A control widget typically interacts with the user to receive and/or display a value of some sort.
A container widget holds a list of child widgets and handles moving, sizing, showing, or hiding them as needed. Fl_Group is the main container widget class in FLTK, and all of the other containers (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.
MyClass(int x, int y, int w, int h, const char *label = 0);This will allow the class to be used in Fluid without problems.
The constructor must call the constructor for the base class and pass the same arguments:
MyClass::MyClass(int x, int y, int w, int h, const char *label)
: Fl_Widget(x, y, w, h, label) {
// do initialization stuff...
}
Fl_Widget's protected constructor sets x(),
y(), w(), h(), and label() to the
passed values and initializes the other instance variables to:
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);
The second form indicates that a region is damaged. If only these calls are done in a window (no calls to 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 damage() unless this is a Fl_Window widget.
The third form returns the bitwise-OR of all damage(n) calls done since the last draw(). The public method redraw() does damage(FL_DAMAGE_ALL), but the implementation of your widget can call the private damage(n).
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.
The third form draws the label anywhere. It acts as though FL_ALIGN_INSIDE has been forced on, the label will appear inside the passed bounding box. This is designed for parent groups to draw labels with.
The second version lets you do this test against an arbitrary string.
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.
If you don't have RTTI you can use the clumsy FLTK mechanisim, by
having type() have a unique value. These unique values must
be greater than the symbol FL_RESERVED_TYPE (which is 100).
Look through the header files for FL_RESERVED_TYPE to find an
unused number. If you make a subclass of Fl_Group you must
use FL_GROUP + n, and if you make a subclass of
Fl_Window you must use FL_WINDOW + n (in both cases
n is in the range 1 to 7).
Handling Events
The virtual method int Fl_Widget::handle(int event) is called
to handle each event passed to the widget. It can:
Events are identified by the integer argument. Other information about
the most recent event is stored in static locations and aquired by
calling the Fl::event_*() functions.
This information remains valid until another event is handled.
Here is a sample handle() method for a widget that acts as a pushbutton and also accepts the keystroke 'x' to cause the callback:
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;
}
}
You must return non-zero if your handle() method uses the
event. If you return zero it indicates to the parent widget that it can
try sending the event to another widget.
damage() contains the bitwise-OR of all the 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 all the bits on if it thinks the entire widget must be redrawn (for instance due to an expose event).
Expose events (and the above damage(b,x,y,w,h)) will cause draw() to be called with FLTK's clipping turned on. You can greatly speed up redrawing in some cases by testing fl_clipped and fl_current_clip and skipping invisible parts.
Besides the protected methods described above, FLTK provide a large number of basic drawing functions, which are described below.
This should not call redraw(), at least if only the x() and y() change. This is because group objects like Fl_Scroll may have a more efficient way of drawing the new position.
Instances of the child widgets may be included in the parent:
class MyClass : public Fl_Group {
Fl_Button the_button;
Fl_Slider the_slider;
...
};
The constructor has to initialize these instances. They are
automatically add()ed to the group, since the
Fl_Group constructor does begin(). Don't forget
to call end() or use the Fl_End
pseudo-class:
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!
}
The child widgets need callbacks. These will be called with a pointer
to the children, but the widget itself may be found in the
parent() pointer of the child. Usually these callbacks can be
static private methods, with a matching private method:
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());
}
If you make the handle() method, you can quickly pass all the
events to the children using the Fl_Group::handle() method.
Note that you don't need to override handle() if your
composite widget does nothing other than pass events to the children:
int MyClass::handle(int event) {
if (Fl_Group::handle(event)) return 1;
... handle events that children don't want ...
}
If you override draw() you need to draw all the children. If
redraw() or damage() is called on a child,
damage(FL_DAMAGE_CHILD) is done to the group, so this bit of
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:
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
}
}
}
Fl_Group provides some protected methods to make drawing easier:
Subclassing Fl_Window is almost exactly like subclassing Fl_Widget, in fact you can easily switch a subclass back and forth. Watch out for the following differences: