FLTK 1.4.0
Handling Events

This chapter discusses the FLTK event model and how to handle events in your program or widget.

The FLTK Event Model

Every time a user moves the mouse pointer, clicks a button, or presses a key, an event is generated and sent to your application. Events can also come from other programs like the window manager.

Events are identified by the integer argument passed to a handle() method that overrides the Fl_Widget::handle() virtual method. Other information about the most recent event is stored in static locations and acquired by calling the Fl::event_*() methods. This static information remains valid until the next event is read from the window system, so it is ok to look at it outside of the handle() method.

Event numbers can be converted to their actual names using the fl_eventnames[] array defined in #include <FL/names.h>; see next chapter for details.

In the next chapter, the MyClass::handle() example shows how to override the Fl_Widget::handle() method to accept and process specific events.

Mouse Events


A mouse button has gone down with the mouse pointing at this widget. You can find out what button by calling Fl::event_button(). You find out the mouse position by calling Fl::event_x() and Fl::event_y().

A widget indicates that it "wants" the mouse click by returning non-zero from its handle() method, as in the MyClass::handle() example. It will then become the Fl::pushed() widget and will get FL_DRAG and the matching FL_RELEASE events. If handle() returns zero then FLTK will try sending the FL_PUSH to another widget.


The mouse has moved with a button held down. The current button state is in Fl::event_state(). The mouse position is in Fl::event_x() and Fl::event_y().

In order to receive FL_DRAG events, the widget must return non-zero when handling FL_PUSH.


A mouse button has been released. You can find out what button by calling Fl::event_button().

In order to receive the FL_RELEASE event, the widget must return non-zero when handling FL_PUSH.


The mouse has moved without any mouse buttons held down. This event is sent to the Fl::belowmouse() widget.

In order to receive FL_MOVE events, the widget must return non-zero when handling FL_ENTER.


The user has moved the mouse wheel. The Fl::event_dx() and Fl::event_dy() methods can be used to find the amount to scroll horizontally and vertically.

Focus Events


The mouse has been moved to point at this widget. This can be used for highlighting feedback. If a widget wants to highlight or otherwise track the mouse, it indicates this by returning non-zero from its handle() method. It then becomes the Fl::belowmouse() widget and will receive FL_MOVE and FL_LEAVE events.


The mouse has moved out of the widget.

In order to receive the FL_LEAVE event, the widget must return non-zero when handling FL_ENTER.


This indicates an attempt to give a widget the keyboard focus.

If a widget wants the focus, it should change itself to display the fact that it has the focus, and return non-zero from its handle() method. It then becomes the Fl::focus() widget and gets FL_KEYDOWN, FL_KEYUP, and FL_UNFOCUS events.

The focus will change either because the window manager changed which window gets the focus, or because the user tried to navigate using tab, arrows, or other keys. You can check Fl::event_key() to figure out why it moved. For navigation it will be the key pressed and for interaction with the window manager it will be zero.


This event is sent to the previous Fl::focus() widget when another widget gets the focus or the window loses focus.

Keyboard Events


A key was pressed (FL_KEYDOWN) or released (FL_KEYUP). FL_KEYBOARD is a synonym for FL_KEYDOWN, and both names are used interchangeably in this documentation.

The key can be found in Fl::event_key(). The text that the key should insert can be found with Fl::event_text() and its length is in Fl::event_length().

If you use the key, then handle() should return 1. If you return zero then FLTK assumes you ignored the key and will then attempt to send it to a parent widget. If none of them want it, it will change the event into a FL_SHORTCUT event. FL_KEYBOARD events are also generated by the character palette/map.

To receive FL_KEYBOARD events you must also respond to the FL_FOCUS and FL_UNFOCUS events by returning 1. This way FLTK knows whether to bother sending your widget keyboard events. (Some widgets don't need them, e.g. Fl_Box.)

If you are writing a text-editing widget you may also want to call the Fl::compose() function to translate individual keystrokes into characters.

FL_KEYUP events are sent to the widget that currently has focus. This is not necessarily the same widget that received the corresponding FL_KEYDOWN event because focus may have changed between events.

Add details on how to detect repeating keys, since on some X servers a repeating key will generate both FL_KEYUP and FL_KEYDOWN, such that to tell if a key is held, you need Fl::event_key(int) to detect if the key is being held down during FL_KEYUP or not.


