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<HTML>
<BODY>
<H1 ALIGN=RIGHT><A NAME="subclassing">7 - Adding and Extending Widgets</A></H1>
This chapter describes how to add your own widgets or extend existing
widgets in FLTK.
<H2>Subclassing</H2>
New widgets are created by <i>subclassing</i> an existing FLTK widget,
typically <tt>Fl_Widget</tt> for controls and <tt>Fl_Group</tt> for
containers.
<p>A control widget typically interacts with the user to receive and/or
display a value of some sort.
<p>A container widget holds a list of child widgets and handles moving,
sizing, showing, or hiding them as needed. <tt>Fl_Group</tt> is the
main container widget class in FLTK, and all of the other containers
(<tt>Fl_Pack</tt>, <tt>Fl_Scroll</tt>, <tt>Fl_Tabs</tt>, <tt>Fl_Tile</tt>,
and <tt>Fl_Window</tt>) are subclasses of it.
<p>You can also subclass other existing widgets to provide a different look
or user-interface. For example, the button widgets are all subclasses of
<tt>Fl_Button</tt> 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.
<H2>Making a Subclass of Fl_Widget</H2>
Your subclasses can directly descend from <tt>Fl_Widget</tt> or any
subclass of <tt>Fl_Widget</tt>. <tt>Fl_Widget</tt> has only four
virtual methods, and overriding some or all of these may be necessary.
<H2>The Constructor</H2>
The constructor should access the following arguments:
<ul><pre>
MyClass(int x, int y, int w, int h, const char *label = 0);
</pre></ul>
This will allow the class to be used in <a href="#fluid">Fluid</a> without
problems.
<p>The constructor must call the constructor for the base class and
pass the same arguments:
<ul><pre>
MyClass::MyClass(int x, int y, int w, int h, const char *label)
: Fl_Widget(x, y, w, h, label) {
// do initialization stuff...
}
</pre></ul>
<tt>Fl_Widget</tt>'s protected constructor sets <tt>x()</tt>,
<tt>y()</tt>, <tt>w()</tt>, <tt>h()</tt>, and <tt>label()</tt> to the
passed values and initializes the other instance variables to:
<ul><pre>
type(0);
box(FL_NO_BOX);
color(FL_GRAY);
selection_color(FL_GRAY);
labeltype(FL_NORMAL_LABEL);
labelstyle(FL_NORMAL_STYLE);
labelsize(FL_NORMAL_SIZE);
labelcolor(FL_BLACK);
align(FL_ALIGN_CENTER);
callback(default_callback,0);
flags(ACTIVE|VISIBLE);
</pre></ul>
<H2>Protected Methods of Fl_Widget</H2>
The following methods are provided for subclasses to use:
<ul>
<li><a name="#clear_visible">clear_visible</a>
<li><a name="#damage">damage</a>
<li><a name="#draw_box">draw_box</a>
<li><a name="#draw_label">draw_label</a>
<li><a name="#set_flag">set_flag</a>
<li><a name="#set_visible">set_visible</a>
<li><a name="#test_shortcut">test_shortcut</a>
<li><a name="#type">type</a>
</ul>
<H3><a name="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()</a></H3>
The first form indicates that a partial update of the object is
needed. The bits in mask are OR'd into <tt>damage()</tt>. Your
<tt>draw()</tt> routine can examine these bits to limit what it is
drawing. The public method <tt>Fl_Widget::redraw()</tt> simply does
<tt>Fl_Widget::damage(FL_DAMAGE_ALL)</tt>.
<p>The second form indicates that a region is damaged. If only these
calls are done in a window (no calls to <tt>damage(n)</tt>) 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 <tt>damage()</tt> unless this is a <tt>Fl_Window</tt> widget.
<p>The third form returns the bitwise-OR of all <tt>damage(n)</tt>
calls done since the last <tt>draw()</tt>. The public method
<tt>redraw()</tt> does <tt>damage(FL_DAMAGE_ALL)</tt>, but the
implementation of your widget can call the private <tt>damage(n)</tt>.
<H3><a name="draw_box">void Fl_Widget::draw_box() const<br>
</a>void Fl_Widget::draw_box(Fl_Boxtype b, ulong c) const</H3>
The first form draws this widget's <tt>box()</tt>, using the dimensions
of the widget.
The second form uses <tt>b</tt> as the box type and <tt>c</tt> as the
color for the box.
<H3><a name="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</a></H3>
This is the usual function for a <tt>draw()</tt> 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).
<p>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, such as a moving slider.
<p>The third form draws the label anywhere. It acts as though
<tt>FL_ALIGN_INSIDE</tt> has been forced on, the label will appear
inside the passed bounding box. This is designed for parent groups to
draw labels with.
<H3><a name="set_flag">void Fl_Widget::set_flag(SHORTCUT_LABEL)</a></H3>
If your constructor calls this it modifies <tt>draw_label()</tt> so
that '&' characters cause an underscore to be printed under the next
letter.
