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XEmacs / man / lispref / menus.texi

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@c -*-texinfo-*-
@c This is part of the XEmacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1997 Free Software Foundation, Inc.
@c Copyright (C) 1995 Sun Microsystems.
@c See the file lispref.texi for copying conditions.
@setfilename ../../info/menu.info
@node Menus, Dialog Boxes, Keymaps, Top
@chapter Menus
@cindex menu

@menu
* Menu Format::         Format of a menu description.
* Menubar Format::      How to specify a menubar.
* Menubar::             Functions for controlling the menubar.
* Modifying Menus::     Modifying a menu description.
* Pop-Up Menus::        Functions for specifying pop-up menus.
* Menu Filters::        Filter functions for the default menubar.
* Menu Accelerators::   Using and controlling menu accelerator keys
* Buffers Menu::        The menu that displays the list of buffers.
@end menu

@node Menu Format
@section Format of Menus
@cindex menu format
@cindex format of menus

A menu is described using a @dfn{menu description}, which is a list of
menu items, keyword-value pairs, strings, and submenus.  The menu
description specifies which items are present in the menu, what function
each item invokes, and whether the item is selectable or not.  Pop-up
menus are directly described with a menu description, while menubars are
described slightly differently (see below).

The first element of a menu must be a string, which is the name of the
menu.  This is the string that will be displayed in the parent menu or
menubar, if any.  This string is not displayed in the menu itself,
except in the case of the top level pop-up menu, where there is no
parent.  In this case, the string will be displayed at the top of the
menu if @code{popup-menu-titles} is non-@code{nil}.

Immediately following the first element there may optionally be up
to five keyword-value pairs, as follows:

@table @code
@item :included @var{form}
This can be used to control the visibility of a menu.  The form is
evaluated and the menu will be omitted if the result is @code{nil}.

@item :config @var{symbol}
This is an efficient shorthand for @code{:included (memq @var{symbol}
menubar-configuration)}.  See the variable @code{menubar-configuration}.

@item :filter @var{function}
A menu filter is used to sensitize or incrementally create a submenu
only when it is selected by the user and not every time the menubar is
activated.  The filter function is passed the list of menu items in the
submenu and must return a list of menu items to be used for the menu.
It is called only when the menu is about to be displayed, so other menus
may already be displayed.  Vile and terrible things will happen if a
menu filter function changes the current buffer, window, or frame.  It
also should not raise, lower, or iconify any frames.  Basically, the
filter function should have no side-effects.

@item :accelerator @var{key}
A menu accelerator is a keystroke which can be pressed while the menu is
visible which will immediately activate the item.  @var{key} must be a char
or the symbol name of a key. 

@item :active @var{form}
@var{form} will be evaluated when the submenu is about to be displayed,
and the menu contents will be displayed only if the result is non-@code{nil}.
If the submenu is inactive, it will display the word "Invalid", grayed-out.
@xref{Menu Accelerators}.
@end table

The rest of the menu consists of elements as follows:

@itemize @bullet
@item
A @dfn{menu item}, which is a vector in the following form:

@example
@code{[ @var{name} @var{callback} @var{:keyword} @var{value} @var{:keyword} @var{value} ... ]}
@end example

@var{name} is a string, the name of the menu item; it is the string to
display on the menu.  It is filtered through the resource database, so
it is possible for resources to override what string is actually
displayed.

@var{callback} is a form that will be invoked when the menu item is
selected.  If the callback of a menu item is a symbol, then it must name
a command.  It will be invoked with @code{call-interactively}.  If it is
a list, then it is evaluated with @code{eval}.

The valid keywords and their meanings are described below.

Note that for compatibility purposes, the form

@example
@code{[ @var{name} @var{callback} @var{active-p} ]}
@end example

is also accepted and is equivalent to

@example
@code{[ @var{name} @var{callback} :active @var{active-p} ]}
@end example

and the form

@example
@code{[ @var{name} @var{callback} @var{active-p} @var{suffix}]}
@end example

is accepted and is equivalent to

@example
@code{[ @var{name} @var{callback} :active @var{active-p} :suffix @var{suffix}]}
@end example

However, these older forms are deprecated and should generally not be used.

@item
If an element of a menu is a string, then that string will be presented
in the menu as unselectable text.

@item
If an element of a menu is a string consisting solely of hyphens, then
that item will be presented as a solid horizontal line.

