Overview of the drawing model [src]

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.

Windows and events

Applications that use a windowing system generally create rectangular regions in the screen called surfaces (GTK is following the Wayland terminology, other windowing systems such as X11 may call these windows). Traditional windowing systems do not automatically save the graphical content of surfaces, and instead ask applications to provide new content whenever it is needed. For example, if a window that is stacked below other windows gets raised to the top, then the application has to repaint it, so the previously obscured area can be shown. When the windowing system asks an application to redraw a window, it sends a frame event (expose event in X11 terminology) for that window.

Each GTK toplevel window or dialog is associated with a windowing system surface. Child widgets such as buttons or entries don’t have their own surface; they use the surface of their toplevel.

Generally, the drawing cycle begins when GTK receives a frame event from the underlying windowing system: if the user drags a window over another one, the windowing system will tell the underlying surface 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 an entry widget, the entry asks GTK to queue a redraw operation for itself.

The windowing system generates frame events for surfaces. The GDK interface to the windowing system translates such events into emissions of the ::render signal on the affected surfaces. The GTK toplevel window connects to that signal, and reacts appropriately.

The following sections describe how GTK decides which widgets need to be repainted in response to such events, and how widgets work internally in terms of the resources they use from the windowing system.

The frame clock

All GTK applications are mainloop-driven, which means that most of the time the app is idle inside a loop that just waits for something to happen and then calls out to the right place when it does. On top of this GTK has a frame clock that gives a “pulse” to the application. This clock beats at a steady rate, which is tied to the framerate of the output (this is synced to the monitor via the window manager/compositor). A typical refresh rate is 60 frames per second, so a new “pulse” happens roughly every 16 milliseconds.

The clock has several phases:

  • Events
  • Update
  • Layout
  • Paint

The phases happens in this order and we will always run each phase through before going back to the start.

The Events phase is a stretch of time between each redraw where GTK processes input events from the user and other events (like e.g. network I/O). Some events, like mouse motion are compressed so that only a single mouse motion event per clock cycle needs to be handled.

Once the Events phase is over, external events are paused and the redraw loop is run. First is the Update phase, where all animations are run to calculate the new state based on the estimated time the next frame will be visible (available via the frame clock). This often involves geometry changes which drive the next phase, Layout. If there are any changes in widget size requirements the new layout is calculated for the widget hierarchy (i.e. sizes and positions for all widgets are determined). Then comes the Paint phase, where we redraw the regions of the window that need redrawing.

If nothing requires the Update/Layout/Paint phases we will stay in the Events phase forever, as we don’t want to redraw if nothing changes. Each phase can request further processing in the following phases (e.g. the Update phase will cause there to be layout work, and layout changes cause repaints).

There are multiple ways to drive the clock, at the lowest level you can request a particular phase with gdk_frame_clock_request_phase() which will schedule a clock beat as needed so that it eventually reaches the requested phase. However, in practice most things happen at higher levels:

  • If you are doing an animation, you can use gtk_widget_add_tick_callback() which will cause a regular beating of the clock with a callback in the Update phase until you stop the tick.
  • If some state changes that causes the size of your widget to change you call gtk_widget_queue_resize() which will request a Layout phase and mark your widget as needing relayout.
  • If some state changes so you need to redraw some area of your widget you use the normal gtk_widget_queue_draw() set of functions. These will request a Paint phase and mark the region as needing redraw.

There are also a lot of implicit triggers of these from the CSS layer (which does animations, resizes and repaints as needed).

The scene graph

The first step in “drawing” a window is that GTK creates render nodes for all the widgets in the window. The render nodes are combined into a tree that you can think of as a scene graph describing your window contents.

Render nodes belong to the GSK layer, and there are various kinds of them, for the various kinds of drawing primitives you are likely to need when translating widget content and CSS styling. Typical examples are text nodes, gradient nodes, texture nodes or clip nodes.

In the past, all drawing in GTK happened via cairo. It is still possible to use cairo for drawing your custom widget contents, by using a cairo render node.

A GSK renderer takes these render nodes, transforms them into rendering commands for the drawing API it targets, and arranges for the resulting drawing to be associated with the right surface. GSK has renderers for OpenGL, Vulkan and cairo.

Hierarchical drawing

During the Paint phase GTK receives a single ::render signal on the toplevel surface. The signal handler will create a snapshot object (which is a helper for creating a scene graph) and call the GtkWidget snapshot() vfunc, which will propagate down the widget hierarchy. This lets each widget snapshot its content at the right place and time, correctly handling things like partial transparencies and overlapping widgets.

During the snapshotting of each widget, GTK automatically handles the CSS rendering according to the CSS box model. It snapshots first the background, then the border, then the widget content itself, and finally the outline.

To avoid excessive work when generating scene graphs, GTK caches render nodes. Each widget keeps a reference to its render node (which in turn, will refer to the render nodes of children, and grandchildren, and so on), and will reuse that node during the Paint phase. Invalidating a widget (by calling gtk_widget_queue_draw()) discards the cached render node, forcing the widget to regenerate it the next time it needs to produce a snapshot.