[~andy/gtk] / docs / reference / gtk / drawing-model.xml

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<refentry id="chap-drawing-model">
<refmeta>
<refentrytitle>The GTK+ Drawing Model</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo>GTK Library</refmiscinfo>
</refmeta>

<refnamediv>
<refname>The GTK+ Drawing Model</refname>
<refpurpose>
    The GTK+ drawing model in detail
</refpurpose>
</refnamediv>


  <refsect1 id="drawing-overview">
    <title>Overview of the drawing model</title>

    <para>
      This chapter describes the GTK+ drawing model in detail.  If you
      are interested in the procedure which GTK+ follows to draw its
      widgets and windows, you should read this chapter; this will be
      useful to know if you decide to implement your own widgets.  This
      chapter will also clarify the reasons behind the ways certain
      things are done in GTK+; for example, why you cannot change the
      background color of all widgets with the same method.
    </para>

    <para>
      Programs that run in a windowing system generally create
      rectangular regions in the screen called
      <firstterm>windows</firstterm>.  Traditional windowing systems
      do not automatically save the graphical content of windows, and
      instead ask client programs to repaint those windows whenever it
      is needed.  For example, if a window that is stacked below other
      windows gets raised to the top, then a client program has to
      repaint the area that was previously obscured.  When the
      windowing system asks a client program to redraw part of a
      window, it sends an <firstterm>exposure event</firstterm> to the
      program for that window.
    </para>

    <para>
      Here, "windows" means "rectangular regions with automatic
      clipping", instead of "toplevel application windows".  Most
      windowing systems support nested windows, where the contents of
      child windows get clipped by the boundaries of their parents.
      Although GTK+ and GDK in particular may run on a windowing
      system with no such notion of nested windows, GDK presents the
      illusion of being under such a system.  A toplevel window may
      contain many subwindows and sub-subwindows, for example, one for
      the menu bar, one for the document area, one for each scrollbar,
      and one for the status bar.  In addition, controls that receive
      user input, such as clickable buttons, are likely to have their
      own subwindows as well.
    </para>

    <para>
      Generally, the drawing cycle begins when GTK+ receives an
      exposure event from the underlying windowing system:  if the
      user drags a window over another one, the windowing system will
      tell the underlying window that it needs to repaint itself.  The
      drawing cycle can also be initiated when a widget itself decides
      that it needs to update its display.  For example, when the user
      types a character in a <link
      linkend="GtkEntry"><classname>GtkEntry</classname></link>
      widget, the entry asks GTK+ to queue a redraw operation for
      itself.
    </para>

    <para>
      The following sections describe how GTK+ decides which widgets
      need to be repainted, and how widgets work internally in terms
      of the resources they use from the windowing system.
    </para>

    <refsect2 id="window-no-window-widgets">
      <title>Window and no-window widgets</title>

      <para>
        A <link linkend="GdkWindow"><classname>GdkWindow</classname></link>
        represents a window from the underlying windowing system on which GTK+
        is running.  For example, on X11 it corresponds to a
        <type>Window</type>; on Win32, it corresponds to a <type>HANDLE</type>.
        The windowing system generates events for these windows.  The GDK
        interface to the windowing system translates such native events into
        <link linkend="GdkEvent"><structname>GdkEvent</structname></link>
        structures and sends them on to the GTK layer.  In turn, the GTK layer
        finds the widget that corresponds to a particular
        <classname>GdkWindow</classname> and emits the corresponding event
        signals on that widget.
      </para>

      <para>
        When the program needs to redraw a region of a
        <classname>GdkWindow</classname>, GDK generates an event of
        type <link
        linkend="GDK_EVENT_EXPOSE"><constant>GDK_EVENT_EXPOSE</constant></link>
        for that window.  The GTK+ widget layer in turn finds the
        widget that corresponds to that window, and emits the <link
        linkend="GtkWidget-expose-event">expose-event signal</link>
        for that widget.
      </para>

