396 lines
17 KiB
HTML
396 lines
17 KiB
HTML
<HTML><BODY>
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<H1 ALIGN=RIGHT><A NAME=subclassing>7 - Adding and Extending Widgets</A></H1>
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This chapter describes how to add your own widgets or extend existing
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widgets in FLTK.
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<H2>Subclassing</H2>
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New widgets are created by <I>subclassing</I> an existing FLTK widget,
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typically <TT>Fl_Widget</TT> for controls and <TT>Fl_Group</TT> for
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composite widgets.
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<P>A control widget typically interacts with the user to receive and/or
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display a value of some sort. </P>
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<P>A composite widget widget holds a list of child widgets and handles moving,
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sizing, showing, or hiding them as needed. <TT>Fl_Group</TT> is the
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main composite widget widget class in FLTK, and all of the other composite widgets (<TT>
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Fl_Pack</TT>, <TT>Fl_Scroll</TT>, <TT>Fl_Tabs</TT>, <TT>Fl_Tile</TT>,
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and <TT>Fl_Window</TT>) are subclasses of it. </P>
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<P>You can also subclass other existing widgets to provide a different
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look or user-interface. For example, the button widgets are all
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subclasses of <TT>Fl_Button</TT> since they all interact with the user
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via a mouse button click. The only difference is the code that draws
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the face of the button. </P>
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<H2>Making a Subclass of Fl_Widget</H2>
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Your subclasses can directly descend from <TT>Fl_Widget</TT> or any
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subclass of <TT>Fl_Widget</TT>. <TT>Fl_Widget</TT> has only four
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virtual methods, and overriding some or all of these may be necessary.
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<H2>The Constructor</H2>
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The constructor should have the following arguments:
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<UL>
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<PRE>
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MyClass(int x, int y, int w, int h, const char *label = 0);
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</PRE>
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</UL>
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This will allow the class to be used in <A href=fluid.html#fluid>FLUID</A>
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without problems.
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<P>The constructor must call the constructor for the base class and
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pass the same arguments: </P>
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<UL>
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<PRE>
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MyClass::MyClass(int x, int y, int w, int h, const char *label)
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: Fl_Widget(x, y, w, h, label) {
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// do initialization stuff...
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}
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</PRE>
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</UL>
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<TT>Fl_Widget</TT>'s protected constructor sets <TT>x()</TT>, <TT>y()</TT>,
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<TT>w()</TT>, <TT>h()</TT>, and <TT>label()</TT> to the passed values
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and initializes the other instance variables to:
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<UL>
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<PRE>
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type(0);
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box(FL_NO_BOX);
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color(FL_GRAY);
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selection_color(FL_GRAY);
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labeltype(FL_NORMAL_LABEL);
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labelstyle(FL_NORMAL_STYLE);
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labelsize(FL_NORMAL_SIZE);
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labelcolor(FL_BLACK);
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align(FL_ALIGN_CENTER);
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callback(default_callback,0);
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flags(ACTIVE|VISIBLE);
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</PRE>
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</UL>
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<H2>Protected Methods of Fl_Widget</H2>
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The following methods are provided for subclasses to use:
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<UL>
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<LI><A href=#clear_visible><TT>Fl_Widget::clear_visible</TT></A></LI>
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<LI><A href=#damage><TT>Fl_Widget::damage</TT></A></LI>
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<LI><A href=#draw_box><TT>Fl_Widget::draw_box</TT></A></LI>
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<LI><A href=#draw_label><TT>Fl_Widget::draw_label</TT></A></LI>
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<LI><A href=#set_flag><TT>Fl_Widget::set_flag</TT></A></LI>
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<LI><A href=#set_visible><TT>Fl_Widget::set_visible</TT></A></LI>
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<LI><A href=#test_shortcut><TT>Fl_Widget::test_shortcut</TT></A></LI>
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<LI><A href=#type><TT>Fl_Widget::type</TT></A></LI>
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</UL>
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<H3><A name=damage>void Fl_Widget::damage(uchar mask)
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<BR> void Fl_Widget::damage(uchar mask, int x, int y, int w, int h)
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<BR> uchar Fl_Widget::damage()</A></H3>
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The first form indicates that a partial update of the object is
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needed. The bits in mask are OR'd into <TT>damage()</TT>. Your <TT>
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draw()</TT> routine can examine these bits to limit what it is
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drawing. The public method <TT>Fl_Widget::redraw()</TT> simply does
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<TT> Fl_Widget::damage(FL_DAMAGE_ALL)</TT>, but the implementation of
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your widget can call the private <TT>damage(n)</TT>.
