|  |  |  | GTK+ 3 Reference Manual |  | 
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| The GTK+ Drawing ModelThe GTK+ Drawing Model — The GTK+ drawing model in detail | 
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.
Programs that run in a windowing system generally create rectangular regions in the screen called windows. 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 exposure event to the program for that window.
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.
      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 GtkEntry
      widget, the entry asks GTK+ to queue a redraw operation for
      itself.
    
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.
      A GdkWindow
      represents a window from the underlying windowing system on which GTK+
      is running.  For example, on X11 it corresponds to a
      Window; on Win32, it corresponds to a HANDLE.
      The windowing system generates events for these windows.  The GDK
      interface to the windowing system translates such native events into
      GdkEvent
      structures and sends them on to the GTK layer.  In turn, the GTK layer
      finds the widget that corresponds to a particular
      GdkWindow and emits the corresponding event
      signals on that widget.
    
        When the program needs to redraw a region of a
        GdkWindow, generates an event of
        type GDK_EVENT_EXPOSE
        for that window, specifying the region to redraw in the process.
      
When generating the event, GDK also sets up double buffering to avoid the flickering that would result from each widget drawing itself in turn. the section called “Double buffering” describes the double buffering mechanism in detail.
        When the GTK+ widget layer receives the event, it finds the widget that
        corresponds to the window, and causes it to render itself using the
        widget's #GtkWidget::draw signal. For this purpose it creates a
        cairo context. It then clips the context
        to the area that needs to be drawn. This makes sure that the minimal
        amount of work is done if only a small part of the widget needs to be
        repainted. After translating the context so that its (0, 0) coordinate
        corresponds to the top left corner of the widget, it effectively calls
        the widget's gtk_widget_draw function.
      
        gtk_widget_draw takes care of drawing the widget
        to the cairo context. It first checks that the widget actually needs to
        be drawn. Widgets might for example be empty or outside of the cairo
        context's clipped area, which would make drawing them not do anything.
        Usually they will need to be drawn. In this case, the context will be
        clipped to the widget's allocated size and the
        draw signal will be emitted on 
        the widget which will finally draw the widget.
      
	In principle, each widget could have a
	GdkWindow 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
	GdkWindow, the GTK layer would simply
	emit the #GtkWidget::draw signal for that widget.  The signal
	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.
      
	However, in practice it is convenient to have widgets which do
	not have a GdkWindow of their own, but
	rather share the one from their parent widget.  Such widgets
	have called gtk_widget_set_has_window to
	disable it; this can be tested easily with the gtk_widget_get_has_window()
	function.  As such, these are called no-window
	widgets.
      
No-window widgets are useful for various reasons:
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. Figure 1, “Windowed label vs. no-window label” shows this undesirable effect.
Reducing the number of windows creates less traffic between GTK+ and the underlying windowing system, especially when getting events.
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.
	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 GTK_NO_WINDOW flag turned
	off (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
	GTK_NO_WINDOW children to paint
	themselves.  This process is applied recursively for all the
	GTK_NO_WINDOW descendants of the original
	widget.
      
	Note that this process does not get propagated to widgets
	which have windows of their own, that is, to widgets which
	have the GTK_NO_WINDOW 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
	propagate the exposure to them on the
	GTK+ side.
      
	Figure 2, “Hierarchical drawing order” shows how a simple toplevel window would
	paint itself when it contains only GTK_NO_WINDOW descendants:
	
	      The outermost, thick rectangle is a toplevel GtkWindow,
	      which is not a GTK_NO_WINDOW widget —
	      as such, it does receive its exposure event as it comes from GDK.
	      First the GtkWindow would paint its own
	      background.  Then, it would ask its only child to paint itself,
	      numbered 2.
	    
	      The dotted rectangle represents a GtkVBox, which
	      has been made the sole child of the
	      GtkWindow.  Boxes are just layout
	      containers that do not paint anything by themselves, so this
	      GtkVBox would draw nothing, but rather ask
	      its children to draw themselves.  The children are numbered 3 and
	      6.
	    
