The GTK+ Drawing Model

## The GTK+ Drawing Model The GTK+ Drawing Model — The GTK+ drawing model in detail |

## Overview of the drawing model

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.

### Emission of the draw event

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.

### Window and no-window widgets

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.

  Figure 1. Windowed label vs. no-window label

- 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.

### Hierarchical drawing

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:

1. 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.

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.

3. 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.

4. The frame label has no children, so it just draws its text: "Frame Label".

5. The internal label has no children, so it just draws its text: "This is some text inside the frame!".

6. 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.

7. 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.

8. This is a text label which has no children, so it just draws its own text: "Cancel".

9. 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.

10. Similar to number 8, this is a text label which has no children, so it just draws its own text: "OK".

Figure 2. Hierarchical drawing order

## Double buffering

When the GTK layer receives an exposure event from GDK, it first finds the `!`GTK_NO_WINDOW`` 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_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.

### Automatic double buffering

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

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    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.

## App-paintable widgets

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:

1. Your own draw handler gets run. It paints something on the window or the event box.

2. 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.

3. 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.