@item
If an element of a menu is a string beginning with @samp{--:}, then
a particular sort of horizontal line will be displayed, as follows:

@table @samp
@item "--:singleLine"
A solid horizontal line.  This is equivalent to a string consisting
solely of hyphens.
@item "--:doubleLine"
A solid double horizontal line.
@item "--:singleDashedLine"
A dashed horizontal line.
@item "--:doubleDashedLine"
A dashed double horizontal line.
@item "--:noLine"
No line (but a small space is left).
@item "--:shadowEtchedIn"
A solid horizontal line with a 3-d recessed appearance.
@item "--:shadowEtchedOut"
A solid horizontal line with a 3-d pushed-out appearance.
@item "--:shadowDoubleEtchedIn"
A solid double horizontal line with a 3-d recessed appearance.
@item "--:shadowDoubleEtchedOut"
A solid double horizontal line with a 3-d pushed-out appearance.
@item "--:shadowEtchedInDash"
A dashed horizontal line with a 3-d recessed appearance.
@item "--:shadowEtchedOutDash"
A dashed horizontal line with a 3-d pushed-out appearance.
@item "--:shadowDoubleEtchedInDash"
A dashed double horizontal line with a 3-d recessed appearance.
@item "--:shadowDoubleEtchedOutDash"
A dashed double horizontal line with a 3-d pushed-out appearance.
@end table

@item
If an element of a menu is a list, it is treated as a submenu.  The name
of that submenu (the first element in the list) will be used as the name
of the item representing this menu on the parent.
@end itemize

The possible keywords are as follows:

@table @asis
@item :active @var{form}
@var{form} will be evaluated when the menu that this item is a part of
is about to be displayed, and the item will be selectable only if the
result is non-@code{nil}.  If the item is unselectable, it will
usually be displayed grayed-out to indicate this.

@item :suffix @var{form}
@var{form} will be evaluated when the menu that this item is a part of
is about to be displayed, and the resulting string is appended to the
displayed name.  This provides a convenient way of adding the name of a
command's ``argument'' to the menu, like @samp{Kill Buffer NAME}.

@item :keys @var{string}
Normally, the keyboard equivalents of commands in menus are displayed
when the ``callback'' is a symbol.  This can be used to specify keys for
more complex menu items.  It is passed through
@code{substitute-command-keys} first.

@item :style @var{style}
Specifies what kind of object this menu item is.  @var{style} be one
of the symbols

@table @code
@item nil
A normal menu item.
@item toggle
A toggle button.
@item radio
A radio button.
@item button
A menubar button.
@end table

The only difference between toggle and radio buttons is how they are
displayed.  But for consistency, a toggle button should be used when
there is one option whose value can be turned on or off, and radio
buttons should be used when there is a set of mutually exclusive options.
When using a group of radio buttons, you should arrange for no more than
one to be marked as selected at a time.

@item :selected @var{form}
Meaningful only when @var{style} is @code{toggle}, @code{radio} or
@code{button}.  This specifies whether the button will be in the
selected or unselected state.  @var{form} is evaluated, as for
@code{:active}.

@item :included @var{form}
This can be used to control the visibility of a menu item.  The form is
evaluated and the menu item is only displayed if the result is
non-@code{nil}.  Note that this is different from @code{:active}: If
@code{:active} evaluates to @code{nil}, the item will be displayed
grayed out, while if @code{:included} evaluates to @code{nil}, the item
will be omitted entirely.

@item :config @var{symbol}
This is an efficient shorthand for @code{:included (memq @var{symbol}
menubar-configuration)}.  See the variable @code{menubar-configuration}.

@item :accelerator @var{key}
A menu accelerator is a keystroke which can be pressed while the menu is
visible which will immediately activate the item.  @var{key} must be a char
or the symbol name of a key.  @xref{Menu Accelerators}.
@end table

@defvar menubar-configuration
This variable holds a list of symbols, against which the value of the
@code{:config} tag for each menubar item will be compared.  If a menubar
item has a @code{:config} tag, then it is omitted from the menubar if
that tag is not a member of the @code{menubar-configuration} list.
@end defvar

For example:

@example
 ("File"
  :filter file-menu-filter      ; file-menu-filter is a function that takes
                                ; one argument (a list of menu items) and
                                ; returns a list of menu items
  [ "Save As..."    write-file]
  [ "Revert Buffer" revert-buffer :active (buffer-modified-p) ]
  [ "Read Only"     toggle-read-only :style toggle :selected buffer-read-only ]
  )
@end example

@node Menubar Format
@section Format of the Menubar
@cindex menubar format
@cindex format of the menubar

A menubar is a list of menus, menu items, and strings.  The format is
similar to that of a menu, except:

@itemize @bullet
@item
The first item need not be a string, and is not treated specially.

