/**

 \page  subclassing Adding and Extending Widgets


This chapter describes how to add your own widgets or extend existing
widgets in FLTK.

\section subclassing_subclassing Subclassing

New widgets are created by \e subclassing an existing FLTK widget,
typically Fl_Widget for controls and Fl_Group for composite widgets.

A control widget typically interacts with the user to receive and/or
display a value of some sort.

A composite widget holds a list of child widgets and handles moving,
sizing, showing, or hiding them as needed. Fl_Group is the main
composite widget class in FLTK, and all of the other composite widgets
(Fl_Pack, Fl_Scroll, Fl_Tabs, Fl_Tile, and Fl_Window) are subclasses of it.

You can also subclass other existing widgets to provide a different
look or user-interface. For example, the button widgets are all
subclasses of Fl_Button since they all interact with the user
via a mouse button click.  The only difference is the code that draws
the face of the button.

\section subclassing_fl_widget Making a Subclass of Fl_Widget

Your subclasses can directly descend from Fl_Widget or any
subclass of Fl_Widget. Fl_Widget has only four
virtual methods, and overriding some or all of these may be necessary.

\section subclassing_constructor The Constructor

The constructor should have the following arguments:

\code
MyClass(int x, int y, int w, int h, const char *label = 0);
\endcode

This will allow the class to be used in
\ref fluid "FLUID"
without problems.

The constructor must call the constructor for the base class and
pass the same arguments:

\code
MyClass::MyClass(int x, int y, int w, int h, const char *label)
: Fl_Widget(x, y, w, h, label) {
// do initialization stuff...
}
\endcode

Fl_Widget's protected constructor sets \p x(), \p y(),
\p w(), \p h(), and \p label() to the passed values
and initializes the other instance variables to:

\code
type(0);
box(FL_NO_BOX);
color(FL_BACKGROUND_COLOR);
selection_color(FL_BACKGROUND_COLOR);
labeltype(FL_NORMAL_LABEL);
labelstyle(FL_NORMAL_STYLE);
labelsize(FL_NORMAL_SIZE);
labelcolor(FL_FOREGROUND_COLOR);
align(FL_ALIGN_CENTER);
callback(default_callback,0);
flags(ACTIVE|VISIBLE);
image(0);
deimage(0);
\endcode

\section subclassing_protected Protected Methods of Fl_Widget

The following methods are provided for subclasses to use:

\li \ref subclassing_clear_visible "clear_visible()"
\li \ref subclassing_damage "damage()"
\li \ref subclassing_draw_box "draw_box()"
\li \ref subclassing_draw_focus "draw_focus()"
\li \ref subclassing_draw_label "draw_label()"
\li \ref subclassing_set_flag "set_flag()"
\li \ref subclassing_set_visible "set_visible()"
\li \ref subclassing_test_shortcut "test_shortcut()"
\li \ref subclassing_type "type()"

\anchor subclassing_damage
void Fl_Widget::damage(uchar mask) <br>
void Fl_Widget::damage(uchar mask, int x, int y, int w, int h) <br>
uchar Fl_Widget::damage()

\par
The first form indicates that a partial update of the object is
needed.  The bits in mask are OR'd into
\ref subclassing_damage "damage()".
Your \p draw() routine can examine these bits to limit what it is
drawing.  The public method Fl_Widget::redraw() simply does
\p Fl_Widget::damage(FL_DAMAGE_ALL),
but the implementation of your widget can call the public
\p damage(n).

\par
The second form indicates that a region is damaged.  If only these
calls are done in a window (no calls to \p damage(n)) then FLTK
will clip to the union of all these calls before drawing anything.
This can greatly speed up incremental displays.  The mask bits are
OR'd into \p damage() unless this is a Fl_Window widget.

\par
The third form returns the bitwise-OR of all \p damage(n)
calls done since the last \p draw().

\par
<I>When redrawing your widgets you should look at the damage bits to
see what parts of your widget need redrawing.</I> The \p handle()
method can then set individual damage bits to limit the amount of drawing
that needs to be done:
\code
MyClass::handle(int event) {
  ...
  if (change_to_part1) damage(1);
  if (change_to_part2) damage(2);
  if (change_to_part3) damage(4);
}

MyClass::draw() {
  if (damage() & FL_DAMAGE_ALL) {
    ... draw frame/box and other static stuff ...
  }

  if (damage() & (FL_DAMAGE_ALL | 1)) draw_part1();
  if (damage() & (FL_DAMAGE_ALL | 2)) draw_part2();
  if (damage() & (FL_DAMAGE_ALL | 4)) draw_part3();
}
\endcode

\todo Clarify Fl_Window::damage(uchar) handling - seems confused/wrong?
      ORing value doesn't match setting behaviour in FL_Widget.H!

