B - The Fl:: Functions

This appendix describes all of the Fl::xyz functions. These functions mostly are concerned with event handling and waiting for events. Fl:: should be considered a namespace, although it is defined as a C++ class in the header file to work with older C++ compilers.

FLTK also provides a number of global functions with fl_xyz names. The majority of these are concerned with drawing or graphics state, and are described in the Drawing appendix. The others are a set of modal popup utility functions for asking questions described in the Utility appendix.

Also see the description of all the classes listed in the Classes appendix.

void Fl::add_fd(int fd, void (*cb)(int, void *), void* = 0)
void Fl::add_fd(int fd, int when, void (*cb)(int, void*), void* = 0)

void Fl::remove_fd(int)

Add file descriptor fd to listen to. When the fd becomes ready for reading Fl::wait() will call the callback and then return. The callback is passed the fd and the arbitrary void* argument.

The second version takes a when bitfield, with the bits FL_READ, FL_WRITE, and FL_EXCEPT defined, to indicate when the callback should be done.

There can only be one callback of each type for a file descriptor. Fl::remove_fd() gets rid of all the callbacks for a given file descriptor.

Under UNIX any file descriptor can be monitored (files, devices, pipes, sockets, etc.) Due to limitations in Microsoft Windows, WIN32 applications can only monitor sockets.

void Fl::add_handler(int (*f)(int))

Install a function to parse unrecognized events. If FLTK cannot figure out what to do with an event, it calls each of these functions (most recent first) until one of them returns non-zero. If none of them returns non zero then the event is ignored. Events that cause this to be called are:

Fl::add_idle(void (*cb)(void*), void*)

Adds a callback function that is called every time by Fl::wait() and also makes it act as though the timeout is zero (this makes Fl::wait() return immediately, so if it is in a loop it is called repeatedly, and thus the idle fucntion is called repeatedly). The idle function can be used to get background processing done.

You can have multiple idle callbacks. To remove an idle callback use Fl::remove_idle().

Fl::wait() and Fl::check() call idle callbacks, but Fl::ready() does not.

The idle callback can call any FLTK functions, including Fl::wait(), Fl::check(), and Fl::ready(). Fltk will not recursively call the idle callback.

static void Fl::add_timeout(float t, void (*cb)(void*),void*v=0)

Add a one-shot timeout callback. The function will be called by Fl::wait() at t seconds after this function is called. The optional void* argument is passed to the callback.

static void Fl::repeat_timeout(float t, void (*cb)(void*),void*v=0)

Inside a timeout callback you can call this to add another timeout. Rather than the time being measured from "now", it is measured from when the system call elapsed that caused this timeout to be called. This will result in far more accurate spacing of the timeout callbacks, it also has slightly less system call overhead. (It will also use all your machine time if your timeout code and fltk's overhead take more than t seconds, as the real timeout will be reduced to zero).

It is undefined what this does if called from outside a timeout callback.

This code will print "TICK" each second on stdout, with a fair degree of accuracy:

static void Fl::add_check(void (*cb)(void*),void*v=0)

Fltk will call this callback just before it flushes the display and waits for events. This is different than an idle callback because it is only called once, then fltk calls the system and tells it not to return until an event happens.

This can be used by code that wants to monitor the application's state, such as to keep a display up to date. The advantage of using a check callback is that it is called only when no events are pending. If events are coming in quickly, whole blocks of them will be processed before this is called once. This can save significant time and avoid the application falling behind the events.

Sample code:

int Fl::arg(int argc, char **argv, int &i)

Consume a single switch from argv, starting at word i. Returns the number of words eaten (1 or 2, or 0 if it is not recognized) and adds the same value to i. You can use this function if you prefer to control the incrementing through the arguments yourself.

int Fl::args(int argc, char **argv, int &i, int (*callback)(int, char**,int &)=0)
void Fl::args(int argc, char **argv)

FLTK provides an entirely optional command-line switch parser. You don't have to call it if you don't like them! Everything it can do can be done with other calls to FLTK.

To use the switch parser, call Fl::args(...) near the start of your program. This does not open the display, instead switches that need the display open are stashed into static variables. Then you must display your first window by calling window->show(argc,argv), which will do anything stored in the static variables.

callback lets you define your own switches. It is called with the same argc and argv, and with i the index of each word. The callback should return zero if the switch is unrecognized, and not change i. It should return non-zero if the switch is recognized, and add at least 1 to i (it can add more to consume words after the switch). This function is called before any other tests, so you can override any FLTK switch (this is why fltk can use very short switches instead of the long ones all other toolkits force you to use).

On return i is set to the index of the first non-switch. This is either:

The return value is i unless an unrecognized switch is found, in which case it is zero. If your program takes no arguments other than switches you should produce an error if the return value is less than argc.