If the Fl::focus() widget is zero or ignores an FL_KEYBOARD event then FLTK tries sending this event to every widget it can, until one of them returns non-zero. FL_SHORTCUT is first sent to the Fl::belowmouse() widget, then its parents and siblings, and eventually to every widget in the window, trying to find an object that returns non-zero. FLTK tries really hard to not to ignore any keystrokes!

You can also make "global" shortcuts by using Fl::add_handler(). A global shortcut will work no matter what windows are displayed or which one has the focus.

Since version 1.4, FLTK has 3 default global shortcuts (Ctrl/+/-/0/ [Cmd/+/-/0/ under macOS]) that change the value of the GUI scaling factor. Ctrl+ zooms-in all app windows of the focussed display (all displays under macOS); Ctrl- zooms-out these windows; Ctrl 0 restores the initial value of the scaling factor. If any window of the display is fullscreen or maximized, scaling shortcuts have no effect. It's possible to deactivate FLTK's default scaling shortcuts with function Fl::keyboard_screen_scaling().

Option Fl::OPTION_SIMPLE_ZOOM_SHORTCUT can facilitate the typing necessary to trigger the zoom-in operation with those keyboard layouts where character '+' is located in the shifted position of its key: when this option is On it's not necessary to press also the Shift key to zoom-in.

These scaling shortcuts are installed when the FLTK library opens the display. They have a lower priority than any shortcut defined in any menu and than any user-provided event handler (see Fl::add_handler()) installed after FLTK opened the display. Therefore, if a menu item of an FLTK app is given FL_COMMAND+'+' as shortcut, that item's callback rather than FLTK's default zooming-in routine is triggered when Ctrl+ (Cmd+ under macOS) is pressed.

FLTK sends the FL_ZOOM_EVENT when the scaling factor value changes, to which a callback can be associated with Fl::add_handler(). By default, FLTK displays the new scaling factor value in a yellow, transient window. This can be changed with option Fl::OPTION_SHOW_SCALING.

Widget Events


This widget is no longer active, due to deactivate() being called on it or one of its parents. Please note that although active() may still return true for this widget after receiving this event, it is only truly active if active() is true for both it and all of its parents. (You can use active_r() to check this).


This widget is now active, due to activate() being called on it or one of its parents.


This widget is no longer visible, due to hide() being called on it or one of its parents, or due to a parent window being minimized. Please note that although visible() may still return true for this widget after receiving this event, it is only truly visible if visible() is true for both it and all of its parents. (You can use visible_r() to check this).


This widget is visible again, due to show() being called on it or one of its parents, or due to a parent window being restored. A child Fl_Window will respond to this by actually creating the window if not done already, so if you subclass a window, be sure to pass FL_SHOW to the base class handle() method!

The events in this chapter ("Widget Events"), i.e. FL_ACTIVATE, FL_DEACTIVATE, FL_SHOW, and FL_HIDE, are the only events deactivated and invisible widgets can usually get, depending on their states. Under certain circumstances, there may also be FL_LEAVE or FL_UNFOCUS events delivered to deactivated or hidden widgets.

Clipboard Events


You should get this event some time after you call Fl::paste(). The contents of Fl::event_text() is the text to insert and the number of characters is in Fl::event_length().


The Fl::selection_owner() will get this event before the selection is moved to another widget. This indicates that some other widget or program has claimed the selection. Motif programs used this to clear the selection indication. Most modern programs ignore this.

Drag and Drop Events

FLTK supports drag and drop of text and files from any application on the desktop to an FLTK widget. Text is transferred using UTF-8 encoding.

See Fl::dnd() for drag and drop from an FLTK widget.

The drag and drop data is available in Fl::event_text() at the concluding FL_PASTE. On some platforms, the event text is also available for the FL_DND_* events, however application must not depend on that behavior because it depends on the protocol used on each platform.

FL_DND_* events cannot be used in widgets derived from Fl_Group or Fl_Window.

Dropped filenames

Files are received as a list of full path and file names.

  • On some X11 platforms, files are received as a URL-encoded UTF-8 string, that is, non-ASCII bytes (and a few others such as space and %) are replaced by the 3 bytes "%XY" where XY are the byte's hexadecimal value. The fl_decode_uri() function can be used to transform in-place the received string into a proper UTF-8 string. On these platforms, strings corresponding to dropped files are further prepended by file:// (or other prefixes such as computer://).
  • Other X11 situations put all dropped filenames in a single line, separated by spaces.
  • On non-X11 platforms, including Wayland, files dropped are received one pathname per line, with no '\n' after the last pathname.


