mutter/doc/cookbook/layouts.xml

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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<chapter id="layouts"
xmlns:xi="http://www.w3.org/2003/XInclude">
<title>Layout management</title>
<epigraph>
<attribution>Abigail Adams, wife of John Adams, in a letter to John
Thaxter (1778-09-29)</attribution>
<para>If we do not lay out ourselves in the service of mankind,
whom should we serve?</para>
</epigraph>
<section id="layouts-introduction">
<title>Introduction</title>
<para>Layout management in Clutter controls how an actor and
children "inside" that actor are sized and positioned. More
specifically, layouts are managed by associating a parent with a
<type>ClutterLayoutManager</type>; the parent is usually either a
composite <type>ClutterActor</type> (composed of several
<type>ClutterActors</type>) or a <type>ClutterContainer</type>
(containing child <type>ClutterActors</type>). The
<type>ClutterLayoutManager</type> then manages:</para>
<orderedlist>
<listitem>
<para>The <emphasis>size requisition</emphasis>
(determination of the desired height and width) of the
parent.</para>
</listitem>
<listitem>
<para>The <emphasis>allocation</emphasis> (size and position)
assigned to each composed or child ClutterActor.</para>
</listitem>
</orderedlist>
<note>
<para>To make this more concrete, imagine you have a sheet of
paper and some coloured squares to place on it. Someone stands
next to you telling you how big the piece of paper should be,
how big the squares should be, and where to put each square on the
piece of paper.</para>
<para>The sheet of paper is analogous to the container or
composite actor; the squares are analogous to the child
<type>ClutterActors</type>; and the person giving you instructions
is analogous to the layout manager.</para>
</note>
<para>The following sections give an overview of how layout
management works in Clutter.</para>
<section>
<title>Using layouts</title>
<para>Although Clutter provides plenty of flexibility in how you
can use layout management, the simplest way to get started is to
use the built-in <type>ClutterBox</type> class with one of the
provided <type>ClutterLayoutManager</type> implementations.</para>
<para>The pattern for doing this is:</para>
<itemizedlist>
<listitem>
<para>Create an instance of one of the
<type>ClutterLayoutManager</type> implementations (see
<link linkend="layouts-introduction-manager-types">the
following section</link>).</para>
</listitem>
<listitem>
<para>Configure the layout manager's default policies
(e.g. how actors are aligned by default, whether to pack
actors horizontally or vertically, spacing between actors
in the layout).</para>
</listitem>
<listitem>
<para>Create a <type>ClutterBox</type>, setting its layout
manager to the one you just created.</para>
</listitem>
<listitem>
<para>Pack actors into the <type>ClutterBox</type>,
setting layout properties (if required) as each is added.</para>
</listitem>
<listitem>
<para>Modify layout properties of child actors using
<function>clutter_layout_manager_child_set()</function>
(if required).</para>
</listitem>
</itemizedlist>
<para>Individual recipes in this section give more examples of
how to make use of the different layout manager
implementations.</para>
<note>
<para>It is not possible to use a layout manager with an arbitrary
<type>ClutterContainer</type>: you must use a <type>ClutterActor</type>
subclass which can delegate its layout to a layout manager (either
use <type>ClutterBox</type> or write your own).</para>
</note>
</section>
<section id="layouts-introduction-manager-types">
<title>Types of layout manager</title>
<para>Clutter provides a range of layout managers suitable
for different use cases:</para>
<itemizedlist>
<listitem>
<para><type>ClutterFixedLayout</type> arranges actors
at fixed positions on the stage. No alignment options are
available, so you have to manually compute and manage the
coordinates (or use <type>ClutterConstraints</type>) which
will align actors how you want them.</para>
</listitem>
<listitem>
<para><type>ClutterBinLayout</type> arranges actors in a
depth-ordered stack on top of each other, aligned to the container.