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<P>The second form indicates that a region is damaged. If only these
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calls are done in a window (no calls to <TT>damage(n)</TT>) then FLTK
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will clip to the union of all these calls before drawing anything.
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This can greatly speed up incremental displays. The mask bits are
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OR'd into <TT>damage()</TT> unless this is a <TT>Fl_Window</TT> widget. </P>
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<P>The third form returns the bitwise-OR of all <TT>damage(n)</TT>
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calls done since the last <TT>draw()</TT>.</P>
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<P><I>When redrawing your widgets you should look at the damage bits to
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see what parts of your widget need redrawing.</I> The <tt>handle()</tt>
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method can then set individual damage bits to limit the amount of drawing
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that needs to be done:
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<UL><PRE>
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MyClass::handle(int event) {
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...
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if (change_to_part1) damage(1);
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if (change_to_part2) damage(2);
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if (change_to_part3) damage(4);
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}
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MyClass::draw() {
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if (damage() & FL_DAMAGE_ALL) {
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... draw frame/box and other static stuff ...
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}
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if (damage() & (FL_DAMAGE_ALL | 1)) draw_part1();
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if (damage() & (FL_DAMAGE_ALL | 2)) draw_part2();
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if (damage() & (FL_DAMAGE_ALL | 4)) draw_part3();
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}
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</PRE></UL>
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<H3><A name=draw_box>void Fl_Widget::draw_box() const
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<BR></A>void Fl_Widget::draw_box(Fl_Boxtype b, ulong c) const</H3>
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The first form draws this widget's <TT>box()</TT>, using the
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dimensions of the widget. The second form uses <TT>b</TT> as the box
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type and <TT>c</TT> as the color for the box.
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<H3><A name=draw_label>void Fl_Widget::draw_label() const
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<BR> void Fl_Widget::draw_label(int x, int y, int w, int h) const
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<BR> void Fl_Widget::draw_label(int x, int y, int w, int h, Fl_Align
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align) const</A></H3>
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This is the usual function for a <TT>draw()</TT> method to call to
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draw the widget's label. It does not draw the label if it is supposed
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to be outside the box (on the assumption that the enclosing group will
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draw those labels).
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<P>The second form uses the passed bounding box instead of the widget's
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bounding box. This is useful so "centered" labels are aligned with some
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feature, like a moving slider. </P>
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<P>The third form draws the label anywhere. It acts as though <TT>
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FL_ALIGN_INSIDE</TT> has been forced on so the label will appear inside
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the passed bounding box. This is designed for parent groups to draw
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labels with. </P>
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<H3><A name=set_flag>void Fl_Widget::set_flag(SHORTCUT_LABEL)</A></H3>
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Modifies <TT>draw_label()</TT> so that '&' characters cause an underscore
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to be printed under the next letter.
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<H3><A name=set_visible>void Fl_Widget::set_visible()</A>
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<BR><A name=clear_visible>void Fl_Widget::clear_visible()</A></H3>
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Fast inline versions of <TT>Fl_Widget::hide()</TT> and <TT>
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Fl_Widget::show()</TT>. These do not send the <TT>FL_HIDE</TT> and <TT>
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FL_SHOW</TT> events to the widget.
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<H3><A name=test_shortcut>int Fl_Widget::test_shortcut() const
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<BR> static int Fl_Widget::test_shortcut(const char *s)</A></H3>
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The first version tests <TT>Fl_Widget::label()</TT> against the
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current event (which should be a <TT>FL_SHORTCUT</TT> event). If the
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label contains a '&' character and the character after it matches the key
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press, this returns true. This returns false if the <TT>SHORTCUT_LABEL</TT>
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flag is off, if the label is <TT>NULL</TT> or does not have a
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'&' character in it, or if the keypress does not match the character.