      <para>
        In principle, each widget could have a
        <classname>GdkWindow</classname> of its own.  With such a
        scheme, the drawing cycle would be trivial:  when GDK notifies
        the GTK layer about an exposure event for a
        <classname>GdkWindow</classname>, the GTK layer would simply
        emit the <link linkend="GtkWidget-expose-event">expose-event
        signal</link> for that widget.  The widget's expose event
        handler would subsequently repaint the widget.  No further
        work would be necessary; the windowing system would generate
        exposure events for each window that needs it, and then each
        corresponding widget would draw itself in turn.
      </para>

      <para>
        However, in practice it is convenient to have widgets which do
        not have a <classname>GdkWindow</classname> of their own, but
        rather share the one from their parent widget.  Such widgets
        have the <constant>GTK_NO_WINDOW</constant> <link
        linkend="GtkWidgetFlags">widget flag</link> turned on; this
        can be tested easily with the <link
        linkend="GTK-WIDGET-NO-WINDOW-CAPS"><function>GTK_WIDGET_NO_WINDOW()</function></link>
        macro.  As such, these are called <firstterm>no-window
        widgets</firstterm>.
      </para>

      <para>
        No-window widgets are useful for various reasons:
      </para>

      <itemizedlist>
        <listitem>
          <para>
            Some widgets may want the parent's background to show through, even
            when they draw on parts of it.  For example, consider a theme that
            uses textured backgrounds, such as gradients or repeating
            patterns.  If each widget had its own window, and in turn its own
            gradient background, labels would look bad because there would be a
            visible break with respect to their surroundings.  <xref
              linkend="figure-windowed-label"/> shows this undesirable effect.
          </para>

          <figure id="figure-windowed-label">
            <title>Windowed label vs. no-window label</title>

            <graphic fileref="figure-windowed-label.png" format="png"/>
          </figure>
        </listitem>

        <listitem>
          <para>
            Reducing the number of windows creates less traffic between GTK+ and
            the underlying windowing system, especially when getting events.
          </para>
        </listitem>
      </itemizedlist>

      <para>
        On the other hand, widgets that would benefit from having a "hard"
        clipping region may find it more convenient to create their own
        windows.  Also, widgets which want to receive events resulting from
        user interaction may find it convenient to use windows of their own as
        well.  Widgets may have more than one window if they want to
        define different regions for capturing events.
      </para>
    </refsect2>

    <refsect2 id="hierarchical-drawing">
      <title>Hierarchical drawing</title>

      <para>
        When the GTK layer receives an exposure event from GDK, it
        finds the widget that corresponds to the window which received
        the event.  By definition, this corresponds to a widget that
        has the <constant>GTK_NO_WINDOW</constant> flag turned
        <emphasis>off</emphasis> (otherwise, the widget wouldn't own
        the window!).  First this widget paints its background, and
        then, if it is a container widget, it tells each of its
        <constant>GTK_NO_WINDOW</constant> children to paint
        themselves.  This process is applied recursively for all the
        <constant>GTK_NO_WINDOW</constant> descendants of the original
        widget.
      </para>

      <para>
        Note that this process does not get propagated to widgets
        which have windows of their own, that is, to widgets which
        have the <constant>GTK_NO_WINDOW</constant> flag turned off.
        If such widgets require redrawing, then the windowing system
        will already have sent exposure events to their corresponding
        windows.  As such, there is no need to
        <firstterm>propagate</firstterm> the exposure to them on the
        GTK+ side.
      </para>

      <para>
        <xref
        linkend="figure-hierarchical-drawing"/> shows how a simple toplevel window would
        paint itself when it contains only <constant>GTK_NO_WINDOW</constant> descendants:

        <orderedlist>
          <listitem>
            <para>
              The outermost, thick rectangle is a toplevel <link
                linkend="GtkWindow"><classname>GtkWindow</classname></link>,
              which is not a <constant>GTK_NO_WINDOW</constant> widget &mdash;
              as such, it does receive its exposure event as it comes from GDK.
              First the <classname>GtkWindow</classname> would paint its own
              background.  Then, it would ask its only child to paint itself,
              numbered 2.
            </para>
          </listitem>
          <listitem>
            <para>
              The dotted rectangle represents a <link
                linkend="GtkVBox"><classname>GtkVBox</classname></link>, which
              has been made the sole child of the
              <classname>GtkWindow</classname>.  Boxes are just layout
              containers that do not paint anything by themselves, so this
              <classname>GtkVBox</classname> would draw nothing, but rather ask
              its children to draw themselves.  The children are numbered 3 and
              6.
            </para>
          </listitem>
          <listitem>
            <para>
              The thin rectangle is a <link
                linkend="GtkFrame"><classname>GtkFrame</classname></link>,
              which has two children:  a label for the frame, numbered 4, and
              another label inside, numbered 5.  First the frame would draw its
              own beveled box, then ask the frame label and its internal child to
              draw themselves.
            </para>
          </listitem>
          <listitem>
            <para>
              The frame label has no children, so it just draws its text:  "Frame&nbsp;Label".
            </para>
          </listitem>
          <listitem>
            <para>
              The internal label has no children, so it just draws its text:  "This
              is some text inside the frame!".
            </para>
          </listitem>
          <listitem>
            <para>
              The dotted rectangle represents a <link
                linkend="GtkHBox"><classname>GtkHBox</classname></link>.  Again,
              this does not draw anything by itself, but rather asks its children
              to draw themselves.  The children are numbered 7 and 9.
            </para>
          </listitem>
          <listitem>
            <para>
              The thin rectangle is a <link
                linkend="GtkButton"><classname>GtkButton</classname></link> with
              a single child, numbered 8.  First the button would draw its
              beveled box, and then it would ask its child to draw itself.
            </para>
          </listitem>
          <listitem>
            <para>
              This is a text label which has no children, so it just draws its
              own text:  "Cancel".
            </para>
          </listitem>
          <listitem>
            <para>
              Similar to number 7, this is a button with a single child, numbered
              10.  First the button would draw its beveled box, and then it would
              ask its child to draw itself.
            </para>
          </listitem>
          <listitem>
            <para>
              Similar to number 8, this is a text label which has no children,
              so it just draws its own text:  "OK".
            </para>
          </listitem>
        </orderedlist>
      </para>

      <figure id="figure-hierarchical-drawing">
        <title>Hierarchical drawing order</title>

        <graphic fileref="figure-hierarchical-drawing.png" format="png"/>
      </figure>

      <para>
        To avoid the flickering that would result from each widget drawing
        itself in turn, GTK+ uses a double-buffering mechanism.  The following
        sections describe this mechanism in detail.
      </para>
    </refsect2>

    <refsect2 id="notes-on-drawing-no-window-widgets">
      <title>Notes on drawing no-window widgets</title>

      <para>
        Remember that the coordinates in a <link
        linkend="GdkEventExpose">GdkEventExpose</link> are relative to
        the <classname>GdkWindow</classname> that received the event,
        <emphasis>not</emphasis> to the widget whose expose-event
        handler is being called.  If your widget owns the window, then
        these coordinates are probably what you expect.  However, if
        you have a <constant>GTK_NO_WINDOW</constant> widget that
        shares its parent's window, then the event's coordinates will
        be offset by your widget's allocation:  remember that the
        allocation is always relative to the parent
        <emphasis>window</emphasis> of the widget, not to the parent
        <emphasis>widget</emphasis> itself.
      </para>

      <para>
        For example, if you have a no-window widget whose allocation
        is {&nbsp;x=5,&nbsp;y=6,
        <replaceable>width</replaceable>,&nbsp;<replaceable>height</replaceable>&nbsp;},
        then your drawing origin should be at (5,&nbsp;6), not at
        (0,&nbsp;0).
      </para>
    </refsect2>

    <refsect2 id="include-inferiors">
      <title>Drawing over child windows</title>

