forked from AuroraMiddleware/gtk
234 lines
9.7 KiB
XML
234 lines
9.7 KiB
XML
<?xml version="1.0"?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
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"http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [
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]>
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<refentry id="chap-drawing-model">
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<refmeta>
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<refentrytitle>The GTK Drawing Model</refentrytitle>
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<manvolnum>3</manvolnum>
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<refmiscinfo>GTK Library</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>The GTK Drawing Model</refname>
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<refpurpose>
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The GTK drawing model in detail
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</refpurpose>
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</refnamediv>
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<refsect1 id="drawing-overview">
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<title>Overview of the drawing model</title>
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<para>
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This chapter describes the GTK drawing model in detail. If you
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are interested in the procedure which GTK follows to draw its
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widgets and windows, you should read this chapter; this will be
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useful to know if you decide to implement your own widgets. This
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chapter will also clarify the reasons behind the ways certain
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things are done in GTK.
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</para>
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<refsect2 id="drawing model windows">
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<title>Windows and events</title>
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<para>
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Applications that use a windowing system generally create
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rectangular regions in the screen called <firstterm>surfaces</firstterm>
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(GTK is following the Wayland terminology, other windowing systems
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such as X11 may call these <firstterm>windows</firstterm>).
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Traditional windowing systems do not automatically save the
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graphical content of surfaces, and instead ask applications to
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provide new content whenever it is needed.
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For example, if a window that is stacked below other
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windows gets raised to the top, then the application has to
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repaint it, so the previously obscured area can be shown.
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When the windowing system asks an application to redraw
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a window, it sends a <firstterm>frame event</firstterm>
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(<firstterm>expose event</firstterm> in X11 terminology)
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for that window.
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</para>
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<para>
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Each GTK toplevel window or dialog is associated with a
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windowing system surface. Child widgets such as buttons or
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entries don't have their own surface; they use the surface
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of their toplevel.
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</para>
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<para>
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Generally, the drawing cycle begins when GTK receives
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a frame event from the underlying windowing system: if the
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user drags a window over another one, the windowing system will
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tell the underlying surface that it needs to repaint itself. The
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drawing cycle can also be initiated when a widget itself decides
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that it needs to update its display. For example, when the user
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types a character in an entry widget, the entry asks GTK to queue
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a redraw operation for itself.
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</para>
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<para>
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The windowing system generates frame events for surfaces. The GDK
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interface to the windowing system translates such events into
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emissions of the ::render signal on the affected surfaces.
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The GTK toplevel window connects to that signal, and reacts appropriately.
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</para>
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<para>
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The following sections describe how GTK decides which widgets
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need to be repainted in response to such events, and how widgets
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work internally in terms of the resources they use from the
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windowing system.
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</para>
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</refsect2>
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<refsect2 id="frameclock">
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<title>The frame clock</title>
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<para>
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All GTK applications are mainloop-driven, which means that most
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of the time the app is idle inside a loop that just waits for
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something to happen and then calls out to the right place when
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it does. On top of this GTK has a frame clock that gives a
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“pulse” to the application. This clock beats at a steady rate,
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which is tied to the framerate of the output (this is synced to
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the monitor via the window manager/compositor). A typical
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refresh rate is 60 frames per second, so a new “pulse” happens
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roughly every 16 milliseconds.
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</para>
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<para>
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The clock has several phases:
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<itemizedlist>
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<listitem><para>Events</para></listitem>
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<listitem><para>Update</para></listitem>
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<listitem><para>Layout</para></listitem>
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<listitem><para>Paint</para></listitem>
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</itemizedlist>
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The phases happens in this order and we will always run each
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phase through before going back to the start.
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</para>
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<para>
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The Events phase is a stretch of time between each redraw where
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GTK processes input events from the user and other events
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(like e.g. network I/O). Some events, like mouse motion are
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compressed so that only a single mouse motion event per clock
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cycle needs to be handled.
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</para>
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<para>
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Once the Events phase is over, external events are paused and
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the redraw loop is run. First is the Update phase, where all
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animations are run to calculate the new state based on the
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estimated time the next frame will be visible (available via
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the frame clock). This often involves geometry changes which
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drive the next phase, Layout. If there are any changes in
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widget size requirements the new layout is calculated for the
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widget hierarchy (i.e. sizes and positions for all widgets are
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determined). Then comes the Paint phase, where we redraw the
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regions of the window that need redrawing.
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</para>
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<para>
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If nothing requires the Update/Layout/Paint phases we will
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stay in the Events phase forever, as we don’t want to redraw
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if nothing changes. Each phase can request further processing
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in the following phases (e.g. the Update phase will cause there
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to be layout work, and layout changes cause repaints).
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</para>
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<para>
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There are multiple ways to drive the clock, at the lowest level
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you can request a particular phase with
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gdk_frame_clock_request_phase() which will schedule a clock beat
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as needed so that it eventually reaches the requested phase.
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However, in practice most things happen at higher levels:
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<itemizedlist>
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<listitem><para>
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If you are doing an animation, you can use
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gtk_widget_add_tick_callback() which will cause a regular
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beating of the clock with a callback in the Update phase
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until you stop the tick.
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</para></listitem>
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<listitem><para>
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If some state changes that causes the size of your widget
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to change you call gtk_widget_queue_resize() which will
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request a Layout phase and mark your widget as needing
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relayout.
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</para></listitem>
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<listitem><para>
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If some state changes so you need to redraw some area of
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your widget you use the normal gtk_widget_queue_draw()
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set of functions. These will request a Paint phase and
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mark the region as needing redraw.
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</para></listitem>
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</itemizedlist>
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There are also a lot of implicit triggers of these from the
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CSS layer (which does animations, resizes and repaints as needed).
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</para>
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</refsect2>
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<refsect2 id="scene-graph">
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<title>The scene graph</title>
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<para>
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The first step in “drawing” a window is that GTK creates
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<firstterm>render nodes</firstterm> for all the widgets
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in the window. The render nodes are combined into a tree
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that you can think of as a <firstterm>scene graph</firstterm>
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describing your window contents.
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</para>
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<para>
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Render nodes belong to the GSK layer, and there are various kinds
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of them, for the various kinds of drawing primitives you are likely
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to need when translating widget content and CSS styling. Typical
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examples are text nodes, gradient nodes, texture nodes or clip nodes.
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<para>
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<para>
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In the past, all drawing in GTK happened via cairo. It is still possible
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to use cairo for drawing your custom widget contents, by using a cairo
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render node.
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</para>
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</para>
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A GSK <firstterm>renderer</firstterm> takes these render nodes, transforms
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them into rendering commands for the drawing API it targets, and arranges
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for the resulting drawing to be associated with the right surface. GSK has
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renderers for OpenGL, Vulkan and cairo.
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</para>
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</refsect2>
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<refsect2 id="hierarchical-drawing">
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<title>Hierarchical drawing</title>
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<para>
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During the Paint phase GTK receives a single ::render signal on the toplevel
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window. The signal handler will create a snapshot object (which is a
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helper for creating a scene graph) and emit a GtkWidget::snapshot() signal,
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which will propagate down the widget hierarchy. This lets each widget
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snapshot its content at the right place and time, correctly handling things
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like partial transparencies and overlapping widgets.
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</para>
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<para>
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To avoid excessive work when generating scene graphs, GTK caches render nodes.
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Each widget keeps a reference to its render node (which in turn, will refer to
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the render nodes of children, and grandchildren, and so on), and will reuse
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that node during the Paint phase. Invalidating a widget (by calling
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gtk_widget_queue_draw()) discards the cached render node, forcing the widget
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to regenerate it the next time it needs to handle a ::snapshot.
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</para>
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</refsect2>
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</refsect1>
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</refentry>
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<!--
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Local variables:
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mode: xml
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sgml-parent-document: ("gtk-docs.sgml" "book" "part" "refentry")
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End:
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-->
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