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<P>The second version lets you do this test against an arbitrary
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string. </P>
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<H3><A name=type>uchar Fl_Widget::type() const
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<BR> void Fl_Widget::type(uchar t)</A></H3>
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The property <TT>Fl_Widget::type()</TT> can return an arbitrary 8-bit
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identifier, and can be set with the protected method <TT>type(uchar t)</TT>
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. This value had to be provided for Forms compatibility, but you can
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use it for any purpose you want. Try to keep the value less than 100
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to not interfere with reserved values.
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<P>FLTK does not use RTTI (Run Time Typing Infomation), to enhance
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portability. But this may change in the near future if RTTI becomes
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standard everywhere. </P>
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<P>If you don't have RTTI you can use the clumsy FLTK mechanisim, by
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having <TT>type()</TT> use a unique value. These unique values must
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be greater than the symbol <TT>FL_RESERVED_TYPE</TT> (which is 100).
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Look through the header files for <TT>FL_RESERVED_TYPE</TT> to find an
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unused number. If you make a subclass of <TT>Fl_Window</TT>
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you must use <TT>FL_WINDOW + n</TT> (<TT>n</tt> must be in the
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range 1 to 7). </P>
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<H2>Handling Events</H2>
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The virtual method <TT>int Fl_Widget::handle(int event)</TT> is called
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to handle each event passed to the widget. It can:
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<UL>
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<LI>Change the state of the widget. </LI>
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<LI>Call <A href=Fl_Widget.html#Fl_Widget.redraw><TT>Fl_Widget::redraw()</TT>
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</A> if the widget needs to be redisplayed. </LI>
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<LI>Call <A href=Fl_Widget.html#Fl_Widget.damage><TT>
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Fl_Widget::damage(n)</TT></A> if the widget needs a partial-update
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(assumming you provide support for this in your <TT>Fl_Widget::draw()</TT>
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method). </LI>
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<LI>Call <A href=Fl_Widget.html#Fl_Widget.do_callback><TT>
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Fl_Widget::do_callback()</TT></A> if a callback should be generated. </LI>
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<LI>Call <TT>Fl_Widget::handle()</TT> on child widgets. </LI>
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</UL>
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Events are identified by the integer argument. Other information
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about the most recent event is stored in static locations and aquired
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by calling the <A href=events.html#events><TT>Fl::event_*()</TT></A>
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functions. This information remains valid until another event is
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handled.
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<P>Here is a sample <TT>handle()</TT> method for a widget that acts as
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a pushbutton and also accepts the keystroke 'x' to cause the callback: </P>
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<UL>
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<PRE>
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int MyClass::handle(int event) {
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switch(event) {
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case FL_PUSH:
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highlight = 1;
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redraw();
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return 1;
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case FL_DRAG: {
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int t = Fl::event_inside(this);
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if (t != highlight) {
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highlight = t;
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redraw();
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}
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}
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return 1;
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case FL_RELEASE:
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if (highlight) {
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highlight = 0;
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redraw();
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do_callback();
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// never do anything after a callback, as the callback
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// may delete the widget!
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}
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return 1;
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case FL_SHORTCUT:
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if (Fl::event_key() == 'x') {
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do_callback();
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return 1;
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}
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return 0;
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default:
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return 0;
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}
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}
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</PRE>
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</UL>
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You must return non-zero if your <TT>handle()</TT> method uses the
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event. If you return zero it indicates to the parent widget that it can
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try sending the event to another widget.
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<H2>Drawing the Widget</H2>
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The <TT>draw()</TT> virtual method is called when FLTK wants you to
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redraw your widget. It will be called if and only if <TT>damage()</TT>
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is non-zero, and <TT>damage()</TT> will be cleared to zero after it
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returns. <TT>draw()</TT> should be declared protected, so that it can't
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be called from non-drawing code.