All switches except -bg2 may be abbreviated one letter and case is ignored:

The second form of Fl::args() is useful if your program does not have command line switches of its own. It parses all the switches, and if any are not recognized it calls Fl::abort(Fl::help).

Fl_Widget *Fl::belowmouse() const
void Fl::belowmouse(Fl_Widget *)

Get or set the widget that is below the mouse. This is for highlighting buttons. It is not used to send FL_PUSH or FL_MOVE directly, for several obscure reasons, but those events typically go to this widget. This is also the first widget tried for FL_SHORTCUT events.

If you change the belowmouse widget, the previous one and all parents (that don't contain the new widget) are sent FL_LEAVE events. Changing this does not send FL_ENTER to this or any widget, because sending FL_ENTER is supposed to test if the widget wants the mouse (by it returning non-zero from handle()).

static int Fl::check()

Same as Fl::wait(0). Calling this during a big calculation will keep the screen up to date and the interface responsive:

int Fl::damage()

If true then flush() will do something.

void Fl::display(const char *)

Sets the X display to use for all windows. Actually this just sets the environment variable $DISPLAY to the passed string, so this only works before you show() the first window or otherwise open the display, and does nothing useful under WIN32.

int Fl::event()

Returns the most recent event handled, such as FL_PUSH or FL_KEYBOARD. This is useful so callbacks can find out why they were called.

int Fl::event_x()
int Fl::event_y()

Returns the mouse position of the event relative to the Fl_Window it was passed to.

int Fl::event_x_root()
int Fl::event_y_root()

Returns the mouse position on the screen of the event. To find the absolute position of an Fl_Window on the screen, use the difference between event_x_root(),event_y_root() and event_x(),event_y().

int Fl::event_button()

Returns which mouse button was pressed. This returns garbage if the most recent event was not a FL_PUSH or FL_RELEASE event.

int Fl::event_clicks()
void Fl::event_clicks(int)

The first form returns non-zero if the most recent FL_PUSH or FL_KEYBOARD was a "double click". Returns N-1 for N clicks. A double click is counted if the same button is pressed again while event_is_click() is true.

The second form directly sets the number returned by Fl::event_clicks(). This can be used to set it to zero so that later code does not think an item was double-clicked.

int Fl::event_inside(int x, int y, int w, int h)

Returns non-zero if the current event_x and event_y put it inside the passed box. You should always call this rather than doing your own comparison so you are consistent about edge effects.

int Fl::event_is_click()
void Fl::event_is_click(0)

The first form returns non-zero if the mouse has not moved far enough and not enough time has passed since the last FL_PUSH or FL_KEYBOARD event for it to be considered a "drag" rather than a "click". You can test this on FL_DRAG, FL_RELEASE, and FL_MOVE events. The second form clears the value returned by Fl::event_is_click(). Useful to prevent the next click from being counted as a double-click or to make a popup menu pick an item with a single click. Don't pass non-zero to this.

int Fl::event_key()
int Fl::event_key(int)

int Fl::get_key(int)

Fl::event_key() returns which key on the keyboard was last pushed. It returns zero if the last event was not a key press or release.

Fl::event_key(int) returns true if the given key was held down (or pressed) during the last event. This is constant until the next event is read from the server.

Fl::get_key(int) returns true if the given key is held down now. Under X this requires a round-trip to the server and is much slower than Fl::event_key(int).

Keys are identified by the unshifted values. FLTK defines a set of symbols that should work on most modern machines for every key on the keyboard:

On X Fl::get_key(FL_Button+n) does not work.

On WIN32 Fl::get_key(FL_KP_Enter) and Fl::event_key(FL_KP_Enter) do not work.

char *Fl::event_length()

Returns the length of the text in Fl::event_text(). There will always be a nul at this position in the text. However there may be a nul before that if the keystroke translates to a nul character or you paste a nul character.

ulong Fl::event_state()
unsigned int Fl::event_state(ulong)

This is a bitfield of what shift states were on and what mouse buttons were held down during the most recent event. The second version returns non-zero if any of the passed bits are turned on. The legal bits are: X servers do not agree on shift states, and FL_NUM_LOCK, FL_META, and FL_SCROLL_LOCK may not work. The values were selected to match the XFree86 server on Linux. In addition there is a bug in the way X works so that the shift state is not correctly reported until the first event after the shift key is pressed or released.

char *Fl::event_text()

Returns the ASCII text (in the future this may be UTF-8) produced by the last FL_KEYBOARD or FL_PASTE or possibly other event. A zero-length string is returned for any keyboard function keys that do not produce text. This pointer points at a static buffer and is only valid until the next event is processed.