This is useful for arranging actors inside composites (e.g.
creating a button widget from a <type>ClutterTexture</type>
with a <type>ClutterText</type> on top of it).</para>
</listitem>
<listitem>
<para><type>ClutterBoxLayout</type> arranges actors in a
single horizontal row or vertical column. This type of layout is
common in UI elements like toolbars and menus.</para>
</listitem>
<listitem>
<para><type>ClutterFlowLayout</type> arranges actors
in reflowing columns and rows. If the container's allocation
changes, the child actors are rearranged to fit inside its
new allocation. This can be useful for arranging actors
where you're not sure how many there might be; or where
new ones are going to be added into the UI, perhaps displacing
others. An example might be a photo viewer or an
RSS feed display.</para>
</listitem>
</itemizedlist>
</section>
<section id="layouts-introduction-layout-properties">
<title>Layout properties</title>
<para>How actors are sized and positioned inside a container
associated with a layout manager depends on two things:</para>
<orderedlist>
<listitem>
<formalpara>
<title>Properties which apply to all actors added to the layout</title>
<para>There will be one setting at the layout level which can't
be overridden per actor. This includes properties like spacing
between rows and columns, whether the layout is homogenous
(each actor gets the same allocation), etc.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title>Properties for each actor added to the layout</title>
<para>These are properties of the relationship between the
layout, the container associated with the layout, and the
children of the container. Each layout/container/actor
combination can have different settings for each of these
properties.</para>
</formalpara>
</listitem>
</orderedlist>
<para>Each layout manager implementation supports a subset of the
following layout properties; different managers may have different
names or functions for setting them, but the functionality remains
the same. Individual recipes give more details about which
properties can be set for each layout manager implementation.</para>
<itemizedlist>
<listitem>
<formalpara>
<title>Alignment</title>
<para>How an actor aligns to the container's axes, e.g.
aligned to the container's left, right, or center. For some
layouts (like <type>ClutterBinLayout</type>) alignment
is also used to set expand and fill properties.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title>Horizontal/vertical orientation</title>
<para>Whether actors are arranged in a horizontal row or
vertical column.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title>Homogenous rows and columns</title>
<para>Grid-like layouts (e.g. <type>ClutterFlowLayout</type>)
can be configured to have uniform rows and/or columns,
expanding to fit the largest actor they contain.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title>Row height and column width</title>
<para>Grid-like layouts arranged in rows and columns
can be configured with maximum and minimum row height and
column width.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title>Row and column spacing</title>
<para>Grid-like layouts enable you to define a space (in pixels)
between rows and columns.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title>Expand</title>
<para>Some layouts can be configured to minimize their size request
to fit the actors they contain (<emphasis>expand is FALSE</emphasis>);
or to increase the allocation of actors they contain so
that all available space in the layout is used
(<emphasis>expand is TRUE</emphasis>). In the latter case, you'd
also need to set a size for the container associated with
the layout, otherwise the container will just fit itself to the
actors inside it.</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title>Fill</title>
<para>This property only has an effect when
<emphasis>expand</emphasis> is on. The <emphasis>fill</emphasis>
setting controls whether actors are resized to fill their
allocation (<emphasis>fill is TRUE</emphasis>); or if the
space around the actor is increased (<emphasis>fill is
FALSE</emphasis>).</para>
</formalpara>
</listitem>
<listitem>
<formalpara>
<title>Pack at start/end</title>
<para>This controls whether actors at prepended or appended
to the layout.</para>
<itemizedlist>
<listitem>
<para>If the orientation is vertical, prepended
actors are added to the top of the layout and appended
actors to the bottom.</para>
</listitem>
<listitem>
<para>If the orientation is horizontal, prepended
actors are added at the left of the layout and appended actors
on the right.</para>
</listitem>
</itemizedlist>
</formalpara>
</listitem>
</itemizedlist>
<section>
<title>Setting layout properties</title>
<para>Layout properties can be set in one or more of the following ways
(depending on the type of property and the layout manager):</para>
<orderedlist>
<listitem>
<para>By setting a default value for the property on the
layout manager (e.g. using
<function>clutter_bin_layout_set_alignment()</function>,
<function>clutter_box_layout_set_expand()</function>). Any
actor added to the layout gets this value for the property,
unless it is overridden for that actor.</para>
</listitem>
<listitem>
<para>When adding an actor to a <type>ClutterBox</type> container
using <function>clutter_box_pack()</function>, you can set
properties on the actor which you're adding.</para>
</listitem>
<listitem>
<para>When adding an actor to a layout you can use a function
which enables setting properties simultaneously (e.g.
