5;
  text = "Another button";
  clickText = "CLICLED ME!!";
  onClick = function()

  end;
}

local timer = {
  index = false;
  timer = false;
}

while true do
  button()
  local e = {os.pullEvent()}
  if e[1] == "mouse_click" then
    local index = button:checkClick(e[3],e[4])
    timer.index = index
    timer.timer = os.startTimer(1)
  elseif e[1] == "timer" and e[2] == timer.timer then
    button.buttons[timer.index].isClicked = false
    timer = {}
  end
end