BMenuItem

Derived from: (none)
Mix-in classes: BInvoker, BArchivable
Declared in:  be/interface/MenuItem.h
Library: libbe.so
Summary:  more...

A BMenuItem object displays one item within a menu and contains the state associated with that item. By default, menu items display text; a derived class can reimplement the Draw() and DrawContent() hook functions to draw something else.

Each BMenuItem object can have its own invocation message and target. A menu item needn't send any message—it can be used simply for its visual presence (see BSeparatorItem for an example).

Menu items can't be used outside of a menu; to add a BMenuItem to a BMenu, call BMenu::AddItem().

Kinds of Items

Some menu items set up the menu hierarchy by giving users access to submenus. A submenu remains hidden until the user operates the item that controls it.

Other items accomplish specific actions. When the user invokes the item, it sends a message to a target BLooper and BHandler, usually the window where the menu at the root of the hierarchy (a BMenuBar object) is displayed. The action that the item initiates, or the state that it sets, depends entirely on the message and the target's response to it.

Shortcuts and Triggers

Any menu item (except for those that control submenus) can be associated with a keyboard shortcut, a character the user can type in combination with a Command key (and possibly other modifiers) to invoke the item. The shortcut character is displayed in the item to the right of the label.

A shortcut works even when the item it invokes isn't visible on-screen. It, therefore, has to be unique within the window (within the entire menu hierarchy).

Every menu item is also associated with a trigger, a character that the user can type (without the Command key) to invoke the item. The trigger works only while the menu is both open on-screen and can be operated using the keyboard. It therefore must be unique only within a particular branch of the menu hierarchy (within the menu).

The trigger is one of the characters that's displayed within the item—either the keyboard shortcut or a character in the label. When it's possible for the trigger to invoke the item, the character is underlined. Like shortcuts, triggers are case-insensitive.

For an item to have a keyboard shortcut, the application must explicitly assign one. However, by default, the Interface Kit chooses and assigns triggers for all items. The default choice can be altered by the SetTrigger() function.

Marked Items

An item can also be marked (with a check mark drawn to the left of the label) in order to indicate that the state it sets is currently in effect. Items are marked by the SetMarked() function. A menu can be set up so that items are automatically marked when they're selected and exactly one item is marked at all times. (See SetRadioMode() in the BMenu class.)

Disabled Items

Items can also be enabled or disabled (by the SetEnabled() function). A disabled item is drawn in muted tones to indicate that it doesn't work. It can't be selected or invoked. If the item controls a specific action, it won't post the message that initiates the action. If it controls a submenu, it will still bring the submenu to the screen, but all the items in submenu will be disabled. If an item in the submenu brings its own submenu to the screen, items in that submenu will also be disabled. Disabling the superitem for a submenu in effect disables a whole branch of the menu hierarchy.

Hook Functions

Draw()

<B><TT>DrawContent()TT>B>   

GetContentSize()

<B><TT>Highlight()

TruncateLabel()

Constructor and Destructor

BMenuItem()

BMenuItem(const char *label, BMessage *message,       char shortcut = 0, uint32 modifiers = 0) BMenuItem(BMenu *submenu, BMessage *message = NULL) BMenuItem(BMessage *archive)

Creates a new BMenuItem and sets its label, its invocation message, both of which can be NULL. When the user invokes the menu item, message is copied and the following fields

Whenever the user invokes the item, the model message is copied and the copy is posted and marked for delivery to the target handler. Three pieces of information are added to the copy before it's posted:

Data name	Type code	Description
"when"	B_INT64_TYPE	The time the item was invoked, as measured by the number of microseconds since 12:00:00 AM January 1, 1970.
"source"	B_POINTER_TYPE	A pointer to the BMenuItem object.
"index"	B_INT32_TYPE	The index of the item, its ordinal position in the menu. Indices begin at 0.

By default, the target of the message is the window that the contains the menu that the item is part of. Use SetTarget() to set a different target.

The constructor can also set a keyboard shortcut for the item. The character that's passed as the shortcut parameter will be displayed to the right of the item's label. Upper case characters are mapped to lower case characters; if you want the user to have to type an upper case character, you must include B_SHIFT_KEY in the modifers mask.

The modifiers mask determines which modifier keys the user must hold down for the shortcut to work. The mask can be formed by combining any of the modifiers constants, especially these:

B_SHIFT_KEY

B_CONTROL_KEY

B_OPTION_KEY

B_COMMAND_KEY is required for all keyboard shortcuts; it doesn't have to be explicitly included in the mask.

If the BMenuItem is constructed to control a submenu, it can't take a shortcut and it typically doesn't post messages—its role is to bring up the submenu. However, it can be assigned a model message if the application must take some collateral action when the submenu is opened. The item's initial label will be taken from the name of the submenu. It can be changed after construction by calling SetLabel().

~BMenuItem()

virtual ~BMenuItem()

Frees the item's label and its model BMessage object. If the item controls a submenu, that menu and all its items are also freed. Deleting a BMenuItem destroys the entire menu hierarchy under that item.