	      The thin rectangle is a GtkFrame,
	      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.
	    
The frame label has no children, so it just draws its text: "Frame Label".
The internal label has no children, so it just draws its text: "This is some text inside the frame!".
	      The dotted rectangle represents a GtkHBox.  Again,
	      this does not draw anything by itself, but rather asks its children
	      to draw themselves.  The children are numbered 7 and 9.
	    
	      The thin rectangle is a GtkButton with
	      a single child, numbered 8.  First the button would draw its
	      beveled box, and then it would ask its child to draw itself.
	    
This is a text label which has no children, so it just draws its own text: "Cancel".
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.
Similar to number 8, this is a text label which has no children, so it just draws its own text: "OK".
      When the GTK layer receives an exposure event from GDK, it first finds
      the ! widget that
      corresponds to the event's window.  Then, it emits the
      #GtkWidget::draw signal for that
      widget.  As described above, that widget will first draw its background,
      and then ask each of its GTK_NO_WINDOWGTK_NO_WINDOW children to
      draw themselves.
    
      If each of the drawing calls made by each subwidget's
      draw 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 double
	buffering 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.
    
      Two basic functions in GDK form the core of the double-buffering
      mechanism:  gdk_window_begin_paint_region()
      and gdk_window_end_paint().
      The first function tells a GdkWindow 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.
    
	It would be inconvenient for all widgets to call
	gdk_window_begin_paint_region() and
	gdk_window_end_paint() at the beginning
	and end of their draw handlers.
      
	To make this easier, most GTK+ widgets have the
	GTK_DOUBLE_BUFFERED widget flag turned on by
	default.  When GTK+ encounters such a widget, it automatically
	calls gdk_window_begin_paint_region()
	before emitting the #GtkWidget::draw signal for the widget, and
	then it calls gdk_window_end_paint()
	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.
      
	However, some widgets may prefer to disable this kind of
	automatic double buffering and do things on their own.  To do
        this, call the
        gtk_widget_set_double_buffered() function
	in your widget's constructor.
      
Example 5. Disabling automatic double buffering
| 1 2 3 4 5 6 7 8 9 | static void my_widget_init (MyWidget *widget) { ... gtk_widget_set_double_buffered (widget, FALSE); ... } | 
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 not 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.
	Even if you turn off double buffering on a widget, you
	can still call
	gdk_window_begin_paint_region() and
	gdk_window_end_paint() by hand to use
	temporary drawing buffers.
      
      Generally, applications use the pre-defined widgets in GTK+ and
      they do not draw extra things on top of them (the exception
      being GtkDrawingArea).  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.
    
      GtkWindow and
      GtkEventBox are the two widgets that allow
      turning off drawing of default contents by calling
      gtk_widget_set_app_paintable(). If you call
      this function, they will not draw their contents and let you do
      it instead.
    
Since the #GtkWidget::draw signal runs user-connected handlers before the widget's default handler, what usually happens is this:
Your own draw handler gets run. It paints something on the window or the event box.
	  The widget's default draw handler gets run.  If
          gtk_widget_set_app_paintable() has not
          been called to turn off widget drawing (this
	  is the default), your drawing will be
            overwritten.  An app paintable widget will not
          draw its default contents however and preserve your drawing
          instead.
	
	  The draw handler for the parent class gets run.
	  Since both GtkWindow and
	  GtkEventBox are descendants of
	  GtkContainer, their no-window
	  children will be asked to draw themselves recursively, as
	  described in the section called “Hierarchical drawing”.
	
Summary of app-paintable widgets. 
        Call gtk_widget_set_app_paintable() if you
	intend to draw your own content directly on a
	GtkWindow and
	GtkEventBox.  You seldom need to draw
	on top of other widgets, and
	GtkDrawingArea ignores this flag, as it
	is intended to be drawn on.