Under X this is the result of calling XLookupString().

int Fl::compose(int& del)

Use of this function is very simple. Any text editing widget should call this for each FL_KEYBOARD event.

If true is returned, then it has modified the Fl::event_text() and Fl::event_length() to a set of bytes to insert (it may be of zero length!). It will also set the del parameter to the number of bytes to the left of the cursor to delete, this is used to delete the results of the previous call to Fl::compose().

If false is returned, the keys should be treated as function keys. You could insert the text anyways, if you don't know what else to do, del is set to zero and the Fl::event_text() and Fl::event_length() are left unchanged, length is zero for any function keys.

Though the current implementation returns immediately, future versions may take quite awhile, as they may pop up a window or do other user-interface things to allow characters to be selected.

int Fl::compose_reset()

If the user moves the cursor, be sure to call Fl::compose_reset(). The next call to Fl::compose() will start out in an initial state. In particular it will not set "del" to non-zero. This call is very fast so it is ok to call it many times and in many places.

Fl_Window *Fl::first_window()

Returns the first visible() top-level window. This is the modal() window if it exists, if there are no modal windows it is the most recent window to get an event. Returns NULL if no windows are visible.

void Fl::first_window(Fl_Window*)

Since first_window() is used by default as the "parent" of modal windows, changing it is useful. This removes window from wherever it is in the list and inserts it at the top, as though it received an event. Code that then shows a modal window or calls Fl_Window::exec() will make this window the "parent".

Fl_Window *Fl::next_window(Fl_Window *)

Returns the next visible() top-level window, returns NULL after the last one. You can use this and first_window() to iterate through all the visible windows.

void Fl::flush()

Causes all the windows that need it to be redrawn and graphics forced out through the pipes. This is what wait() does before looking for events.

Fl_Widget *Fl::focus() const;

Returns the widgets that will receive FL_KEYBOARD events. This is NULL if the application does not have focus now, or if no widgets accepted focus.

void Fl::focus(Fl_Widget *)

Change Fl::focus() to the given widget, the previous widget and all parents (that don't contain the new widget) are sent FL_UNFOCUS events, the new widget and all parents that don't contain the old widget are sent FL_FOCUS events. focus() is set whether or not the applicaton has the focus or if the widgets accept the focus. You may want to use Fl_Widget::take_focus() instead, it will test first.

void Fl::get_mouse(int &x, int &y)

Return where the mouse is on the screen by doing a round-trip query to the server. You should use Fl::event_x_root() and Fl::event_y_root() if possible, but this is necessary if you are not sure if a mouse event has been processed recently (such as to position your first window). If the display is not open, this will open it.

void grab(int (*)(int, void*), void*);
void release();
int (*Fl::grab())(int, void*);

This is used when pop-up menu systems are active. All user events are sent to the passed function, rather than going to widgets. The void* argument can be used to pass arbitrary data to this function.

This messes with the window system so you get the events no matter where the cursor is on the screen. Under both X and WIN32 some window must be mapped because the system interface needs a window id. Be careful that your program does not enter an infinite loop while grab() is on. On X this will lock up your screen!

To turn off grabbing do Fl::release().

Fl::grab() returns the current function, this is useful for checking if a grab is active.

int Fl::h()

Returns the height of the screen in pixels.

int Fl::handle(int, Fl_Window *)

Sends the event to a window for processing. Returns non-zero if any widget uses the event.

const char *Fl::help

This is the usage string that is displayed if Fl::args() detects an invalid argument on the command-line.

Fl_Window *Fl::modal()

Returns the top-most modal() window currently shown. This is the most recently shown() window with modal() true, or NULL if there are no modal() windows shown(). The modal() window has its handle() method called for all events, and no other windows will have handle() called (grab() overrides this).

void Fl::own_colormap()

Makes FLTK use its own colormap. This may make FLTK display better and will reduce conflicts with other programs that want lots of colors. However the colors may flash as you move the cursor between windows.

This does nothing if the current visual is not colormapped.

void Fl::paste(Fl_Widget *receiver)

Set things up so the receiver widget will be called with an FL_PASTE event some time in the future. The reciever should be prepared to be called directly by this, or for it to happen later, or possibly not at all. This allows the window system to take as long as necessary to retrieve the paste buffer (or even to screw up completely) without complex and error-prone synchronization code in FLTK.

Fl_Widget *Fl::pushed() const
void Fl::pushed(Fl_Widget *)

Get or set the widget that is being pushed. FL_DRAG or FL_RELEASE (and any more FL_PUSH) events will be sent to this widget.