Member Functions

ContentLocation()

protected: BPoint ContentLocation(void) const

Returns the left top corner of the content area of the item, in the coordinate system of the BMenu to which it belongs. The content area of an item is the area where it displays its label (or whatever graphic substitutes for the label). It doesn't include the part of the item where a check mark or a keyboard shortcut could be displayed, nor the border and background around the content area.

You would need to call this function only if you're implementing a DrawContent() function to draw the contents of the menu item. The content rectangle can be calculated from the point returned by this function and the size specified by GetContentSize().

If the item isn't part of a menu, the return value is indeterminate.

Draw() , DrawContent()

protected: virtual void Draw(void) protected: virtual void DrawContent(void)

These functions draw the menu item and highlight it if it's currently selected. They're called by the Draw() function of the BMenu where the item is located whenever the menu is required to display itself; they don't need to be called from within application code.

However, they can both be overridden by derived classes that display something other than a textual label. The Draw() function is called first. It draws the background for the entire item, then calls DrawContent() to draw the label within the item's content area. After DrawContent() returns, it draws the check mark (if the item is currently marked) and the keyboard shortcut (if any). It finishes by calling Highlight() if the item is currently selected.

Both functions draw by calling functions of the BMenu in which the item is located. For example:

void MyItem::DrawContent() {    . . .    Menu()->DrawBitmap(image);    . . . }

A derived class can override either Draw(), if it needs to draw the entire item, or DrawContent(), if it needs to draw only within the content area. A Draw() function can find the frame rectangle it should draw within by calling the BMenuItem's Frame() function; a DrawContent() function can calculate the content area from the point returned by ContentLocation() and the dimensions provided by GetContentSize().

When DrawContent() is called, the pen is positioned to draw the item's label and the high color is appropriately set. The high color may be a shade of gray, if the item is disabled, or black if it's enabled. If some other distinction is used to distinguish disabled from enabled items, DrawContent() should check the item's current state by calling IsEnabled().

If a derived class implements its own DrawContent() function, but still wants to draw a textual string, it should do so by assigning the string as the BMenuItem's label and calling the inherited version of DrawContent(), not by calling DrawString(). This preserves the BMenuItem's ability to display a trigger character in the string.

See also: Highlight(), Frame(), ContentLocation(), GetContentSize()

Frame()

BRect Frame(void) const

Returns the rectangle that frames the entire menu item, in the coordinate system of the BMenu to which the item belongs. If the item hasn't been added to a menu, the return value is indeterminate.

See also: BMenu::AddItem()

GetContentSize()

protected: virtual void GetContentSize(float *width, float *height)

Writes the size of the item's content area into the variables referred to by width and height. The content area of an item is the area where its label (or whatever substitutes for the label) is drawn.

A BMenu calls GetContentSize() for each of its items as it arranges them in a column or a row; the function is not called for items in a matrix. The information it provides helps determine where each item is located and the overall size of the menu.

GetContentSize() must report a size that's large enough to display the content of the item (and separate one item from another). By default, it reports an area just large enough to display the item's label. This area is calculated from the label and the BMenu's current font.

If you design a class derived from BMenuItem and implement your own Draw() or DrawContent() function, you should also implement a GetContentSize() function to report how much room will be needed to draw the item's contents.

See also: DrawContent(), ContentLocation()

Highlight()

protected: virtual void Highlight(bool flag)

Highlights the menu item when flag is true, and removes the highlighting when flag is false. Highlighting simply inverts all the colors in the item's frame rectangle (except for the check mark).

This function is called by the Draw() function whenever the item is selected and needs to be drawn in its highlighted state. There's no reason to call it yourself, unless you define your own version of Draw(). However, it can be reimplemented in a derived class, if items belonging to that class need to be highlighted in some way other than simple inversion.

See also: Draw()

Invoke()

protected: virtual status_t Invoke(BMessage *message = NULL)

Augments the BInvoker version of Invoke() to ensure that only enabled menu items that are attached to a menu hierarchy can be invoked. This function appropriately marks items when the user invokes them. Before sending a message, it adds "when", "source", and "index" field to it, as explained under the BMenuItem constructor.

See also: BInvoker::Invoke()

	IsEnabled()  see SetEnabled()
	isMarked()  see SetMarked()

IsSelected()

protected: bool IsSelected(void) const

Returns true if the menu item is currently selected, and false if not. Selected items are highlighted.

Label()  see SetLabel()

Menu()

BMenu *Menu(void) const

Returns the menu where the item is located, or NULL if the item hasn't yet been added to a menu.

See also: BMenu::AddItem()

SetEnabled() , IsEnabled()

virtual void SetEnabled(bool enabled) bool IsEnabled(void) const

SetEnabled() enables the BMenuItem if the enabled flag is true, disables it if enabled is false, and updates the item if it's visible on-screen. If the item controls a submenu, this function calls the submenu's SetEnabled() virtual function, passing it the same flag. This ensures that the submenu is enabled or disabled as well.

IsEnabled() returns true if the BMenuItem is enabled, its menu is enabled, and all menus above it in the hierarchy are enabled. It returns false if the item is disabled or any objects above it in the menu hierarchy are disabled.

