# 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 the window system, so 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()`.

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

### FL_RELEASE

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`.

### FL_MOVE

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`.

### FL_MOUSEWHEEL

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

### 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.

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

### 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_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 interaction with the window manager it will be zero.

### FL_UNFOCUS

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

## Keyboard Events

### FL_KEYDOWN, FL_KEYUP

A key was pressed or released. 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 `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.

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_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.

### 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 `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.

## 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.

## Drag And Drop Events

FLTK supports drag and drop of text and files from any application on the desktop. Text is transfered using the current code page. Files are received as a list of full path and file names, separated by newline. On some platforms, path names are prepended with `file://`.

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`.

### FL_DND_ENTER

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.

### FL_DND_DRAG

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.

### FL_DND_LEAVE

The mouse has moved out of the widget.

### FL_DND_RELEASE

The user has released the mouse button dropping data into the widget. If the widget returns 1, it will receive the data in the immediately following FL_PASTE event.

## 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_dx`
- `Fl::event_dy`
- `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 its 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.

The following methods are provided for character composition:

- `Fl::compose()`
- `Fl::compose_reset()`