\anchor subclassing_draw_box
void Fl_Widget::draw_box() const <br>
void Fl_Widget::draw_box(Fl_Boxtype t, Fl_Color c) const

\par
The first form draws this widget's \p box(), using the
dimensions of the widget.  The second form uses \p t as the box
type and \p c as the color for the box.

\anchor subclassing_draw_focus
void Fl_Widget::draw_focus() <br>
void Fl_Widget::draw_focus(Fl_Boxtype t, int x, int y, int w, int h) const

\par
Draws a focus box inside the widget's bounding box. The second
form allows you to specify a different bounding box.

\anchor subclassing_draw_label
void Fl_Widget::draw_label() const <br>
void Fl_Widget::draw_label(int x, int y, int w, int h) const <br>
void Fl_Widget::draw_label(int x, int y, int w, int h, Fl_Align align) const

\par
The first form is the usual function for a \p draw() method to call to
draw the widget's label. It does not draw the label if it is supposed
to be outside the box (on the assumption that the enclosing group will
draw those labels).

\par
The second form uses the passed bounding box instead of the widget's
bounding box. This is useful so "centered" labels are aligned with some
feature, like a moving slider.

\par
The third form draws the label anywhere. It acts as though
\p FL_ALIGN_INSIDE has been forced on so the label will appear inside
the passed bounding box. This is designed for parent groups to draw
labels with.

\anchor subclassing_set_flag
void Fl_Widget::set_flag(int c) <br>

\par
Calling \p set_flag(SHORTCUT_LABEL) modifies the behavior of
\ref subclassing_draw_label "draw_label()" so that '\&' characters
cause an underscore to be printed under the next letter.

\anchor subclassing_clear_visible
\anchor subclassing_set_visible
void Fl_Widget::set_visible() <br>
void Fl_Widget::clear_visible()

\par
Fast inline versions of Fl_Widget::hide() and Fl_Widget::show().
These do not send the \p FL_HIDE and \p FL_SHOW events to the widget.

\anchor subclassing_test_shortcut
int Fl_Widget::test_shortcut() <br>
static int Fl_Widget::test_shortcut(const char *s)

\par
The first version tests Fl_Widget::label() against the current event
(which should be a \p FL_SHORTCUT event).  If the label contains a '&'
character and the character after it matches the keypress, this returns
true. This returns false if the \p SHORTCUT_LABEL flag is off, if the
label is \p NULL, or does not have a '&' character in it, or if the
keypress does not match the character.

\par
The second version lets you do this test against an arbitrary string.

\todo Clarify Fl_Widget::test_shortcut() explanations. Fl_Widget.h
      says Internal Use only, but subclassing chapter gives details!

\anchor subclassing_type
uchar Fl_Widget::type() const <br>
void Fl_Widget::type(uchar t)

\par
The property Fl_Widget::type() can return an arbitrary 8-bit
identifier, and can be set with the protected method <tt>type(uchar t)</tt>.
This value had to be provided for Forms compatibility, but you can
use it for any purpose you want.  Try to keep the value less than 100
to not interfere with reserved values.

\par
FLTK does not use RTTI (Run Time Typing Information) to enhance portability.
But this may change in the near future if RTTI becomes standard everywhere.

\par
If you don't have RTTI you can use the clumsy FLTK mechanism, by
having \p type() use a unique value.  These unique values must
be greater than the symbol \p FL_RESERVED_TYPE (which is 100) and
less than \p FL_WINDOW (unless you make a subclass of Fl_Window).
Look through the header files for \p FL_RESERVED_TYPE to find an
unused number. If you make a subclass of Fl_Window you must use
<tt>FL_WINDOW + n</tt> (where \p n must be in the range 1 to 7).

\section subclassing_events Handling Events

The virtual method Fl_Widget::handle(int event) is called
to handle each event passed to the widget. It can:

\li Change the state of the widget.
\li Call Fl_Widget::redraw() if the widget needs to be redisplayed.
\li Call Fl_Widget::damage(uchar c) if the widget needs a partial-update
    (assuming you provide support for this in your
    \ref subclassing_drawing "draw()"
    method).
\li Call Fl_Widget::do_callback() if a callback should be generated.
\li Call Fl_Widget::handle() on child widgets.

Events are identified by the integer argument.  Other information
about the most recent event is stored in static locations and acquired
by calling the
\ref events_event_xxx.
This information remains valid until another event is handled.