@item
A string consisting solely of hyphens is not treated specially.

@item
If an element of a menubar is @code{nil}, then it is used to represent
the division between the set of menubar items which are flush-left and
those which are flush-right.  (Note: this isn't completely implemented
yet.)
@end itemize

@node Menubar
@section Menubar
@cindex menubar

@defvar current-menubar
This variable holds the description of the current menubar.  This may be
buffer-local.  When the menubar is changed, the function
@code{set-menubar-dirty-flag} has to be called in order for the menubar
to be updated on the screen.
@end defvar

@defvr Constant default-menubar
This variable holds the menubar description of the menubar that is
visible at startup.  This is the value that @code{current-menubar}
has at startup.
@end defvr

@defun set-menubar-dirty-flag
This function tells XEmacs that the menubar widget has to be updated.
Changes to the menubar will generally not be visible until this function
is called.
@end defun

The following convenience functions are provided for setting the
menubar.  They are equivalent to doing the appropriate action to change
@code{current-menubar}, and then calling @code{set-menubar-dirty-flag}.
Note that these functions copy their argument using
@code{copy-sequence}.

@defun set-menubar menubar
This function sets the default menubar to be @var{menubar} (@pxref{Menu
Format}).  This is the menubar that will be visible in buffers that
have not defined their own, buffer-local menubar.
@end defun

@defun set-buffer-menubar menubar
This function sets the buffer-local menubar to be @var{menubar}.  This
does not change the menubar in any buffers other than the current one.
@end defun

Miscellaneous:

@defvar menubar-show-keybindings
If true, the menubar will display keyboard equivalents.  If false, only
the command names will be displayed.
@end defvar

@defvar activate-menubar-hook
Function or functions called before a menubar menu is pulled down.
These functions are called with no arguments, and should interrogate and
modify the value of @code{current-menubar} as desired.

The functions on this hook are invoked after the mouse goes down, but
before the menu is mapped, and may be used to activate, deactivate, add,
or delete items from the menus.  However, using a filter (with the
@code{:filter} keyword in a menu description) is generally a more
efficient way of accomplishing the same thing, because the filter is
invoked only when the actual menu goes down.  With a complex menu,
there can be a quite noticeable and sometimes aggravating delay if
all menu modification is implemented using the @code{activate-menubar-hook}.
See above.

These functions may return the symbol @code{t} to assert that they have
made no changes to the menubar.  If any other value is returned, the
menubar is recomputed.  If @code{t} is returned but the menubar has been
changed, then the changes may not show up right away.  Returning
@code{nil} when the menubar has not changed is not so bad; more
computation will be done, but redisplay of the menubar will still be
performed optimally.
@end defvar

@defvar menu-no-selection-hook
Function or functions to call when a menu or dialog box is dismissed
without a selection having been made.
@end defvar

@node Modifying Menus
@section Modifying Menus

The following functions are provided to modify the menubar of one of its
submenus.  Note that these functions modify the menu in-place, rather
than copying it and making a new menu.

Some of these functions take a @dfn{menu path}, which is a list of
strings identifying the menu to be modified.  For example,
@code{("File")} names the top-level ``File'' menu.  @code{("File"
"Foo")} names a hypothetical submenu of ``File''.

Others take a @dfn{menu item path}, which is similar to a menu path but
also specifies a particular item to be modified.  For example,
@code{("File" "Save")} means the menu item called ``Save'' under the
top-level ``File'' menu.  @code{("Menu" "Foo" "Item")} means the menu
item called ``Item'' under the ``Foo'' submenu of ``Menu''.

@defun add-submenu menu-path submenu &optional before in-menu
This function adds a menu to the menubar or one of its submenus.  If the
named menu exists already, it is changed.

@var{menu-path} identifies the menu under which the new menu should be
inserted.  If @var{menu-path} is @code{nil}, then the menu will be added
to the menubar itself.

@var{submenu} is the new menu to add (@pxref{Menu Format}).

@var{before}, if provided, is the name of a menu before which this menu
should be added, if this menu is not on its parent already.  If the menu
is already present, it will not be moved.

