menuYet another kind of container is a menu widget. It contains a list of
widgets to display inside itself, as a pulldown menu. A simple entry in
a menu widget is a command widget, displayed as an option in the
menu widget, which if chosen executes a Tcl command. Other types of
widgets allowed inside a menu widget are radiobuttons and checkboxes. A
special kind of menu item is a separator that is used to group
together menu items within a menu. (It should be noted that the widgets
inside a menu widget are special to that menu widget and do not have an
independent existence, and so do not have their own Tk name.)
A menu widget is built by first creating an instance of a menu
widget (the container) and then invoking the add method
to make entries into the menu.
An example of a menu widget is as follows:
menu .m
.m add command -label "Open file" -command "open_file"
.m add command -label "Open directory" -command "open_directory"
.m add command -label "Save buffer" -command "save_buffer"
.m add command -label "Save buffer as..." -command "save_buffer_as"
.m add separator
.m add command -label "Make new frame" -command "new_frame"
.m add command -label "Open new display" -command "new_display"
.m add command -label "Delete frame" -command "delete_frame"
which creates a menu widget called .m, which contains eight menu
items, the first four of which are commands, then comes a separator
widget, then the final three command entries. (Some of you will notice
that this menu is a small part of the Files menu from the menubar of
the Emacs text editor.)
An example of a checkbox and some radiobutton widget entries:
.m add checkbox -label "Inverse video" -variable inv_vid
.m add radiobutton -label "black" -variable color
.m add radiobutton -label "blue" -variable color
.m add radiobutton -label "red" -variable color
which gives a checkbox displaying `Inverse video', keeping its
state in the variable inv_vid, and three radiobuttons linked through
the variable color.
Another menu item variant is the cascade variant, which is used
to make cascadable menus, i.e. menus that have submenus.
An example of a cascade entry is the following:
.m add cascade -label "I cascade" -menu .m.c
which adds a cascade entry to the menu .m that displays the text
`I cascade'. If the `I cascade' option is chosen from the .m menu
then the menu .m.c will be displayed.
The cascade option is also used to make menubars at the top of an application window. A menu bar is simply a menu each element of which is a cascade entry, (for example). The menubar menu is attached to the application window through a special configuration option for toplevel widgets, the -menu option. Then a menu is defined for each of the cascade entry in the menubar menu.
There are a large number of other variants to menu widgets: menu items can display bitmaps instead of text; menus can be specified as tear-off menus; accelerator keys can be defined for menu items; and so on.