<H3><a name="set_visible">void Fl_Widget::set_visible()</a><br>
<a name="clear_visible">void Fl_Widget::clear_visible()</a></H3>
Fast inline versions of <tt>Fl_Widget::hide()</tt> and
<tt>Fl_Widget::show()</tt>. These do not send the <tt>FL_HIDE</tt> and
<tt>FL_SHOW</tt> events to the widget.
<H3><a name="test_shortcut">int Fl_Widget::test_shortcut() const<br>
static int Fl_Widget::test_shortcut(const char *s)</a></H3>
The first version tests <tt>Fl_Widget::label()</tt> against the current
event (which should be a <tt>FL_SHORTCUT</tt> event). If the label
contains a '&' character and the character after it matches the key
press, this returns true. This returns false if the
<tt>SHORTCUT_LABEL</tt> flag is off, if the label is <tt>NULL</tt> or
does not have a '&' character in it, or if the keypress does not match
the character.
<p>The second version lets you do this test against an arbitrary string.
<H3><a name="type">uchar Fl_Widget::type() const<br>
void Fl_Widget::type(uchar t)</a></H3>
The property <tt>Fl_Widget::type()</tt> 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.
<p>FLTK does not use RTTI (Run Time Typing Infomation), to enhance
portability. But this may change in the near future if RTTI becomes
standard everywhere.
<p>If you don't have RTTI you can use the clumsy FLTK mechanisim, by
having <tt>type()</tt> have a unique value. These unique values must
be greater than the symbol <tt>FL_RESERVED_TYPE</tt> (which is 100).
Look through the header files for <tt>FL_RESERVED_TYPE</tt> to find an
unused number. If you make a subclass of <tt>Fl_Group</tt> you must
use <tt>FL_GROUP + n</tt>, and if you make a subclass of
<tt>Fl_Window<tt> you must use <tt>FL_WINDOW + n</tt> (in both cases
<tt>n is in the range 1 to 7).
<H2>Handling Events</H2>
The virtual method <tt>int Fl_Widget::handle(int event)</tt> is called
to handle each event passed to the widget. It can:
<ul>
<li>Change the state of the widget.
<li>Call <a href="#Fl_Widget.redraw"><tt>Fl_Widget::redraw()</tt></a>
if the widget needs to be redisplayed.
<li>Call <a
href="#Fl_Widget.damage"><tt>Fl_Widget::damage(n)</tt></a> if
the widget needs a partial-update (assumming you provide
support for this in your <tt>Fl_Widget::draw()</tt> method).
<li>Call <a
href="#Fl_Widget.do_callback"><tt>Fl_Widget::do_callback()</tt></a>
if a callback should be generated.
<li>Call <tt>Fl_Widget::handle()</tt> on child widgets.
</ul>
Events are identified by the integer argument. Other information about
the most recent event is stored in static locations and aquired by
calling the <a href="#events"><tt>Fl::event_*()</tt></a> functions.
This information remains valid until another event is handled.
<p>Here is a sample <tt>handle()</tt> method for a widget that acts as a
pushbutton and also accepts the keystroke 'x' to cause the callback:
<ul><pre>
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 0;
}
}
</pre></ul>
You must return non-zero if your <tt>handle()</tt> method uses the
event. If you return zero it indicates to the parent widget that it can
try sending the event to another widget.
<H2>Drawing the Widget</H2>
The <tt>draw()</tt> virtual method is called when FLTK wants you to
redraw your widget. It will be called if and only if <tt>damage()</tt>
is non-zero, and <tt>damage()</tt> will be cleared to zero after it
returns. <tt>draw()</tt> should be declared protected, so that it can't
be called from non-drawing code.
<p><tt>damage()</tt> contains the bitwise-OR of all the <tt>damage(n)</tt> 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
<i>all</i> the bits on if it thinks the entire widget must be redrawn
(for instance due to an expose event).
<p>Expose events (and the above <tt>damage(b,x,y,w,h)</tt>) will cause
<tt>draw()</tt> to be called with FLTK's <a
href="#clipping">clipping</a> turned on. You can greatly speed up
redrawing in some cases by testing <tt>fl_clipped</tt> and
<tt>fl_current_clip</tt> and skipping invisible parts.
<p>Besides the protected methods described above, FLTK provide a large
number of basic drawing functions, which are described <a
href=#drawing>below</a>.
<H2>Resizing the Widget</H2>
The <tt>resize(int x, int y, int w, int h)</tt> method is called when
the widget is being resized or moved. The arguments are the new
position, width, and height. <tt>x()</tt>, <tt>y()</tt>, <tt>w()</tt>,
and <tt>h()</tt> still remain the old size. You must call
<tt>resize()</tt> on your base class with the same arguments to get the
widget size to actually change.
<p>This should <i>not</i> call <tt>redraw()</tt>, at least if only the
<tt>x()</tt> and <tt>y()</tt> change. This is because group objects
like <a href="#Fl_Scroll"><tt>Fl_Scroll</tt></a> may have a more
efficient way of drawing the new position.
