The Be Book - System Overview - The Interface Kit
A BMenuBar
is a menu that can stand at the root of a menu hierarchy.
Rather than appear on-screen when commanded to do so by a user action, a
BMenuBar
object has a settled location in a window's view hierarchy, just
like other views. Typically, the root menu is the menu bar that's drawn
across the top of the window. It's from this use that the class gets its
name.
However, instances of this class can also be used in other ways. A
BMenuBar
might simply display a list of items arranged in a column
somewhere in a window. Or it might contain just one item, where that item
controls a pop-up menu (a
BPopUpMenu
object). Rather than look like a "menu bar," the
BMenuBar
object would look something like a button.
The "real" menu bar at the top of the window usually represents an extensive menu hierarchy; each of its items typically controls a submenu.
The user should be able to operate this menu bar from the keyboard (using
the arrow keys and Enter). There are two ways that the user can put the
BMenuBar
and its hierarchy in focus for keyboard events:
Clicking an item in the menu bar. If the "click to open" preference is not turned off, this opens the submenu the item controls so that it stays visible on-screen and puts the submenu in focus.
Pressing the Menu key or
Command+Escape.
This puts the BMenuBar
in focus and selects its first item.
Either method opens the entire menu hierarchy to keyboard navigation.
If a window's view hierarchy includes more than one
BMenuBar object, the
Menu key (or
Command+Escape)
must choose one of them to put in focus. By default, it picks the last one
that was attached to the window. However, the
SetKeyMenuBar()
function defined in the
BWindow
class can be called to designate a different
BMenuBar
object as the "key" menu bar for the window.
BMenuBar
inherits most of its functions from the
BMenu
class. It reimplements the
AttachedToWindow(),
Draw(), and
MouseDown(), functions
that set up the object and respond to messages, but these aren't
functions that you'd call from application code; they're called for you.
The only real function (other than the constructor) that the
BMenuBar
class adds to those it inherits is
SetBorder(),
which determines how the list of items is bordered.
Therefore, for most BMenuBar
operations—adding submenus, finding
items, temporarily disabling the menu bar, and so on—you must call
inherited functions and treat the object like the
BMenu that it is.
