A handler assigned to the default change signal. Handlers are called when some event triggers a widget to emit a signal. For each widget some default signal is assumed, and handlers may be assigned to that through addHandlerChanged or at construction time. Handlers are functions whose first argument, h in the documentation, is a list with atleast two components obj, referring to the object emitting the signal and action, which passes in user-specified data to parameterize the function call.

Handlers may also be added via addHandlerXXX methods for the widgets, where XXX indicates the signal, with a default signal mapped to addHandlerChanged (cf. addHandler for a listing). These methods pass back a handler ID that can be used with blockHandler and unblockHandler to suppress temporarily the calling of the handler.

User supplied data passed to the handler when it is called

A parent container. When a widget is created it can be incorporated into the widget heirarchy by passing in a parent container at construction time. (For some toolkits this is not optional, e.g. gWidgets2tcltk or gWidgets2WWW2.)

These values are passed to the add method of the parent container. Examples of values are expand, fill, and anchor, although they're not always supported by a given widget. For more details see add. Occasionally the variable arguments feature has been used to sneak in hidden arguments to toolkit implementations. For example, when using a widget as a menubar object one can specify a parent argument to pass in parent information, similar to how the argument is used with gaction and the dialogs.

Each widget constructor is passed in the toolkit it will use. This is typically done using the default, which will lookup the toolkit through guiToolkit.