<function>clutter_box_layout_pack()</function>,
<function>clutter_bin_layout_add()</function>).</para>
</listitem>
<listitem>
<para>By using
<function>clutter_layout_manager_child_set()</function> on
the child of a layout.</para>
</listitem>
</orderedlist>
</section>
</section>
<section id="layouts-introduction-not-using-layout-managers">
<title>Not using layout managers</title>
<para>It is perfectly possible to arrange <type>ClutterActors</type>
without using layout managers; however, you may have to do
more of your own calculations about actor sizes and positions.</para>
<para>There are two (not mutually-exclusive) approaches you can
take to do this, described below.</para>
<section>
<title>Manual positioning and alignment</title>
<para>This basically means using the <type>ClutterActor</type>
bounding box mechanism (see the <type>ClutterActor</type>
documentation for details) to set actor sizes and positions.
This is the approach you will see in a lot of older Clutter
code (written before layout managers were available).</para>
<para>This approach is simplest where the UI is relatively static
and is composed of a few known actors. It will work in larger,
more complex scenarios, but in those sorts of cases it is better
to make use of layout managers and constraints (see below) instead.</para>
</section>
<section>
<title>Using <type>ClutterConstraint</type></title>
<para>Constraints provide mechanisms for:</para>
<itemizedlist>
<listitem>
<para>Aligning actors with each other
(<type>ClutterAlignConstraint</type>). For example, you
can align the top, bottom or center of one actor with the
top, bottom or center of another (on the <code>y</code>
axis). Similarly, you can align one actor to another
on the <code>x</code> axis.</para>
</listitem>
<listitem>
<para>Binding properties of one actor to those of
another. For example, you could ensure that two actors
always remain the same width; or you could specify
that two actors always have the same <code>x</code>
coordinate. In both these cases and others, you can
specify that the properties should be the same, or the same
+/- some offset.</para>
</listitem>
</itemizedlist>
<note>
<para><type>ClutterConstraints</type> can be used in combination
with some layout managers, but you need to be careful that
constraints don't fight with the layout manager policies.
Unpredictable results could ensue.</para>
</note>
</section>
</section>
</section>
<section id="layouts-stacking">
<title>Stacking actors on top of each other</title>
<section>
<title>Problem</title>
<para>You want to lay out several actors so that they are in
layers on top of each other (e.g. to create a button widget
composed from a rectangle with text on top of it).</para>
</section>
<section id="layouts-stacking-solution">
<title>Solution</title>
<para>The most flexible approach is to use a <type>ClutterBinLayout</type>
associated with a <type>ClutterBox</type>:</para>
<informalexample>
<programlisting>
<![CDATA[
/* define some colors */
const ClutterColor background_color = { 0xaa, 0x99, 0x00, 0xff };
const ClutterColor text_color = { 0xff, 0xff, 0xff, 0xff };
ClutterLayoutManager *layout;
ClutterActor *box;
ClutterActor *background;
ClutterActor *text;
/*
* create a layout, setting the default x and y alignment;
* actors fill the whole allocation of the layout's container
* by default
*/
layout = clutter_bin_layout_new (CLUTTER_BIN_ALIGNMENT_FILL,
CLUTTER_BIN_ALIGNMENT_FILL);
/* create the box whose children the layout will manage */
box = clutter_box_new (layout);
/*
* fill doesn't have much effect here
* unless the container has height and/or width
*/
clutter_actor_set_size (box, 100, 30);
/*
* background for the button; could equally be a texture
* with an image loaded into it or any other ClutterActor
*/
background = clutter_rectangle_new_with_color (&background_color);
/*
* add the background to the container;
* as it should use the default alignment, it can be added
* direct to the container, rather than via the layout
*/
clutter_container_add_actor (CLUTTER_CONTAINER (box), background);
/* text for the button */
text = clutter_text_new_full ("Sans 15px", "Click me", &text_color);
/*
* the text requires a different alignment from the background
* (centered on the box)
* so we add it via the layout so the default
* alignment can be overridden
*/
clutter_bin_layout_add (CLUTTER_BIN_LAYOUT (layout),
text,
CLUTTER_BIN_ALIGNMENT_CENTER,
CLUTTER_BIN_ALIGNMENT_CENTER);
/*
* ensure the actors are arranged in the correct depth order;
* in this case, the text is on top
* (NB this is not strictly necesary here as text is added after
* background)
*/
clutter_actor_raise_top (text);
]]>
</programlisting>
</informalexample>
</section>
<section>
<title>Discussion</title>
<para>This section covers some other aspects of using a
<type>ClutterBinLayout</type>.</para>
<section>
<title>Setting and changing alignment</title>
<para>Alignment is the only
<link linkend="layouts-introduction-layout-properties">layout
property</link> available for <type>ClutterBinLayout</type>. Each
actor can have a different setting for its alignment in one or both
of the <code>x</code> or <code>y</code> axes. However, as shown in the
solution above, alignment can also be used to expand an actor to
fill the container (<constant>CLUTTER_BIN_ALIGNMENT_FILL</constant>)
in one or both axes.</para>
<para>Setting alignment does not have any effect if the container
is the same size as all of the actors inside it: in this case,
every alignment produces the same layout. But if the container
associated with the layout is larger than the actor being aligned,
alignment will have an effect; see
<link linkend="layouts-stacking-size-requisitioning">this
section</link> for more details.</para>
<para>Changing an actor's alignment after it has been added
to a <type>ClutterBinLayout</type> may make the actor "jump"
(without animation) to a new position and/or change its size.
The exception is changing from some other alignment to
<constant>CLUTTER_BIN_ALIGNMENT_FIXED</constant>:
in this case, the actor will retain the position and size it
had before its alignment was fixed.</para>
</section>
<section id="layouts-stacking-size-requisitioning">
<title>Size requisitioning</title>
<para>A container with a <type>ClutterBinLayout</type> will by
default request the width of the widest actor in it, and the
height of the tallest. If you add actors smaller than those
dimensions, they will be aligned inside the container according
to the layout's policies. Here's an example where a
<type>ClutterBinLayout</type> requests a size to encompass the
tallest (light grey rectangle) and widest (dark grey rectangle)
actors inside it, with other actors aligned within
those bounds:</para>
<screenshot>
<mediaobject>
<imageobject>
<imagedata format="PNG"
fileref="images/layouts-stacking-diff-actor-sizes.png" />
</imageobject>
<alt>
<para>Size requisition in a <type>ClutterBinLayout</type></para>
</alt>
</mediaobject>
</screenshot>
<note>
<para>The screenshot also shows the 9 possible combinations
of start, center and end alignments on the <code>x</code> and
<code>y</code> axes. See
<link linkend="layouts-stacking-example-1">the sample
code</link> for more details.</para>
</note>
<para>The white space is the stage visible behind the
<type>ClutterBox</type> holding the coloured rectangles.
Notice that the layout is the width of the widest actor
within it and the height of the tallest.</para>
<para>You can also manually set a size on the container associated
with a layout to override the automatically-computed size
requisition.</para>
</section>
<section>
<title>Depth ordering</title>
<para>Another important consideration is the
<emphasis>depth ordering</emphasis> of actors inside a
<type>ClutterBinLayout</type>. By default, the depth ordering
mirrors the order in which actors are added to the layout: the
earlier an actor is added, the lower down in the depth order it
is. If this isn't what you want, you can fix the depth ordering using
<function>clutter_actor_raise()</function>,
<function>clutter_actor_lower()</function> and their relatives.</para>
</section>
<section>
<title>Other ways to stack actors</title>
<para><type>ClutterBinLayout</type> makes it simple to lay out
large numbers of actors in a stack and align them to the
container; see <link linkend="layouts-stacking-example-2">the
example below</link> which shows layering of many actors on
top of each other.</para>
<para>However, if you have a small number of actors and you
need some simple alignment, an alternative is to use
manual positioning inside a <type>ClutterFixedLayout</type>
(or even a <type>ClutterGroup</type>), possibly combined with
<type>ClutterConstraints</type> to align actors with each other
and bind their widths and heights together. See
<link linkend="layouts-introduction-not-using-layout-managers">this
section</link> for more details.</para>
</section>
</section>
<section>
<title>Full examples</title>
<example id="layouts-stacking-example-1">
<title><type>ClutterBinLayout</type>, with actors in 9
combinations of start, center and end alignment combinations</title>
<programlisting>
<xi:include href="examples/layouts-stacking-diff-sized-actors.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
<example id="layouts-stacking-example-2">
<title>Layering multiple textures on top of each other
inside a <type>ClutterBinLayout</type></title>
<programlisting>
<xi:include href="examples/layouts-stacking.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
</section>
</section>
<section id="layouts-bind-constraint">
<title>Binding the size of one actor to the size of another</title>
<section>
<title>Problem</title>
<para>You want one actor (the "target") to automatically change
its width or height (or both) when the size of another
actor (the "source") changes.</para>
<para>Example use cases:</para>
<itemizedlist>
<listitem>
<para>Making an actor adjust itself to the size of the stage
(particularly when the stage is resizable).</para>
</listitem>
<listitem>
<para>Putting one actor on top of another and keeping their
sizes in sync.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title>Solution</title>
<para>Create a <type>ClutterBindConstraint</type> bound to the
width and/or height of one actor (the "source"). Add that constraint
to an actor (the "target") whose size should follow the
size of the source.</para>
<para>This short example shows how to create and add a constraint;
<varname>source</varname> and <varname>target</varname> can
be any two <type>ClutterActors</type>:</para>
<informalexample>
<programlisting>
<emphasis>ClutterConstraint *width_constraint;</emphasis>
/* create a constraint which binds a target actor's width to 100px less than
* the width of the source actor (use CLUTTER_BIND_HEIGHT to create a
* constraint based on an actor's height)
*
* the third argument is a positive or negative offset from the actor's
* dimension, in pixels; this is added to the height or width of the source
* actor before the constraint is applied to the target actor
*/
<emphasis>width_constraint = clutter_bind_constraint_new (source, CLUTTER_BIND_WIDTH, -100);</emphasis>
/* add the constraint to an actor */
<emphasis>clutter_actor_add_constraint (target, width_constraint);</emphasis>
</programlisting>
</informalexample>
<para>Below is a full example, showing how to incorporate a
constraint into a Clutter application.</para>
<example id="layouts-bind-constraint-example-1">
<title>Constraining the size of a texture to
the size of the stage using <type>ClutterBindConstraint</type></title>
<programlisting>
<xi:include href="examples/layouts-bind-constraint-stage.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
<para>The texture in this example is 100px smaller than the stage,
leaving a border of visible stage around the texture; and the texture
has a tiled image on it. The tiling changes as the texture changes
size. Also note that two <type>ClutterAlignConstraints</type> are
added to center the actor on the stage.</para>
<para>The result looks like this:</para>
<screenshot>
<mediaobject>
<imageobject>
<imagedata format="PNG"
fileref="images/layouts-bind-constraint-stage.png" />
</imageobject>
<alt>
