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(widget)Defining New Widgets


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Defining New Widgets
====================

   You can define specialized widgets with `define-widget'.  It allows
you to create a shorthand for more complex widgets, including specifying
component widgets and new default values for the keyword arguments.

 - Function: define-widget name class doc &rest args
     Define a new widget type named NAME from `class'.

     NAME and class should both be symbols, `class' should be one of
     the existing widget types.

     The third argument DOC is a documentation string for the widget.

     After the new widget has been defined, the following two calls will
     create identical widgets:

        *      (widget-create NAME)

        *      (apply widget-create CLASS ARGS)


   Using `define-widget' just stores the definition of the widget type
in the `widget-type' property of NAME, which is what `widget-create'
uses.

   If you only want to specify defaults for keywords with no complex
conversions, you can use `identity' as your conversion function.

   The following additional keyword arguments are useful when defining
new widgets:
`:convert-widget'
     Function to convert a widget type before creating a widget of that
     type.  It takes a widget type as an argument, and returns the
     converted widget type.  When a widget is created, this function is
     called for the widget type and all the widget's parent types, most
     derived first.

     The following predefined functions can be used here:

      - Function: widget-types-convert-widget widget
          Convert `:args' as widget types in WIDGET.

      - Function: widget-value-convert-widget widget
          Initialize `:value' from `:args' in WIDGET.

`:value-to-internal'
     Function to convert the value to the internal format.  The function
     takes two arguments, a widget and an external value, and returns
     the internal value.  The function is called on the present `:value'
     when the widget is created, and on any value set later with
     `widget-value-set'.

`:value-to-external'
     Function to convert the value to the external format.  The function
     takes two arguments, a widget and an internal value, and returns
     the external value.  The function is called on the present `:value'
     when the widget is created, and on any value set later with
     `widget-value-set'.

`:create'
     Function to create a widget from scratch.  The function takes one
     argument, a widget type, and creates a widget of that type,
     inserts it in the buffer, and returns a widget object.

`:delete'
     Function to delete a widget.  The function takes one argument, a
     widget, and should remove all traces of the widget from the buffer.

`:value-create'
     Function to expand the `%v' escape in the format string.  It will
     be called with the widget as its argument and should insert a
     representation of the widget's value in the buffer.

`:value-delete'
     Should remove the representation of the widget's value from the
     buffer.  It will be called with the widget as its argument.  It
     doesn't have to remove the text, but it should release markers and
     delete nested widgets if such have been used.

     The following predefined function can be used here:

      - Function: widget-children-value-delete widget
          Delete all `:children' and `:buttons' in WIDGET.

`:value-get'
     Function to extract the value of a widget, as it is displayed in
     the buffer.

     The following predefined function can be used here:

      - Function: widget-value-value-get widget
          Return the `:value' property of WIDGET.

`:format-handler'
     Function to handle unknown `%' escapes in the format string.  It
     will be called with the widget and the character that follows the
     `%' as arguments.  You can set this to allow your widget to handle
     non-standard escapes.

     You should end up calling `widget-default-format-handler' to handle
     unknown escape sequences, which will handle the `%h' and any future
     escape sequences, as well as give an error for unknown escapes.

`:action'
     Function to handle user initiated events.  By default, `:notify'
     the parent.

     The following predefined function can be used here:

      - Function: widget-parent-action widget &optional event
          Tell `:parent' of WIDGET to handle the `:action'.  Optional
          EVENT is the event that triggered the action.

`:prompt-value'
     Function to prompt for a value in the minibuffer.  The function
     should take four arguments, WIDGET, PROMPT, VALUE, and UNBOUND and
     should return a value for widget entered by the user.  PROMPT is
     the prompt to use.  VALUE is the default value to use, unless
     UNBOUND is non-nil, in which case there is no default value.  The
     function should read the value using the method most natural for
     this widget, and does not have to check that it matches.

   If you want to define a new widget from scratch, use the `default'
widget as its base.

 - Widget: default
     Widget used as a base for other widgets.

     It provides most of the functionality that is referred to as "by
     default" in this text.


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