If @var{in-menu} is present use that instead of @code{current-menubar}
as the menu to change.
@end defun

@defun add-menu-button menu-path menu-leaf &optional before in-menu
This function adds a menu item to some menu, creating the menu first if
necessary.  If the named item exists already, it is changed.

@var{menu-path} identifies the menu under which the new menu item should
be inserted.

@var{menu-leaf} is a menubar leaf node (@pxref{Menu Format}).

@var{before}, if provided, is the name of a menu before which this item
should be added, if this item is not on the menu already.  If the item
is already present, it will not be moved.

If @var{in-menu} is present use that instead of @code{current-menubar}
as the menu to change.
@end defun

@defun delete-menu-item menu-item-path &optional from-menu
This function removes the menu item specified by @var{menu-item-path}
from the menu hierarchy.

If @var{from-menu} is present use that instead of @code{current-menubar}
as the menu to change.
@end defun

@defun enable-menu-item menu-item-path
This function makes the menu item specified by @var{menu-item-path} be
selectable.
@end defun

@defun disable-menu-item menu-item-path
This function makes the menu item specified by @var{menu-item-path} be
unselectable.
@end defun

@defun relabel-menu-item menu-item-path new-name
This function changes the string of the menu item specified by
@var{menu-item-path}.  @var{new-name} is the string that the menu item
will be printed as from now on.
@end defun

The following function can be used to search for a particular item in
a menubar specification, given a path to the item.

@defun find-menu-item menubar menu-item-path &optional parent
This function searches @var{menubar} for the item given by
@var{menu-item-path} starting from @var{parent} (@code{nil} means start
at the top of @var{menubar}).  This function returns @code{(@var{item}
. @var{parent})}, where @var{parent} is the immediate parent of the item
found (a menu description), and @var{item} is either a vector, list, or
string, depending on the nature of the menu item.

This function signals an error if the item is not found.
@end defun

The following deprecated functions are also documented, so that
existing code can be understood.  You should not use these functions
in new code.

@defun add-menu menu-path menu-name menu-items &optional before
This function adds a menu to the menubar or one of its submenus.  If the
named menu exists already, it is changed.  This is obsolete; use
@code{add-submenu} instead.

@var{menu-path} identifies the menu under which the new menu should be
inserted.  If @var{menu-path} is @code{nil}, then the menu will be added
to the menubar itself.

@var{menu-name} is the string naming the menu to be added;
@var{menu-items} is a list of menu items, strings, and submenus.  These
two arguments are the same as the first and following elements of a menu
description (@pxref{Menu Format}).

@var{before}, if provided, is the name of a menu before which this
menu should be added, if this menu is not on its parent already.  If the
menu is already present, it will not be moved.
@end defun

@defun add-menu-item menu-path item-name function enabled-p &optional before
This function adds a menu item to some menu, creating the menu first if
necessary.  If the named item exists already, it is changed.  This is
obsolete; use @code{add-menu-button} instead.

@var{menu-path} identifies the menu under which the new menu item should
be inserted. @var{item-name}, @var{function}, and @var{enabled-p} are
the first, second, and third elements of a menu item vector (@pxref{Menu
Format}).

@var{before}, if provided, is the name of a menu item before which this
item should be added, if this item is not on the menu already.  If the
item is already present, it will not be moved.
@end defun

@node Menu Filters
@section Menu Filters
@cindex menu filters

The following filter functions are provided for use in
@code{default-menubar}.  You may want to use them in your own menubar
description.

@defun file-menu-filter menu-items
This function changes the arguments and sensitivity of these File menu items:

@table @samp
@item Delete Buffer
Has the name of the current buffer appended to it.
@item Print Buffer
Has the name of the current buffer appended to it.
@item Pretty-Print Buffer
Has the name of the current buffer appended to it.
@item Save Buffer
Has the name of the current buffer appended to it, and is sensitive only
when the current buffer is modified.
@item Revert Buffer
Has the name of the current buffer appended to it, and is sensitive only
when the current buffer has a file.
@item Delete Frame
Sensitive only when there is more than one visible frame.
@end table
@end defun

@defun edit-menu-filter menu-items
This function changes the arguments and sensitivity of these Edit menu items:

@table @samp
@item Cut
Sensitive only when XEmacs owns the primary X Selection (if
@code{zmacs-regions} is @code{t}, this is equivalent to saying that
there is a region selected).
@item Copy
Sensitive only when XEmacs owns the primary X Selection.
@item Clear
Sensitive only when XEmacs owns the primary X Selection.
@item Paste
Sensitive only when there is an owner for the X Clipboard Selection.
@item Undo
Sensitive only when there is undo information.  While in the midst of an
undo, this is changed to @samp{Undo More}.
@end table
@end defun

@defun buffers-menu-filter menu-items
This function sets up the Buffers menu.  @xref{Buffers Menu}, for
more information.
@end defun

@node Pop-Up Menus
@section Pop-Up Menus
@cindex pop-up menu

@defun popup-menu menu-description &optional event
This function pops up a menu specified by @var{menu-description}, which
is a menu description (@pxref{Menu Format}).  The menu is displayed at
the current mouse position.
@end defun

@defun popup-menu-up-p
This function returns @code{t} if a pop-up menu is up, @code{nil}
otherwise.
@end defun

@defvar popup-menu-titles
If true (the default), pop-up menus will have title bars at the top.
@end defvar

Some machinery is provided that attempts to provide a higher-level
mechanism onto pop-up menus.  This only works if you do not redefine
the binding for button3.

@deffn Command popup-mode-menu
This function pops up a menu of global and mode-specific commands.  The
menu is computed by combining @code{global-popup-menu} and
@code{mode-popup-menu}.  This is the default binding for button3.
You should generally not change this binding.
@end deffn

@defvar global-popup-menu
This holds the global popup menu.  This is present in all modes.
(This is @code{nil} by default.)
@end defvar

@defvar mode-popup-menu
The mode-specific popup menu.  Automatically buffer local.
This is appended to the default items in @code{global-popup-menu}.
@end defvar

@defvr Constant default-popup-menu
This holds the default value of @code{mode-popup-menu}.
@end defvr

@defvar activate-popup-menu-hook
Function or functions run before a mode-specific popup menu is made
visible.  These functions are called with no arguments, and should
interrogate and modify the value of @code{global-popup-menu} or
@code{mode-popup-menu} as desired.  Note: this hook is only run if you
use @code{popup-mode-menu} for activating the global and mode-specific
commands; if you have your own binding for button3, this hook won't be
run.
@end defvar

The following convenience functions are provided for displaying
pop-up menus.

@deffn Command popup-buffer-menu event
This function pops up a copy of the @samp{Buffers} menu (from the menubar)
where the mouse is clicked.  It should be bound to a mouse button event.
@end deffn

@deffn Command popup-menubar-menu event
This function pops up a copy of menu that also appears in the menubar.
It should be bound to a mouse button event.
@end deffn

@node Menu Accelerators
@section Menu Accelerators
@cindex menu accelerators
@cindex keyboard menu accelerators

Menu accelerators are keyboard shortcuts for accessing the menubar.
Accelerator keys can be specified for menus as well as for menu items.  An
accelerator key for a menu is used to activate that menu when it appears as a
submenu of another menu.  An accelerator key for a menu item is used to
activate that item.

@menu
* Creating Menu Accelerators::  How to add accelerator keys to a menu.
* Keyboard Menu Traversal::     How to use and modify the keys which are used
                                to traverse the menu structure.
* Menu Accelerator Functions::  Functions for working with menu accelerators.
@end menu

@node Creating Menu Accelerators
@subsection Creating Menu Accelerators

Menu accelerators are specified as part of the menubar format using the
:accelerator tag to specify a key or by placing "%_" in the menu or menu item
name prior to the letter which is to be used as the accelerator key.  The
advantage of the second method is that the menu rendering code then knows to
draw an underline under that character, which is the canonical way of
indicating an accelerator key to a user.

For example, the command

@example
(add-submenu nil '("%_Test"
                   ["One" (insert "1") :accelerator ?1 :active t]
                   ["%_Two" (insert "2")]
                   ["%_3" (insert "3")]))
@end example

will add a new menu to the top level menubar.  The new menu can be reached
by pressing "t" while the top level menubar is active.  When the menu is
active, pressing "1" will activate the first item and insert the character
"1" into the buffer.  Pressing "T" will activate the second item and insert
the character "2" into the buffer.  Pressing "3" will activate the third item
and insert the character "3" into the buffer.

It is possible to activate the top level menubar itself using accelerator keys.
@xref{Menu Accelerator Functions}.