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<P><TT>damage()</TT> contains the bitwise-OR of all the <TT>damage(n)</TT>
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calls to this widget since it was last drawn. This can be used for
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minimal update, by only redrawing the parts whose bits are set. FLTK
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will turn on the <TT>FL_DAMAGE_ALL</TT> bit if it thinks the entire widget
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must be redrawn (e.g. for an expose event). </P>
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<P>Expose events (and the above <TT>damage(b,x,y,w,h)</TT>) will cause <TT>
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draw()</TT> to be called with FLTK's <A href=drawing.html#clipping>
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clipping</A> turned on. You can greatly speed up redrawing in some
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cases by testing <TT>fl_clipped</TT> and <TT>fl_current_clip</TT> and
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skipping invisible parts. </P>
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<P>Besides the protected methods described above, FLTK provides a large
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number of basic drawing functions, which are described <A href=drawing.html#drawing>
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below</A>. </P>
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<H2>Resizing the Widget</H2>
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The <TT>resize(int x, int y, int w, int h)</TT> method is called when
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the widget is being resized or moved. The arguments are the new
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position, width, and height. <TT>x()</TT>, <TT>y()</TT>, <TT>w()</TT>,
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and <TT>h()</TT> still remain the old size. You must call <TT>resize()</TT>
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on your base class with the same arguments to get the widget size to
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actually change.
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<P>This should <I>not</I> call <TT>redraw()</TT>, at least if only the <TT>
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x()</TT> and <TT>y()</TT> change. This is because composite widgets like <A href=Fl_Scroll.html#Fl_Scroll>
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<TT>Fl_Scroll</TT></A> may have a more efficient way of drawing the new
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position. </P>
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<H2>Making a Composite Widget</H2>
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A "composite" widget contains one or more "child" widgets.
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To make a composite widget you should subclass <A href=Fl_Group.html#Fl_Group><TT>Fl_Group</TT></A>
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. It is possible to make a composite object that is not a subclass of <TT>
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Fl_Group</TT>, but you'll have to duplicate the code in <TT>Fl_Group</TT>
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anyways.
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<P>Instances of the child widgets may be included in the parent: </P>
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<UL>
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<PRE>
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class MyClass : public Fl_Group {
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Fl_Button the_button;
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Fl_Slider the_slider;
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...
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};
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</PRE>
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</UL>
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The constructor has to initialize these instances. They are
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automatically <TT>add()</TT>ed to the group, since the <TT>Fl_Group</TT>
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constructor does <TT>begin()</TT>. <I>Don't forget to call <TT>end()</TT>
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or use the <A href=Fl_End.html#Fl_End><TT>Fl_End</TT></A> pseudo-class:</I>
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<UL>
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<PRE>
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MyClass::MyClass(int x, int y, int w, int h) :
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Fl_Group(x, y, w, h),
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the_button(x + 5, y + 5, 100, 20),
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the_slider(x, y + 50, w, 20)
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{
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...(you could add dynamically created child widgets here)...
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end(); // don't forget to do this!
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}
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</PRE>
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</UL>
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The child widgets need callbacks. These will be called with a pointer
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to the children, but the widget itself may be found in the <TT>parent()</TT>
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pointer of the child. Usually these callbacks can be static private
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methods, with a matching private method:
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<UL>
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<PRE>
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void MyClass::slider_cb(Fl_Widget* v, void *) { // static method
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((MyClass*)(v->parent())->slider_cb();
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}
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void MyClass::slider_cb() { // normal method
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use(the_slider->value());
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}
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</PRE>
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</UL>
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If you make the <TT>handle()</TT> method, you can quickly pass all the
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events to the children using the <TT>Fl_Group::handle()</TT> method.
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You don't need to override <TT>handle()</TT> if your composite widget
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does nothing other than pass events to the children:
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<UL>
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<PRE>
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int MyClass::handle(int event) {
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if (Fl_Group::handle(event)) return 1;
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... handle events that children don't want ...
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}
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</PRE>
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</UL>
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If you override <TT>draw()</TT> you need to draw all the children. If <TT>
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redraw()</TT> or <TT>damage()</TT> is called on a child, <TT>
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damage(FL_DAMAGE_CHILD)</TT> is done to the group, so this bit of <TT>
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damage()</TT> can be used to indicate that a child needs to be drawn.