<para>A texture bound to the height and width of the
stage using <type>ClutterBindConstraint</type></para>
</alt>
</mediaobject>
</screenshot>
</section>
<section>
<title>Discussion</title>
<para>Sizing constraints are a good solution in these cases:</para>
<itemizedlist>
<listitem>
<para>Where you can't use a layout manager. For
example, you can't apply a layout manager to the stage
directly; so if you want to control the size of an actor
based on the size of the stage (as in
<link linkend="layouts-bind-constraint-example-1">the example
above</link>), constraints are a good substitute for a layout
manager .</para>
</listitem>
<listitem>
<para>Where the layout of a UI is fairly simple (perhaps
up to half a dozen actors) and fairly static. An example
might be something like a text editor, where the arrangement
of the UI (menu bar, toolbar, editing panel, footer) changes
infrequently. Of course, it is possible to arrange top-level
components using constraints, but still use layout
managers inside individual components (e.g. a flow layout
manager to manage buttons in the toolbar).</para>
</listitem>
<listitem>
<para>Where you have an actor whose size can change erratically,
but you still want to be able to track its size to control
another actor's size. An example might be an application like
a drawing program, where a user can create their own actors:
you might want the user to be able to describe loose, custom
constraints between actors like "keep these actors at the
same width", then allow those actors to be moved around and
resized in a free-form way as a group. In this situation, a
layout manager is too rigid and not appropriate;
but adding <type>ClutterConstraints</type> to actors
in response to user actions could work well.</para>
<para>The <link linkend="layouts-bind-constraint-example-2">sample
code in the appendix</link> is the kind of thing you might include
in a drawing program: you can resize a texture with a key press
(<code>+</code> to increase size, <code>-</code> to decrease), and
click on the actor to select/deselect it (a semi-transparent overlay is
toggled on the texture). The size of the overlay is bound and
aligned to the texture, so that it covers and slightly overlaps the
texture regardless of its size.</para>
</listitem>
</itemizedlist>
<note>
<para>You can bind an actor to a single dimension (just height or
depth) of another actor: you don't have to bind both height
and width. Also, you don't have to bind both dimensions of the
target to the same source: for example, you could bind the target's
height to one source (actor A) and its width to another source
(actor B).</para>
<para>A <type>ClutterBindConstraint</type> can also be used to
constrain a target actor's position on the <code>x</code> and
<code>y</code> axes to the position of a source actor. This is
covered in another recipe.</para>
</note>
<section>
<title>Another way to bind actors' sizes together</title>
<para>There is another way to control the size of a target
actor, based on the size of a source: you can create a handler
for the <code>allocation-changed</code> signal
of the source, emitted when its size and/or position
changes. This signal includes all the data
about the source's new allocation (height, width, x and y
coordindates), which the handler function can then use to
resize the target.</para>
<para>Alternatively, if you're only interested in
a change to width or height, you can create a handler
for the <code>notify::width</code> or
<code>notify::height</code> signal (respectively), and modify
the target's width/height in the handler.</para>
<para>This approach may be useful if you need a type of
control over alignment and size which is not possible using
constraints alone (e.g. one actor's size should be
a proportion of another's). See
<link linkend="layouts-bind-constraint-example-3">the code in
this section</link> for an example where the size
of one actor is dynamically set to 10% more than the
size of another.</para>
<note>
<para><link linkend="actors-allocation-notify">This recipe</link>
explains more about monitoring changes to an actor's size.</para>
</note>
</section>
</section>
<section>
<title>Full examples</title>
<example id="layouts-bind-constraint-example-2">
<title>Creating an automatically-resizing overlay for a
texture using <type>ClutterBindConstraint</type></title>
<programlisting>
<xi:include href="examples/layouts-bind-constraint-overlay.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
<example id="layouts-bind-constraint-example-3">
<title>Using the <code>allocation-changed</code>
signal of one actor to trigger proportional size changes in
another</title>
<programlisting>
<xi:include href="examples/layouts-bind-constraint-allocation.c" parse="text">
<xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
</programlisting>
</example>
</section>
</section>
</chapter>