Items and menus are enabled by default.

When using these functions, keep in mind that:

• Disabling a BMenuItem that controls a submenu serves to disable the entire menu hierarchy under the item.

• Passing an argument of true to SetEnabled() is not sufficient to enable the item if it's located in a disabled branch of the menu hierarchy. It can only undo a previous SetEnabled() call (with an argument of false) on the same item.

See also: BMenu::SetEnabled()

SetLabel() , Label()

virtual void SetLabel(const char *string) const char *Label(void) const

SetLabel() frees the item's current label and copies string to replace it. If the menu is visible on-screen, it will be redisplayed with the item's new label. If necessary, the menu will become wider (or narrower) so that it fits the new label.

The Interface Kit calls this virtual function to:

• Set the initial label of an item that controls a submenu to the name of the submenu, and

• Subsequently set the item's label to match the marked item in the submenu, if the submenu was set up to have this feature.

Label() returns a pointer to the current label.

See also: BMenu::SetLabelFromMarked(), the BMenuItem constructor

SetMarked() , IsMarked()

virtual void SetMarked(bool flag) bool IsMarked(void) const

SetMarked() adds a check mark to the left of the item label if flag is true, or removes an existing mark if flag is false. If the menu is visible on-screen. it's redisplayed with or without the mark.

IsMarked() returns whether the item is currently marked.

See also: BMenu::SetLabelFromMarked(), BMenu::FindMarked()

SetShortcut() , Shortcut()

virtual void SetShortcut(char shortcut, uint32 modifiers) char Shortcut(uint32 *modifiers = NULL) const

SetShortcut() sets the shortcut character that's displayed at the right edge of the menu item and the set of modifiers that are associated with the character. These two arguments work just like the arguments passed to the BMenuItem constructor. See the constructor for a more complete description.

Shortcut() returns the character that's used as the keyboard shortcut for invoking the item, and writes a mask of all the modifier keys the shortcut requires to the variable referred to by modifiers. Since the Command key is required to operate the keyboard shortcut for any menu item, B_COMMAND_KEY will always be part of the modifiers mask. The mask can also be tested against the B_CONTROL_KEY, B_OPTION_KEY, and B_SHIFT_KEY constants.

The shortcut is initially set by the BMenuItem constructor.

SetTrigger() , Trigger()

virtual void SetTrigger(char trigger) char Trigger(void) const

SetTrigger() sets the trigger character that the user can type to invoke the item while the item's menu is open on-screen. If a trigger is not set, the Interface Kit will select one for the item, so it's not necessary to call SetTrigger().

The character passed to this function has to match a character displayed in the item—either the keyboard shortcut or a character in the label. The case of the character doesn't matter; lowercase arguments will match uppercase characters in the item and uppercase arguments will match lowercase characters. When the item can be invoked by its trigger, the trigger character is underlined.

If more than one character in the item matches the character passed, SetTrigger() tries first to mark the keyboard shortcut. Failing that, it tries to mark an uppercase letter at the beginning of a word. Failing that, it marks the first instance of the character in the label.

If the trigger doesn't match any characters in the item, the item won't have a trigger, not even one selected by the system.

Trigger() returns the character set by SetTrigger(), or NULL if SetTrigger() didn't succeed or if SetTrigger() was never called and the trigger is selected automatically.

See also: BMenu::SetTriggersEnabled()

Shortcut()  see SetShortcut()

Submenu()

BMenu *Submenu(void) const

Returns the BMenu object that the item controls, or NULL if the item doesn't control a submenu.

See also: the BMenuItem constructor, the BMenu class

Trigger()  see SetTrigger()

TruncateLabel()

protected: virtual void TruncateLabel(float maxWidth, char *newLabel)

Removes characters from the middle of the item label and replaces them with an ellipsis. This is done so that the label will fit within maxWidth coordinate units. The shortened string is copied into the newLabel buffer.

This function is called by the BMenuItem when it draws the item's label, but only if it's necessary to fit a long item into a smaller space. It can be reimplemented by derived classes to do a better job of shortening the string based on the actual content of the label.

Your version of TruncateLabel() should be careful to not cut the trigger character from the string.

See also: BFont::GetTrunctatedStrings()

Archived Fields

The Archive() function adds the following fields to its BMessage argument:

Field	Type code	Meaning
"_label"	B_STRING_TYPE	Text label of the menu item.
"_disable"	B_BOOL_TYPE	true if menu item is disabled.
"_marked"	B_BOOL_TYPE	true if menu item is marked.
"_user_trig"	B_INT32_TYPE	User-defined trigger character.
"_shortcut"	B_INT32_TYPE	Shortcut character.
"_mods"	B_INT32_TYPE	Shortcut modifiers.
"_msg"	B_MESSAGE_TYPE	Model message for the item.
"_submenu"	B_MESSAGE_TYPE	Submenu (only in deep copy).

Some of these fields may not be present if the setting they represent isn't used, or is the default value. For example, if no shortcut key was defined, the "_shortcut" and "_mods" fields won't be found in the archive.