      <para>
        When you draw on a <classname>GdkWindow</classname>, your
        drawing gets clipped by any child windows that it may
        intersect.  Sometimes you need to draw over your child windows
        as well; for example, when drawing a drag-handle to resize
        something.  In this case, turn on the <link
        linkend="GDK-INCLUDE-INFERIORS:CAPS">GDK_INCLUDE_INFERIORS</link>
        subwindow mode for the <link
        linkend="gdk-Graphics-Contexts">GdkGC</link> which you use for
        drawing.
      </para>
    </refsect2>
  </refsect1>

  <refsect1 id="double-buffering">
    <title>Double buffering</title>

    <para>
      When the GTK layer receives an exposure event from GDK, it first finds
      the <literal>!<constant>GTK_NO_WINDOW</constant></literal> widget that
      corresponds to the event's window.  Then, it emits the <link
        linkend="GtkWidget-expose-event">expose-event signal</link> for that
      widget.  As described above, that widget will first draw its background,
      and then ask each of its <constant>GTK_NO_WINDOW</constant> children to
      draw themselves.
    </para>

    <para>
      If each of the drawing calls made by each subwidget's
      <literal>expose-event</literal> handler were sent directly to the
      windowing system, flicker could result.  This is because areas may get
      redrawn repeatedly:  the background, then decorative frames, then text
      labels, etc.  To avoid flicker, GTK+ employs a <firstterm>double
        buffering</firstterm> system at the GDK level.  Widgets normally don't
      know that they are drawing to an off-screen buffer; they just issue their
      normal drawing commands, and the buffer gets sent to the windowing system
      when all drawing operations are done.
    </para>

    <!-- FIXME: figure with a timeline of non-double-buffered and
         double-buffered paints:

         onscreen:
         [garbage]
         [background]
         [button-frame]
         [icon]
         [label]


         onscreen:             offscreen:
         [garbage]
                               [background]
                               [button-frame]
                               [icon]
                               [label]
         [final result]
    -->

    <para>
      Two basic functions in GDK form the core of the double-buffering
      mechanism:  <link
      linkend="gdk_window_begin_paint_region"><function>gdk_window_begin_paint_region()</function></link>
      and <link
      linkend="gdk_window_end_paint"><function>gdk_window_end_paint()</function></link>.
      The first function tells a <classname>GdkWindow</classname> to
      create a temporary off-screen buffer for drawing.  All
      subsequent drawing operations to this window get automatically
      redirected to that buffer.  The second function actually paints
      the buffer onto the on-screen window, and frees the buffer.
    </para>

    <refsect2 id="automatic-double-buffering">
      <title>Automatic double buffering</title>

      <para>
        It would be inconvenient for all widgets to call
        <function>gdk_window_begin_paint_region()</function> and
        <function>gdk_window_end_paint()</function> at the beginning
        and end of their expose-event handlers.
      </para>

      <para>
        To make this easier, most GTK+ widgets have the
        <constant>GTK_DOUBLE_BUFFERED</constant> <link
        linkend="GtkWidgetFlags">widget flag</link> turned on by
        default.  When GTK+ encounters such a widget, it automatically
        calls <function>gdk_window_begin_paint_region()</function>
        before emitting the expose-event signal for the widget, and
        then it calls <function>gdk_window_end_paint()</function>
        after the signal has been emitted.  This is convenient for
        most widgets, as they do not need to worry about creating
        their own temporary drawing buffers or about calling those
        functions.
      </para>

      <para>
        However, some widgets may prefer to disable this kind of
        automatic double buffering and do things on their own.  To do
        this, turn off the <constant>GTK_DOUBLE_BUFFERED</constant>
        flag in your widget's constructor.
      </para>

      <example id="disabling-double-buffering">
        <title>Disabling automatic double buffering</title>

        <programlisting>
static void
my_widget_init (MyWidget *widget)
{
  ...