Here is a sample \p handle() method for a widget that acts as
a pushbutton and also accepts the keystroke \p 'x' to cause the callback:

\code
int MyClass::handle(int event) {
  switch(event) {
    case FL_PUSH:
      highlight = 1;
      redraw();
      return 1;
    case FL_DRAG: {
        int t = Fl::event_inside(this);
        if (t != highlight) {
          highlight = t;
          redraw();
        }
      }
      return 1;
    case FL_RELEASE:
      if (highlight) {
        highlight = 0;
        redraw();
        do_callback();
        // never do anything after a callback, as the callback
        // may delete the widget!
      }
      return 1;
    case FL_SHORTCUT:
      if (Fl::event_key() == 'x') {
        do_callback();
        return 1;
      }
      return 0;
    default:
      return Fl_Widget::handle(event);
  }
}
\endcode

You must return non-zero if your \p handle() method
uses the event. If you return zero, the parent widget will try
sending the event to another widget.

For debugging purposes, event numbers can be printed as their actual event names
using the \ref fl_eventnames[] array, e.g.:
\code
  #include <FL/names.h>         // defines fl_eventnames[]
  [..]
  int MyClass::handle(int e) {
      printf("Event was %s (%d)\n", fl_eventnames[e], e);     // e.g. "Event was FL_PUSH (1)"
      [..]
\endcode

\section subclassing_drawing Drawing the Widget

The \p draw() virtual method is called when FLTK wants
you to redraw your widget. It will be called if and only if
\p damage() is non-zero, and \p damage() will be
cleared to zero after it returns. The \p draw() method
should be declared protected so that it can't be called from
non-drawing code.

The \p damage() value contains the bitwise-OR of all
the \p damage(n) calls to this widget since it was last
drawn. This can be used for minimal update, by only redrawing
the parts whose bits are set. FLTK will turn on the
\p FL_DAMAGE_ALL bit if it thinks the entire widget must
be redrawn, e.g. for an expose event.

Expose events (and the
\ref subclassing_damage "damage(mask,x,y,w,h)" function described
above) will cause \p draw() to be called with FLTK's
\ref drawing_clipping "clipping"
turned on.  You can greatly speed up redrawing in some
cases by testing \p fl_not_clipped(x,y,w,h) or \p %fl_clip_box()
and skipping invisible parts.

Besides the protected methods described above, FLTK provides a large
number of basic drawing functions, which are described in the chapter
\ref drawing.

\section subclassing_resizing Resizing the Widget

The \p resize(x,y,w,h) method is called when
the widget is being resized or moved.  The arguments are the new
position, width, and height. \p x(), \p y(), \p w(),
and \p h() still remain the old size.  You must call \p resize()
on your base class with the same arguments to get the widget size to
actually change.

This should \e not call \p redraw(), at least if only the
\p x() and \p y() change.  This is because composite widgets like
Fl_Scroll may have a more efficient way of drawing the new position.

\section subclassing_composite Making a Composite Widget

A "composite" widget contains one or more "child" widgets.
To make a composite widget you should subclass Fl_Group.
It is possible to make a composite object that is not a subclass of
Fl_Group, but you'll have to duplicate the code in Fl_Group
anyways.

Instances of the child widgets may be included in the parent:

\code
class MyClass : public Fl_Group {
  Fl_Button the_button;
  Fl_Slider the_slider;
  ...
};
\endcode

The constructor has to initialize these instances. They are automatically
added to the group, since the Fl_Group constructor does
Fl_Group::begin().
<I>Don't forget to call Fl_Group::end() or use the Fl_End pseudo-class:</I>

\code
MyClass::MyClass(int x, int y, int w, int h) :
  Fl_Group(x, y, w, h),
  the_button(x + 5, y + 5, 100, 20),
  the_slider(x, y + 50, w, 20)
{
  ...(you could add dynamically created child widgets here)...
  end(); // don't forget to do this!
}
\endcode

The child widgets need callbacks.  These will be called with a pointer
to the children, but the widget itself may be found in the \p parent()
pointer of the child.  Usually these callbacks can be static private
methods, with a matching private method:

\code
void MyClass::static_slider_cb(Fl_Widget* v, void *) { // static method
  ((MyClass*)(v->parent())->slider_cb();
}
void MyClass::slider_cb() { // normal method
  use(the_slider->value());
}
\endcode

If you make the \p handle() method, you can quickly pass all the
events to the children using the Fl_Group::handle() method.
You don't need to override \p handle() if your composite widget
does nothing other than pass events to the children:

\code
int MyClass::handle(int event) {
  if (Fl_Group::handle(event)) return 1;
  ... handle events that children don't want ...
}
\endcode

If you override \p draw() you need to draw all the children.
If \p redraw() or \p damage() is called on a child,
\p damage(FL_DAMAGE_CHILD) is done to the group,
so this bit of \p damage() can be used to indicate
that a child needs to be drawn. It is fastest if you avoid
drawing anything else in this case:

\code
int MyClass::draw() {
  Fl_Widget *const*a = array();
  if (damage() == FL_DAMAGE_CHILD) { // only redraw some children
    for (int i = children(); i --; a ++) update_child(**a);
  } else { // total redraw
    ... draw background graphics ...
    // now draw all the children atop the background:
    for (int i = children_; i --; a ++) {
      draw_child(**a);
      draw_outside_label(**a); // you may not need to do this
    }
  }
}
\endcode

Fl_Group provides some protected methods to make drawing easier:

\li \ref subclassing_draw_child "draw_child()"
\li \ref subclassing_draw_children "draw_children()"
\li \ref subclassing_draw_outside_label "draw_outside_label()"
\li \ref subclassing_update_child "update_child()"

\anchor subclassing_draw_child
void Fl_Group::draw_child(Fl_Widget &widget) const

\par
This will force the child's \p damage() bits all to one and call
\p draw() on it, then clear the \p damage().  You should call
this on all children if a total redraw of your widget is requested, or
if you draw something (like a background box) that damages the child.
Nothing is done if the child is not \p visible() or if it is
clipped.

\anchor subclassing_draw_children
void Fl_Group::draw_children()

\par
A convenience function that draws all children of the group.
This is useful if you derived a widget from Fl_Group and want to draw
a special border or background. You can call \p draw_children() from the
derived  \p draw() method after drawing the box, border, or background.

\anchor subclassing_draw_outside_label
void Fl_Group::draw_outside_label(const Fl_Widget &widget) const

\par
Draw the labels that are \e not drawn by
\ref subclassing_draw_label "draw_label()".
If you want more control over the label positions you might want to call
<tt>child->draw_label(x,y,w,h,a)</tt>.

\anchor subclassing_update_child
void Fl_Group::update_child(Fl_Widget& widget) const

\par
Draws the child only if its \p damage() is non-zero.  You
should call this on all the children if your own damage is equal to
\p FL_DAMAGE_CHILD.  Nothing is done if the child is not \p visible()
or if it is clipped.

\section subclassing_cutnpaste Cut and Paste Support

FLTK provides routines to cut and paste UTF-8 encoded text between applications:
\li Fl::copy()
\li Fl::paste()
\li Fl::selection()
\li Fl::selection_owner()

It is also possible to copy and paste image data between applications:
\li Fl_Copy_Surface
\li Fl::clipboard_contains()
\li Fl::paste()

It may be possible to cut/paste other kinds of data by using Fl::add_handler().
Note that handling events beyond those provided by FLTK may be operating
system specific. See \ref osissues for more details.

\section subclassing_dragndrop Drag And Drop Support

FLTK provides routines to drag and drop UTF-8 encoded text between applications:

Drag'n'drop operations are initiated by copying data to the
clipboard and calling the function Fl::dnd().

Drop attempts are handled via the following events,
already described under \ref events_dnd in a previous chapter:

\li \p FL_DND_ENTER
\li \p FL_DND_DRAG
\li \p FL_DND_LEAVE
\li \p FL_DND_RELEASE
\li \p FL_PASTE

\section subclassing_fl_window Making a subclass of Fl_Window

You may want your widget to be a subclass of
Fl_Window, Fl_Double_Window, or
Fl_Gl_Window. This can be useful if your widget wants
to occupy an entire window, and can also be used to take
advantage of system-provided clipping, or to work with a library
that expects a system window ID to indicate where to draw.

Subclassing Fl_Window is almost exactly like
subclassing Fl_Group, and in fact you can easily
switch a subclass back and forth. Watch out for the following
differences:

-# Fl_Window is a subclass of Fl_Group so
   <I>make sure your constructor calls</I> \p end()
   unless you actually want children added to your window.
-# When handling events and drawing, the upper-left corner is at
   0,0, not <tt>x(),y()</tt> as in other Fl_Widget's.
   For instance, to draw a box around the widget, call
   <tt>draw_box(0,0,w(),h())</tt>, rather than
   <tt>draw_box(x(),y(),w(),h())</tt>.

You may also want to subclass Fl_Window in order to
get access to different visuals or to change other attributes of
the windows. See the
\ref osissues chapter for more information.


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