<H2>Making a Composite/Group Widget</H2>
A "composite" widget contains one or more "child" widgets. To do this
you should subclass <a href="#Fl_Group"><tt>Fl_Group</tt></a>. It is
possible to make a composite object that is not a subclass of
<tt>Fl_Group</tt>, but you'll have to duplicate the code in <tt>Fl_Group</tt>
anyways.
<p>Instances of the child widgets may be included in the parent:
<ul><pre>
class MyClass : public Fl_Group {
Fl_Button the_button;
Fl_Slider the_slider;
...
};
</pre></ul>
The constructor has to initialize these instances. They are
automatically <tt>add()</tt>ed to the group, since the
<tt>Fl_Group</tt> constructor does <tt>begin()</tt>. <i>Don't forget
to call <tt>end()</tt> or use the <a href="#Fl_End"><tt>Fl_End</tt></a>
pseudo-class:</i>
<ul><pre>
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!
}
</pre></ul>
The child widgets need callbacks. These will be called with a pointer
to the children, but the widget itself may be found in the
<tt>parent()</tt> pointer of the child. Usually these callbacks can be
static private methods, with a matching private method:
<ul><pre>
void MyClass::slider_cb(Fl_Widget* v, void *) { // static method
((MyClass*)(v->parent())->slider_cb();
}
void MyClass::slider_cb() { // normal method
use(the_slider->value());
}
</pre></ul>
If you make the <tt>handle()</tt> method, you can quickly pass all the
events to the children using the <tt>Fl_Group::handle()</tt> method.
Note that you don't need to override <tt>handle()</tt> if your
composite widget does nothing other than pass events to the children:
<ul><pre>
int MyClass::handle(int event) {
if (Fl_Group::handle(event)) return 1;
... handle events that children don't want ...
}
</pre></ul>
If you override <tt>draw()</tt> you need to draw all the children. If
<tt>redraw()</tt> or <tt>damage()</tt> is called on a child,
<tt>damage(FL_DAMAGE_CHILD)</tt> is done to the group, so this bit of
<tt>damage()</tt> can be used to indicate that a child needs to be
drawn. It is fastest if you avoid drawing anything else in this case:
<ul><pre>
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 want to do this
}
}
}
</pre></ul>
<tt>Fl_Group</tt> provides some protected methods to make drawing easier:
<ul>
<li><a href="#draw_child">draw_child</a>
<li><a href="#draw_outside_label">draw_outside_label</a>
<li><a href="#update_child">update_child</a>
</ul>
<H3><a name="draw_child">void Fl_Group::draw_child(Fl_Widget&)</a></H3>
This will force the child's <tt>damage()</tt> bits all to one and call
<tt>draw()</tt> on it, then clear the <tt>damage()</tt>. 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
<tt>visible()</tt> or if it is clipped.
<H3><a name="draw_outside_label">void Fl_Group::draw_outside_label(Fl_Widget&) const</a></H3>
Draw the labels that are <i>not</i> drawn by
<a href="#draw_label"><tt>draw_label()</tt></a>. If you want more control
over the label positions you might want to call
<tt>child->draw_label(x,y,w,h,a)</tt>.
<H3><a name="update_child">void Fl_Group::update_child(Fl_Widget&)</a></H3>
Draws the child only if it's <tt>damage()</tt> is non-zero. You should
call this on all the children if your own damage is equal to
FL_DAMAGE_CHILD. Nothing is done if the child is not
<tt>visible()</tt> or if it is clipped.
<H2>Cut and Paste Support</H2>
FLTK provides routines to cut and paste ASCII text (in the future this
may be UTF-8) between applications:
<ul>
<li><a href="#paste">Fl::paste</a>
<li><a href="#selection">Fl::selection</a>
<li><a href="#selection_length">Fl::selection_length</a>
<li><a href="#selection_owner">Fl::selection_owner</a>
</ul>
It may be possible to cut/paste non-ASCII data by using <a
href="#add_handler"><tt>Fl::add_handler()</tt></a>.
<H2>Making a subclass of Fl_Window</H2>
You may want your widget to be a subclass of <tt>Fl_Window</tt>. 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.
<p>Subclassing <tt>Fl_Window</tt> is almost exactly like subclassing
<tt>Fl_Widget</tt>, in fact you can easily switch a subclass back and
forth. Watch out for the following differences:
<ol>
<li><tt>Fl_Window</tt> is a subclass of <tt>Fl_Group</tt> so
<i>make sure your constructor calls <tt>end()</tt></i> (unless
you actually want children added to your window).
<li>When handling events and drawing, the upper-left corner is
at 0,0, not <tt>x(),y()</tt> as in other <tt>Fl_Widget</tt>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>.
</ol>
You may also want to subclass <tt>Fl_Window</tt> in order to get access to
different visuals or to change other attributes of the windows. See
<a href="#osissues">Appendix F - Operating System Issues</a> for more
information.
</BODY>
</HTML>
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