The mouse has been moved to point at this widget. A widget that is interested in receiving drag'n'drop data must return 1 to receive FL_DND_DRAG, FL_DND_LEAVE and FL_DND_RELEASE events.


The mouse has been moved inside a widget while dragging data. A widget that is interested in receiving drag'n'drop data should indicate the possible drop position.


The mouse has moved out of the widget.


The user has released the mouse button dropping data into the widget. When the receiving widget's handle() method gets the FL_DND_RELEASE event, it should return 1 to accept the dragged data. Processing of this event must not use code that would make unrelated events be sent to the application (opening a dialog window for example) or that would communicate with the dragging process. The next event received by the handle() method will then be an FL_PASTE event. The handle() method should process this FL_PASTE event rapidly to prevent the dragging process from failing with a timeout error.

Other events


Sent whenever the screen configuration changes (a screen is added/removed, a screen resolution is changed, screens are moved). Use Fl::add_handler() to be notified of this event.


The application window has been changed from normal to fullscreen, or from fullscreen to normal. If you are using a X window manager which supports Extended Window Manager Hints, this event will not be delivered until the change has actually happened.

Fl::event_*() methods

FLTK keeps the information about the most recent event in static storage. This information is good until the next event is processed. Thus it is valid inside handle() and callback() methods.

These are all trivial inline functions and thus very fast and small:

Event Propagation

Widgets receive events via the virtual handle() function. The argument indicates the type of event that can be handled. The widget must indicate if it handled the event by returning 1. FLTK will then remove the event and wait for further events from the host. If the widget's handle function returns 0, FLTK may redistribute the event based on a few rules.

Most events are sent directly to the handle() method of the Fl_Window that the window system says they belong to. The window (actually the Fl_Group that Fl_Window is a subclass of) is responsible for sending the events on to any child widgets. To make the Fl_Group code somewhat easier, FLTK sends some events (FL_DRAG, FL_RELEASE, FL_KEYBOARD, FL_SHORTCUT, FL_UNFOCUS, and FL_LEAVE) directly to leaf widgets. These procedures control those leaf widgets:

FLTK propagates events along the widget hierarchy depending on the kind of event and the status of the UI. Some events are injected directly into the widgets, others may be resent as new events to a different group of receivers.

Mouse click events are first sent to the window that caused them. The window then forwards the event down the hierarchy until it reaches the widget that is below the click position. If that widget uses the given event, the widget is marked "pushed" and will receive all following mouse motion (FL_DRAG) events until the mouse button is released.

Mouse motion (FL_MOVE) events are sent to the Fl::belowmouse() widget, i.e. the widget that returned 1 on the last FL_ENTER event.

Mouse wheel events are sent to the window that caused the event. The window propagates the event down the tree, first to the widget that is below the mouse pointer, and if that does not succeed, to all other widgets in the group. This ensures that scroll widgets work as expected with the widget furthest down in the hierarchy getting the first opportunity to use the wheel event, but also giving scroll bars, that are not directly below the mouse a chance.

Keyboard events are sent directly to the widget that has keyboard focus. If the focused widget rejects the event, it is resent as a shortcut event, first to the top-most window, then to the widget below the mouse pointer, propagating up the hierarchy to all its parents. Those send the event also to all widgets that are not below the mouse pointer. Now if that did not work out, the shortcut is sent to all registered shortcut handlers.

If we are still unsuccessful, the event handler flips the case of the shortcut letter and starts over. Finally, if the key is "escape", FLTK sends a close event to the top-most window.

All other events are pretty much sent right away to the window that created the event.

Widgets can "grab" events. The grabbing window gets all events exclusively, but usually by the same rules as described above.

Windows can also request exclusivity in event handling by making the window modal.

FLTK Compose-Character Sequences

The character composition done by Fl_Input widget requires that you call the Fl::compose() function if you are writing your own text editor widget.

Currently, all characters made by single key strokes with or without modifier keys, or by system-defined character compose sequences (that can involve dead keys or a compose key) can be input. You should call Fl::compose() in case any enhancements to this processing are done in the future. The interface has been designed to handle arbitrary UTF-8 encoded text.

The following methods are provided for character composition:

Under Mac OS X, FLTK "previews" partially composed sequences.

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