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(elisp)Extended Menu Items

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Extended Menu Items
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   An extended-format menu item is a more flexible and also cleaner
alternative to the simple format.  It consists of a list that starts
with the symbol `menu-item'.  To define a non-selectable string, the
item looks like this:

     (menu-item ITEM-NAME)

A string starting with two or more dashes specifies a separator line;
see Note: Menu Separators.

   To define a real menu item which can be selected, the extended format
item looks like this:

     (menu-item ITEM-NAME REAL-BINDING
         . ITEM-PROPERTY-LIST)

Here, ITEM-NAME is an expression which evaluates to the menu item
string.  Thus, the string need not be a constant.  The third element,
REAL-BINDING, is the command to execute.  The tail of the list,
ITEM-PROPERTY-LIST, has the form of a property list which contains
other information.  Here is a table of the properties that are
supported:

`:enable FORM'
     The result of evaluating FORM determines whether the item is
     enabled (non-`nil' means yes).  If the item is not enabled, you
     can't really click on it.

`:visible FORM'
     The result of evaluating FORM determines whether the item should
     actually appear in the menu (non-`nil' means yes).  If the item
     does not appear, then the menu is displayed as if this item were
     not defined at all.

`:help HELP'
     The value of this property, HELP, specifies a "help-echo" string
     to display while the mouse is on that item.  This is displayed in
     the same way as `help-echo' text properties (Note: Help display).
     Note that this must be a constant string, unlike the `help-echo'
     property for text and overlays.

`:button (TYPE . SELECTED)'
     This property provides a way to define radio buttons and toggle
     buttons.  The CAR, TYPE, says which: it should be `:toggle' or
     `:radio'.  The CDR, SELECTED, should be a form; the result of
     evaluating it says whether this button is currently selected.

     A "toggle" is a menu item which is labeled as either "on" or "off"
     according to the value of SELECTED.  The command itself should
     toggle SELECTED, setting it to `t' if it is `nil', and to `nil' if
     it is `t'.  Here is how the menu item to toggle the
     `debug-on-error' flag is defined:

          (menu-item "Debug on Error" toggle-debug-on-error
                     :button (:toggle
                              . (and (boundp 'debug-on-error)
                                     debug-on-error)))

     This works because `toggle-debug-on-error' is defined as a command
     which toggles the variable `debug-on-error'.

     "Radio buttons" are a group of menu items, in which at any time one
     and only one is "selected."  There should be a variable whose value
     says which one is selected at any time.  The SELECTED form for
     each radio button in the group should check whether the variable
     has the right value for selecting that button.  Clicking on the
     button should set the variable so that the button you clicked on
     becomes selected.

`:key-sequence KEY-SEQUENCE'
     This property specifies which key sequence is likely to be bound
     to the same command invoked by this menu item.  If you specify the
     right key sequence, that makes preparing the menu for display run
     much faster.

     If you specify the wrong key sequence, it has no effect; before
     Emacs displays KEY-SEQUENCE in the menu, it verifies that
     KEY-SEQUENCE is really equivalent to this menu item.

`:key-sequence nil'
     This property indicates that there is normally no key binding
     which is equivalent to this menu item.  Using this property saves
     time in preparing the menu for display, because Emacs does not
     need to search the keymaps for a keyboard equivalent for this menu
     item.

     However, if the user has rebound this item's definition to a key
     sequence, Emacs ignores the `:keys' property and finds the keyboard
     equivalent anyway.

`:keys STRING'
     This property specifies that STRING is the string to display as
     the keyboard equivalent for this menu item.  You can use the
     `\\[...]' documentation construct in STRING.

`:filter FILTER-FN'
     This property provides a way to compute the menu item dynamically.
     The property value FILTER-FN should be a function of one argument;
     when it is called, its argument will be REAL-BINDING.  The
     function should return the binding to use instead.