# 6 - 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 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 window system (i.e. it is ok to look at it outside of the `handle()` method).

## Mouse Events

### FL_PUSH

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. 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.

### FL_DRAG

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()`.

To receive `FL_DRAG` events you must also respond to the `FL_PUSH` and `FL_RELEASE` events.

### FL_RELEASE

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

### FL_MOVE

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

## Focus Events

### FL_ENTER

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.

### FL_LEAVE

The mouse has moved out of the widget.

### FL_FOCUS

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_KEYBOARD` 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 instructions from the window manager it will be zero.

### FL_UNFOCUS

Sent to the previous `Fl::focus()` widget when another widget gets the focus.

## Keyboard Events

### FL_KEYBOARD

A key press. The key pressed 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 `handle()` should return 1. If you return zero then FLTK assummes you ignored the key. It 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.

To receive `FL_KEYBOARD` events you must also respond to the `FL_FOCUS` and `FL_UNFOCUS` events.

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

### FL_SHORTCUT

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 `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.

## Widget Events

### FL_DEACTIVATE

This widget is no longer active, due to `deactivate()` being called on it or one of its parents. `active()` may still be true after this, the widget is only active if `active()` is true on it and all its parents (use `active_r()` to check this).

### FL_ACTIVATE

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

### FL_HIDE

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. `visible()` may still be true after this, but the widget is visible only if `visible()` is true for it and all its parents (use `visible_r()` to check this).

### FL_SHOW

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. Child `Fl_Window`s 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!

## Clipboard Events

### FL_PASTE

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()`.

### FL_SELECTIONCLEAR

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.

## 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:

- `Fl::event_button`
- `Fl::event_clicks`
- `Fl::event_inside`
- `Fl::event_is_click`
- `Fl::event_key`
- `Fl::event_length`
- `Fl::event_state`
- `Fl::event_text`
- `Fl::event_x`
- `Fl::event_x_root`
- `Fl::event_y`
- `Fl::event_y_root`
- `Fl::get_key`
- `Fl::get_mouse`
- `Fl::test_shortcut`

## Event Propagation

FLTK follows very simple and unchangeable rules for sending events. The major innovation is that widgets can indicate (by returning 0 from the `handle()` method) that they are not interested in an event, and FLTK can then send that event elsewhere. This eliminates the need for "interests" (event masks or tables), and this is probably the main reason FLTK is much smaller than other toolkits.

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:

- `Fl::add_handler`
- `Fl::belowmouse`
- `Fl::focus`
- `Fl::grab`
- `Fl::modal`
- `Fl::pushed`
- `Fl::release`
- `Fl_Widget::take_focus`

## FLTK Compose-Character Sequences

The foreign-letter compose processing done by the `Fl_Input` widget is provided in a function that you can call if you are writing your own text editor widget.

Fltk uses it's own compose processing to allow "preview" of the partially composed sequence, which is impossible with the usual "dead key" processing.

Although currently only characters in the ISO-8859-1 character set are handled, you should call this in case any enhancements to the processing are done in the future. The interface has been designed to handle arbitrary UTF-8 encoded text.

#### `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!). In 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, and del is set to zero. You could insert the text anyways, if you don't know what else to do.

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.