mirror of
https://github.com/brl/mutter.git
synced 2024-12-25 12:32:05 +00:00
e92b186719
The recipe covers how to use ClutterBindConstraint to bind actor sizes together. It gives some examples of where this approach is appropriate, as well as explaining an alternative using allocation-changed or notify::* signals. Three examples are given: 1. Resizing a texture to the stage. 2. Resizing a rectangle to act as a transparent overlay on top of a texture (using constraints). 3. Resizing a rectangle to act as a transparent overlay on top of a texture, but with a size proportional to the texture (using a handler connected to allocation-changed signals emitted by the texture).
811 lines
32 KiB
XML
811 lines
32 KiB
XML
<!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>
|