@node Keyboard Menu Traversal
@subsection Keyboard Menu Traversal

In addition to immediately activating a menu or menu item, the keyboard can
be used to traverse the menus without activating items.  The keyboard arrow
keys, the return key and the escape key are defined to traverse the menus in a
way that should be familiar to users of any of a certain family of popular PC
operating systems.

This behavior can be changed by modifying the bindings in
menu-accelerator-map.  At this point, the online help is your best bet
for more information about how to modify the menu traversal keys.

@node Menu Accelerator Functions
@subsection Menu Accelerator Functions

@deffn Command accelerate-menu
Make the menubar immediately active and place the cursor on the left most entry
in the top level menu.  Menu items can be selected as usual.
@end deffn

@defvar menu-accelerator-enabled
Whether menu accelerator keys can cause the menubar to become active.

If @code{menu-force} or @code{menu-fallback}, then menu accelerator keys can
be used to activate the top level menu.  Once the menubar becomes active, the
accelerator keys can be used regardless of the value of this variable.

@code{menu-force} is used to indicate that the menu accelerator key takes
precedence over bindings in the current keymap(s).  @code{menu-fallback} means
that bindings in the current keymap take precedence over menu accelerator keys.
Thus a top level menu with an accelerator of "T" would be activated on a
keypress of Meta-t if @code{menu-accelerator-enabled} is @code{menu-force}.
However, if @code{menu-accelerator-enabled} is @code{menu-fallback}, then
Meta-t will not activate the menubar and will instead run the function
transpose-words, to which it is normally bound.

The default value is @code{nil}.

See also @code{menu-accelerator-modifiers} and @code{menu-accelerator-prefix}.
@end defvar

@defvar menu-accelerator-map
Keymap consulted to determine the commands to run in response to keypresses
occurring while the menubar is active.  @xref{Keyboard Menu Traversal}.
@end defvar

@defvar menu-accelerator-modifiers
A list of modifier keys which must be pressed in addition to a valid menu
accelerator in order for the top level menu to be activated in response to
a keystroke.  The default value of @code{(meta)} mirrors the usage of the alt key
as a menu accelerator in popular PC operating systems.

The modifier keys in @code{menu-accelerator-modifiers} must match exactly the
modifiers present in the keypress.  The only exception is that the shift
modifier is accepted in conjunction with alphabetic keys even if it is not a
menu accelerator modifier.

See also @code{menu-accelerator-enabled} and @code{menu-accelerator-prefix}.
@end defvar

@defvar menu-accelerator-prefix
Prefix key(s) that must be typed before menu accelerators will be activated.
Must be a valid key descriptor.

The default value is @code{nil}.
@end defvar

@example
(setq menu-accelerator-prefix ?\C-x)
(setq menu-accelerator-modifiers '(meta control))
(setq menu-accelerator-enabled 'menu-force)
(add-submenu nil '("%_Test"
                   ["One" (insert "1") :accelerator ?1 :active t]
                   ["%_Two" (insert "2")]
                   ["%_3" (insert "3")]))
@end example

will add the menu "Test" to the top level menubar.  Pressing C-x followed by
C-M-T will activate the menubar and display the "Test" menu.  Pressing
C-M-T by itself will not activate the menubar.  Neither will pressing C-x
followed by anything else.

@node Buffers Menu
@section Buffers Menu
@cindex buffers menu

The following options control how the @samp{Buffers} menu is displayed.
This is a list of all (or a subset of) the buffers currently in existence,
and is updated dynamically.

@defopt buffers-menu-max-size
This user option holds the maximum number of entries which may appear on
the @samp{Buffers} menu.  If this is 10, then only the ten
most-recently-selected buffers will be shown.  If this is @code{nil},
then all buffers will be shown.  Setting this to a large number or
@code{nil} will slow down menu responsiveness.
@end defopt

@defun format-buffers-menu-line buffer
This function returns a string to represent @var{buffer} in the
@samp{Buffers} menu.  @code{nil} means the buffer shouldn't be listed.
You can redefine this.
@end defun

@defopt complex-buffers-menu-p
If true, the @samp{Buffers} menu will contain several commands, as submenus
of each buffer line.  If this is false, then there will be only one command:
select that buffer.
@end defopt

@defopt buffers-menu-switch-to-buffer-function
This user option holds the function to call to select a buffer from the
@samp{Buffers} menu.  @code{switch-to-buffer} is a good choice, as is
@code{pop-to-buffer}.
@end defopt