FLTK 1.4.0
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FAQ (Frequently Asked Questions)

A list of frequently asked questions about FLTK.

This appendix describes various frequently asked questions regarding FLTK.

Where do I start learning FLTK?

It is assumed you know C++, which is the language all FLTK programs are written in, including FLTK itself.

If you like reading manuals to work your way into things, a good start is the FLTK documentation's Introduction to FLTK. Under the FLTK Basics section there's an example 'hello world' program that includes a line-by-line description.

If you like looking at simple code first to pique your interest, and then read up from there, start with the example programs in the test/ and examples/ directory that is included with the source code. A good place to start is the 'hello world' program in test/hello.cxx. Also do a google search for "FLTK example programs". "Erco's Cheat Page" is one that shows many simple examples of how to do specific things.

If you like to run example programs and look for ones that are like yours and then read them, download and build FLTK from the source, then run the test/demo program. Also, go into the 'examples/' directory and run 'make', then run some of those programs.

If you prefer watching TV to reading books and code, google search for "FLTK video tutorials" which has some introductory examples of how to write FLTK programs in C++ and build them.

How do I make a box with text?

The 'hello world' program shows how to make a box with text. All widgets have labels, so picking a simple widget like Fl_Box and setting its label() and using align() to align the label and labelfont() to set the font, and labelsize() to set the size, you can get text just how you want.

Labels are not selectable though; if you want selectable text, you can use Fl_Output or Fl_Multiline_Output for simple text that doesn't include scrollbars. For more complex text that might want scrollbars and multiple colors/fonts, use either Fl_Text_Display which handles plain text, or Fl_Help_View which handles simple HTML formatted text.

Can I use FLTK to make closed-source commercial applications?

Yes. The FLTK Software License is standard LGPL, but also includes a special clause ("exception") to allow for static linking. Specifically:

    [from the top of the FLTK LGPL License section on exceptions]

    3. Static linking of applications and widgets to the FLTK library does
    not constitute a derivative work and does not require the author to
    provide source code for the application or widget, use the shared FLTK
    libraries, or link their applications or widgets against a user-supplied
    version of FLTK.

    If you link the application or widget to a modified version of FLTK,
    then the changes to FLTK must be provided under the terms of the LGPL
    in sections 1, 2, and 4.

    4. You do not have to provide a copy of the FLTK license with programs
    that are linked to the FLTK library, nor do you have to identify the
    FLTK license in your program or documentation as required by section 6
    of the LGPL.

    However, programs must still identify their use of FLTK.  The following
    example statement can be included in user documentation to satisfy
    this requirement:

        [program/widget] is based in part on the work of the
        FLTK project (https://www.fltk.org).

Hitting the 'Escape' key closes windows - how do I prevent this?

[From FLTK article #378]

  1. FLTK has a "global event handler" that makes Escape try to close the window, the same as clicking the close box. To disable this everywhere you can install your own that pretends it wants the escape key and thus stops the default one from seeing it (this may not be what you want, see below about the callbacks):
static int my_handler(int event) {
if (event == FL_SHORTCUT) return 1; // eat all shortcut keys
return 0;
...in main():
If the Fl::focus() widget is zero or ignores an FL_KEYBOARD event then FLTK tries sending this event ...
Definition Enumerations.H:324
The Fl is the FLTK global (static) class containing state information and global methods for the curr...
Definition Fl.H:137
  1. Attempts to close a window (both clicking the close box or typing Escape) call that window's callback. The default version of the callback does hide(). To make the window not close or otherwise do something different you replace the callback. To make the main window exit the program:
void my_callback(Fl_Widget*, void*) {
Fl_Widget is the base class for all widgets in FLTK.
Definition Fl_Widget.H:104

If you don't want Escape to close the main window and exit you can check for and ignore it. This is better than replacing the global handler because Escape will still close pop-up windows:

void my_callback(Fl_Widget*, void*) {
return; // ignore Escape
#define FL_Escape
The escape key.
Definition Enumerations.H:484
static int event_key()
Gets which key on the keyboard was last pushed.
Definition Fl.H:742
static int event()
Returns the last event that was processed.
Definition Fl.H:612

The reason for calling a window callback can also be found using using the Fl::callback_reason() method:

void my_callback(Fl_Widget*, void*) {
return; // ignore that the user pressed the Escape key
save_and_exit(); // user clicked the Close button in the window decoration
exit(0); // fallback for other callback reasons
a dialog was cancelled
Definition Enumerations.H:454
an item was closed
Definition Enumerations.H:452
static Fl_Callback_Reason callback_reason()
Give the reason for calling a callback.
Definition Fl.cxx:1206

It is very common to ask for confirmation before exiting, this can be done with:

void my_callback(Fl_Widget*, void*) {
if (fl_choice("Are you sure you want to quit?",
"continue", "quit", NULL))

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