If you change the pushed widget, the previous one and all parents (that don't contain the new widget) are sent FL_RELEASE events. Changing this does not send FL_PUSH to this or any widget, because sending FL_PUSH is supposed to test if the widget wants the mouse (by it returning non-zero from handle()).

static int Fl::ready()

This is similar to Fl::check() except this does not call Fl::flush() or any callbacks, which is useful if your program is in a state where such callbacks are illegal. This returns true if Fl::check() would do anything (it will continue to return true until you call Fl::check() or Fl::wait()).

void Fl::redraw()

Redraws all widgets.

static int Fl::has_idle(void (*cb)(void*), void* = 0)

Returns true if the specified idle callback is currently installed.

static void Fl::remove_idle(void (*cb)(void*), void* = 0)

Removes the specified idle callback, if it is installed.

static int Fl::has_timeout(void (*cb)(void*), void* = 0)

Returns true if the timeout exists and has not been called yet.

static void Fl::remove_timeout(void (*cb)(void*), void* = 0)

Removes a timeout callback. It is harmless to remove a timeout callback that no longer exists.

static int Fl::has_check(void (*cb)(void*), void* = 0)

Returns true if the check exists and has not been called yet.

static void Fl::remove_check(void (*cb)(void*), void* = 0)

Removes a check callback. It is harmless to remove a check callback that no longer exists.

static Fl::run()

Calls Fl::wait() in an infinite loop. The only way out is for a callback to call exit() or abort(). A normal program will end main() with return Fl::run();.

void Fl::copy(const char *stuff, int len)
const char* Fl::selection()
int Fl::selection_length()

The first form changes the current selection. The block of text is copied to an internal buffer by FLTK (be careful if doing this in response to an FL_PASTE as this may be the same buffer returned by event_text()). The second form looks at the buffer containing the current selection. The contents of this buffer are undefined if this program does not own the current selection.

Copying the buffer every time the selection is changed is obviously wasteful, especially for large selections. An interface will probably be added in a future version to allow the selection to be made by a callback function. The current interface will be emulated on top of this.

int Fl::test_shortcut(int shortcut)

Test the current event, which must be an FL_KEYBOARD or FL_SHORTCUT, against a shortcut value (described in Fl_Button). Returns non-zero if there is a match. Not to be confused with Fl_Widget::test_shortcut().

const char* Fl::key_name(int shortcut)

Unparse a key name (as returned by Fl::event_key()) or a shortcut value (as used by Fl_Button or Fl_Menu_Item) into a human-readable string like "Alt+N". If the shortcut is zero an empty string is returned. The return value points at a static buffer that is overwritten with each call.

int Fl::visual(int)

This call may be useful on multi-visual X servers to change from the default to a more useful color mode. You must call this before you show() any windows. The integer argument is an 'or' of the following: This returns true if the system has the capabilities by default or FLTK suceeded in turing them on. Your program will still work even if this returns false (it just won't look as good). On non-X systems this just returns true or false indicating if the system supports the passed values.

int Fl::gl_visual(int)

This does the same thing as Fl::visual(int) but also requires OpenGL drawing to work. Doing this on X will reduce colormap flashing at the edges of Fl_Gl_Windows when they are inside regular windows.

See Fl_Gl_Window for a list of additional values for the argument.

int Fl::w()

Returns the width of the screen in pixels.

static int Fl::wait()

Same as Fl::wait(infinity). Call this repeatedly to "run" your program. You can also check what happened each time after this returns, which is quite useful for managing program state.

static int Fl::wait(double time)

Waits until "something happens", or the given time interval passes. It can return much sooner than the time if something happens.

What this really does is call all idle callbacks, all elapsed timeouts, call Fl::flush() to get the screen to update, and then wait some time (zero if there are idle callbacks, the shortest of all pending timeouts, or the given time), for any events from the user or any Fl::add_fd() callbacks. It then handles the events and calls the callbacks and then returns.

The return value is non-zero if there are any visible windows (this may change in future versions of fltk).

The return value is whatever the select() system call returned. This will be negative if there was an error (this will happen on Unix if a signal happens), zero if the timeout occurred, and positive if any events or fd's came in.

On Win32 the return value is zero if nothing happened and time is 0.0. Otherwise 1 is returned.

void (*Fl::warning)(const char *, ...)
void (*Fl::error)(const char *, ...)
void (*Fl::fatal)(const char *, ...)

FLTK will call these to print messages when unexpected conditions occur. By default they fprintf to stderr, and Fl::error and Fl::fatal call exit(1). You can override the behavior by setting the function pointers to your own routines.

Fl::warning means that there was a recoverable problem, the display may be messed up but the user can probably keep working (all X protocol errors call this). Fl::error means there is a recoverable error, but the display is so messed up it is unlikely the user can continue (very little calls this now). Fl::fatal must not return, as FLTK is in an unusable state, however your version may be able to use longjmp or an exception to continue, as long as it does not call FLTK again.