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It is fastest if you avoid drawing anything else in this case:
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<UL>
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<PRE>
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int MyClass::draw() {
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Fl_Widget *const*a = array();
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if (damage() == FL_DAMAGE_CHILD) { // only redraw some children
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for (int i = children(); i --; a ++) update_child(**a);
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} else { // total redraw
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... draw background graphics ...
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// now draw all the children atop the background:
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for (int i = children_; i --; a ++) {
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draw_child(**a);
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draw_outside_label(**a); // you may not want to do this
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}
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}
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}
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</PRE>
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</UL>
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<TT>Fl_Group</TT> provides some protected methods to make drawing
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easier:
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<UL>
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<LI><A href=#draw_child>draw_child</A></LI>
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<LI><A href=#draw_outside_label>draw_outside_label</A></LI>
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<LI><A href=#update_child>update_child</A></LI>
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</UL>
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<H3><A name=draw_child>void Fl_Group::draw_child(Fl_Widget&)</A></H3>
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This will force the child's <TT>damage()</TT> bits all to one and call <TT>
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draw()</TT> on it, then clear the <TT>damage()</TT>. You should call
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this on all children if a total redraw of your widget is requested, or
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if you draw something (like a background box) that damages the child.
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Nothing is done if the child is not <TT>visible()</TT> or if it is
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clipped.
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<H3><A name=draw_outside_label>void
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Fl_Group::draw_outside_label(Fl_Widget&) const</A></H3>
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Draw the labels that are <I>not</I> drawn by <A href=#draw_label><TT>
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draw_label()</TT></A>. If you want more control over the label
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positions you might want to call <TT>child->draw_label(x,y,w,h,a)</TT>.
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<H3><A name=update_child>void Fl_Group::update_child(Fl_Widget&)</A></H3>
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Draws the child only if its <TT>damage()</TT> is non-zero. You
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should call this on all the children if your own damage is equal to
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FL_DAMAGE_CHILD. Nothing is done if the child is not <TT>visible()</TT>
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or if it is clipped.
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<H2>Cut and Paste Support</H2>
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FLTK provides routines to cut and paste ASCII text (in the future this
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may be UTF-8) between applications:
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<UL>
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<LI><A href=functions.html#paste><TT>Fl::paste</TT></A></LI>
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<LI><A href=functions.html#selection><TT>Fl::selection</TT></A></LI>
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<LI><A href=#selection_length><TT>Fl::selection_length</TT></A></LI>
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<LI><A href=functions.html#selection_owner><TT>Fl::selection_owner</TT></A></LI>
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</UL>
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It may be possible to cut/paste non-ASCII data by using <A href=osissues.html#add_handler>
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<TT>Fl::add_handler()</TT></A>.
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<H2>Making a subclass of Fl_Window</H2>
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You may want your widget to be a subclass of <TT>Fl_Window</TT>. This
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can be useful if your widget wants to occupy an entire window, and can
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also be used to take advantage of system-provided clipping, or to work
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with a library that expects a system window ID to indicate where to
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draw.
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<P>Subclassing <TT>Fl_Window</TT> is almost exactly like subclassing <TT>
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Fl_Widget</TT>, and in fact you can easily switch a subclass back and
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forth. Watch out for the following differences: </P>
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<OL>
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<LI><TT>Fl_Window</TT> is a subclass of <TT>Fl_Group</TT> so <I>make
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sure your constructor calls <TT>end()</TT></I> (unless you actually
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want children added to your window). </LI>
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<LI>When handling events and drawing, the upper-left corner is at 0,0,
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not <TT>x(),y()</TT> as in other <TT>Fl_Widget</TT>'s. For instance, to
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draw a box around the widget, call <TT>draw_box(0, 0, w(), h())</TT>,
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rather than <TT>draw_box(x(), y(), w(), h())</TT>. </LI>
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</OL>
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You may also want to subclass <TT>Fl_Window</TT> in order to get
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access to different visuals or to change other attributes of the
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windows. See <A href=osissues.html#osissues>"Appendix F - Operating
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System Issues"</A> for more information. </BODY></HTML>
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