Mapping widgets on the screen Widgets are not automatically mapped on a window. You must create a window, which can be the main window or a toplevel widget, to hold the other widgets, and then explicitly map the window onto the screen. In this way, the users can freely describe their dialogs, even cross-referencing widgets, mapping them when necessary. Writing callbacks Many widgets are active objects, i.e., they execute a command to respond users' actions. For instance, we can create a label and then a button that would change the label text under user's action.
All callback functions have a single table as their argument.
Also, arg.n reports the numbers or parameters being passed. In addition to that, as callbacks are methods, the self variable is always available to access the object instance itself. For that reason, TkLua also accepts the command being given by a string. If the command is a string, TkLua assumes it to be the body of the method. So we can choose one of them to avoid conflict.