If & occurs in label (when label includes a string), it is specially parsed; on Windows and Unix, the character following & is underlined in the displayed control to indicate a keyboard mnemonic. (On Mac OS, mnemonic underlines are not shown.) The underlined mnemonic character must be a letter or a digit. The user can effectively click the button by typing the mnemonic when the control’s top-level-window contains the keyboard focus. The user must also hold down the Meta or Alt key if the keyboard focus is currently in a control that handles normal alphanumeric input. The & itself is removed from label before it is displayed for the control; a && in label is converted to & (with no mnemonic underlining). On Mac OS, a parenthesized mnemonic character is removed (along with any surrounding space) before the label is displayed, since a parenthesized mnemonic is often used for non-Roman languages. Finally, for historical reasons, any text after a tab character is removed on all platforms. All of these rules are consistent with label handling in menu items (see set-label). Mnemonic keyboard events are handled by on-traverse-char (but not on Mac OS).

The callback procedure is called (with the event type 'button) whenever the user clicks the button.

If style includes 'border, the button is drawn with a special border that indicates to the user that it is the default action button (see on-traverse-char). If style includes 'multi-line, the button is drawn in a way that can stretch vertically and accommodate multiple lines in a text label; currently, this style makes a difference only on Mac OS, and it is selected automatically when label is a string that contains #\newline or #\return. If style includes 'deleted, then the button is created as hidden, and it does not affect its parent’s geometry; the button can be made active later by calling parent’s add-child method.

The font argument determines the font for the control. For information about the enabled argument, see window<%>. For information about the horiz-margin and vert-margin arguments, see subarea<%>. For information about the min-width, min-height, stretchable-width, and stretchable-height arguments, see area<%>.

Changed in version 1.47 of package gui-lib: Added the 'multi-line style, and made it selected when label contains #\return.

Otherwise, sets the bitmap label for a bitmap button. Since label is a bitmap, if the bitmap has a mask (see get-loaded-mask in bitmap%) that is the same size as the bitmap, then the mask is used for the label. Modifying a bitmap while it is used as a label has an unspecified effect on the displayed label. The bitmap label is installed only if the control was originally created with a bitmap label.