  GTK_WIDGET_UNSET_FLAGS (widget, GTK_DOUBLE_BUFFERED);

  ...
}
        </programlisting>
      </example>

      <para>
        When is it convenient to disable double buffering?  Generally,
        this is the case only if your widget gets drawn in such a way
        that the different drawing operations do not overlap each
        other.  For example, this may be the case for a simple image
        viewer:  it can just draw the image in a single operation.
        This would <emphasis>not</emphasis> be the case with a word
        processor, since it will need to draw and over-draw the page's
        background, then the background for highlighted text, and then
        the text itself.
      </para>

      <para>
        Even if you turn off the
        <constant>GTK_DOUBLE_BUFFERED</constant> flag on a widget, you
        can still call
        <function>gdk_window_begin_paint_region()</function> and
        <function>gdk_window_end_paint()</function> by hand to use
        temporary drawing buffers.
      </para>
    </refsect2>
  </refsect1>

  <refsect1 id="app-paintable-widgets">
    <title>App-paintable widgets</title>

    <para>
      Generally, applications use the pre-defined widgets in GTK+ and
      they do not draw extra things on top of them (the exception
      being <classname>GtkDrawingArea</classname>).  However,
      applications may sometimes find it convenient to draw directly
      on certain widgets like toplevel windows or event boxes.  When
      this is the case, GTK+ needs to be told not to overwrite your
      drawing afterwards, when the window gets to drawing its default
      contents.
    </para>

    <para>
      <classname>GtkWindow</classname> and
      <classname>GtkEventBox</classname> are the only two widgets
      which will draw their default contents unless you turn on the
      <constant>GTK_APP_PAINTABLE</constant> <link
      linkend="GtkWidgetFlags">widget flag</link>.  If you turn on
      this flag, then they will not draw their contents and let you do
      it instead.
    </para>

    <para>
      The expose-event handler for <classname>GtkWindow</classname> is
      implemented effectively like this:
    </para>

    <programlisting>
static gint
gtk_window_expose (GtkWidget      *widget,
                   GdkEventExpose *event)
{
  if (!GTK_WIDGET_APP_PAINTABLE (widget))
    gtk_paint_flat_box (widget->style, widget->window, GTK_STATE_NORMAL, 
                        GTK_SHADOW_NONE, event->area, widget, "base", 0, 0, -1, -1);

  if (GTK_WIDGET_CLASS (gtk_window_parent_class)->expose_event)
    return GTK_WIDGET_CLASS (gtk_window_parent_class)->expose_event (widget, event);

  return FALSE;
}
    </programlisting>

    <para>
      The expose-event handler for <classname>GtkEventBox</classname>
      is implemented in a similar fashion.
    </para>

    <para>
      Since the <link linkend="GtkWidget-expose-event">expose-event
        signal</link> runs user-connected handlers
      <emphasis>before</emphasis> the widget's default handler, what
      happens is this:
    </para>

    <orderedlist>
      <listitem>
        <para>
          Your own expose-event handler gets run.  It paints something
          on the window or the event box.
        </para>
      </listitem>

      <listitem>
        <para>
          The widget's default expose-event handler gets run.  If
          <constant>GTK_APP_PAINTABLE</constant> is turned off (this
          is the default), <emphasis>your drawing will be
          overwritten</emphasis>.  If that flag is turned on, the
          widget will not draw its default contents and preserve your
          drawing instead.
        </para>
      </listitem>

      <listitem>
        <para>
          The expose-event handler for the parent class gets run.
          Since both <classname>GtkWindow</classname> and
          <classname>GtkEventBox</classname> are descendants of
          <classname>GtkContainer</classname>, their no-window
          children will be asked to draw themselves recursively, as
          described in <xref linkend="hierarchical-drawing"/>.
        </para>
      </listitem>
    </orderedlist>

    <formalpara>
      <title>Summary of app-paintable widgets</title>

      <para>
        Turn on the <constant>GTK_APP_PAINTABLE</constant> flag if you
        intend to draw your own content directly on a
        <classname>GtkWindow</classname> and
        <classname>GtkEventBox</classname>.  You seldom need to draw
        on top of other widgets, and
        <classname>GtkDrawingArea</classname> ignores this flag, as it
        <emphasis>is</emphasis> intended to be drawn on.
      </para>
    </formalpara>
  </refsect1>
</refentry>

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