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3 - Common Widgets and Attributes

This chapter describes many of the widgets that are provided with FLTK and covers how to query and set the standard attributes.


FLTK provides many types of buttons:

For all of these buttons you just need to include the corresponding <FL/Fl_xyz_Button.H> header file. The constructor takes the bounding box of the button and optionally a label string:

Each button has an associated type() which allows it to behave as a push button, toggle button, or radio button: For toggle and radio buttons, the value() method returns the current button state (0 = off, 1 = on). The set() and clear() methods can be used on toggle buttons to turn a toggle button on or off, respectively. Radio buttons can be turned on with the setonly() method; this will also turn off other radio buttons in the same group.


FLTK provides several text widgets for displaying and receiving text: The Fl_Output and Fl_Multiline_Output widgets allow the user to copy text from the output field but not change it.

The value() method is used to get or set the string that is displayed:

The string is copied to the widget's own storage when you set the value() of the widget.


Unlike text widgets, valuators keep track of numbers instead of strings. FLTK provides the following valuators:

The value() method gets and sets the current value of the widget. The minimum() and maximum() methods set the range of values that are reported by the widget.


The Fl_Group widget class is used as a general purpose "container" widget. Besides grouping radio buttons, the groups are used to encapsulate windows, tabs, and scrolled windows. The following group classes are available with FLTK:

Setting the Size and Position of Widgets

The size and position of widgets is usually set when you create them. You can access them with the x(), y(), w(), and h() methods.

You can change the size and position by using the position(), resize(), and size() methods:

If you change a widget's size or position after it is displayed you will have to call redraw() on the widget's parent.


FLTK stores the colors of widgets as an 8-bit number that is an index into a color palette of 256 colors. This is not the X or WIN32 colormap, but instead is an internal table with fixed contents.

There are symbols for naming some of the more common colors:

The widget color can be set using the color() method: Similarly, the label color can be set using the labelcolor() method:

Box Types

The type Fl_Boxtype stored and returned in Fl_Widget::box() is an enumeration defined in <Enumerations.H>:

FL_NO_BOX means nothing is drawn at all, so whatever is already on the screen remains. The FL_..._FRAME types only draw their edges, leaving the interior unchanged. In the above diagram the blue color is the area that is not drawn by the box.

Making your own Boxtypes

Warning: this interface may change in future versions of fltk!

You can define your own boxtypes by making a small function that draws the box and adding it to the table of boxtypes.

The Drawing Function

The drawing function is passed the bounding box and background color for the widget: A simple drawing function might fill a rectangle with the given color and then draw a black outline:

Adding Your Box Type

The Fl::set_boxtype() method adds or replaces the specified box type: The last 4 arguments to Fl::set_boxtype() are the offsets for the bounding box that should be subtracted when drawing the label inside the box.

Labels and Label Types

The label(), align(), labelfont() , labelsize(), and labeltype() methods control the labeling of widgets.


The label() method sets the string that is displayed for the label. For the FL_SYMBOL_LABEL and image label types the string contains the actual symbol or image data.


The align() method positions the label. The following constants are defined (they may be OR'd together as needed):


The labeltype() method sets the type of the label. The following standard label types are included: To make bitmaps or pixmaps you use a method on the Fl_Bitmap or Fl_Pixmap objects.

Making Your Own Label Types

Warning: this interface is changing in FLTK 2.0!

Label types are actually indexes into a table of functions that draw them. The primary purpose of this is to let you reuse the label() pointer as a pointer to arbitrary data such as a bitmap or pixmap. You can also use this to draw the labels in ways inaccessible through the fl_font mechanisim (e.g. FL_ENGRAVED_LABEL ) or with program-generated letters or symbology.

Label Type Functions
To setup your own label type you will need to write two functions to draw and measure the label. The draw function is called with a pointer to a Fl_Label structure containing the label information, the bounding box for the label, and the label alignment: The label should be drawn inside this bounding box, even if FL_ALIGN_INSIDE is not enabled. The function is not called if the label value is NULL.

The measure function is called with a pointer to a Fl_Label structure and references to the width and height:

It should measure the size of the label and set w and h to the size it will occupy.
Adding Your Label Type
The Fl::set_labeltype method creates a label type using your draw and measure functions: The label type number n can be any integer value starting at the constant FL_FREE_LABELTYPE. Once you have added the label type you can use the labeltype() method to select your label type.

The Fl::set_labeltype method can also be used to overload an existing label type such as FL_NORMAL_LABEL.

Symbol Labels

The FL_SYMBOL_LABEL label type uses the label() string to look up a small drawing procedure in a hash table. For historical reasons the string always starts with '@'; if it starts with something else (or the symbol is not found) the label is drawn normally:
The @ sign may be followed by the following optional "formatting" characters, in this order:


Callbacks are functions that are called when the value of a widget changes. A callback function is sent a Fl_Widget pointer of the widget that changed and optionally a pointer to data of some sort: The callback() method sets the callback function for a widget. You can optionally pass a pointer to some data needed for the callback: Normally callbacks are performed only when the value of the widget changes. You can change this using the when() method:


Shortcuts are key sequences that activate widgets (usually buttons or menu items). The shortcut() method sets the shortcut for a widget: The shortcut value is the key event value (the ASCII value or one of the special keys like FL_Enter) combined with any modifiers (like shift, alt, and control).
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