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Expanded the layout introduction and added a recipe about stacking actors using ClutterBinLayout, with two examples.
591 lines
23 KiB
XML
591 lines
23 KiB
XML
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<chapter id="layouts"
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xmlns:xi="http://www.w3.org/2003/XInclude">
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<title>Layout management</title>
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<epigraph>
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<attribution>Abigail Adams, wife of John Adams, in a letter to John
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Thaxter (1778-09-29)</attribution>
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<para>If we do not lay out ourselves in the service of mankind,
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whom should we serve?</para>
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</epigraph>
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<section id="layouts-introduction">
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<title>Introduction</title>
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<para>Layout management in Clutter controls how an actor and
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children "inside" that actor are sized and positioned. More
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specifically, layouts are managed by associating a parent with a
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<type>ClutterLayoutManager</type>; the parent is usually either a
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composite <type>ClutterActor</type> (composed of several
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<type>ClutterActors</type>) or a <type>ClutterContainer</type>
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(containing child <type>ClutterActors</type>). The
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<type>ClutterLayoutManager</type> then manages:</para>
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<orderedlist>
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<listitem>
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<para>The <emphasis>size requisition</emphasis>
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(determination of the desired height and width) of the
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parent.</para>
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</listitem>
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<listitem>
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<para>The <emphasis>allocation</emphasis> (size and position)
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assigned to each composed or child ClutterActor.</para>
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</listitem>
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</orderedlist>
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<note>
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<para>To make this more concrete, imagine you have a sheet of
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paper and some coloured squares to place on it. Someone stands
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next to you telling you how big the piece of paper should be,
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how big the squares should be, and where to put each square on the
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piece of paper.</para>
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<para>The sheet of paper is analogous to the container or
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composite actor; the squares are analogous to the child
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<type>ClutterActors</type>; and the person giving you instructions
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is analogous to the layout manager.</para>
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</note>
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<para>The following sections give an overview of how layout
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management works in Clutter.</para>
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<section>
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<title>Using layouts</title>
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<para>Although Clutter provides plenty of flexibility in how you
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can use layout management, the simplest way to get started is to
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use the built-in <type>ClutterBox</type> class with one of the
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provided <type>ClutterLayoutManager</type> implementations.</para>
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<para>The pattern for doing this is:</para>
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<itemizedlist>
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<listitem>
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<para>Create an instance of one of the
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<type>ClutterLayoutManager</type> implementations (see
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<link linkend="layouts-introduction-manager-types">the
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following section</link>).</para>
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</listitem>
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<listitem>
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<para>Configure the layout manager's default policies
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(e.g. how actors are aligned by default, whether to pack
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actors horizontally or vertically, spacing between actors
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in the layout).</para>
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</listitem>
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<listitem>
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<para>Create a <type>ClutterBox</type>, setting its layout
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manager to the one you just created.</para>
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</listitem>
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<listitem>
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<para>Pack actors into the <type>ClutterBox</type>,
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setting layout properties (if required) as each is added.</para>
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</listitem>
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<listitem>
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<para>Modify layout properties of child actors using
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<function>clutter_layout_manager_child_set()</function>
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(if required).</para>
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</listitem>
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</itemizedlist>
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<para>Individual recipes in this section give more examples of
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how to make use of the different layout manager
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implementations.</para>
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<note>
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<para>It is not possible to use a layout manager with an arbitrary
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<type>ClutterContainer</type>: you must use a <type>ClutterActor</type>
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subclass which can delegate its layout to a layout manager (either
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use <type>ClutterBox</type> or write your own).</para>
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</note>
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</section>
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<section id="layouts-introduction-manager-types">
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<title>Types of layout manager</title>
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<para>Clutter provides a range of layout managers suitable
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for different use cases:</para>
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<itemizedlist>
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<listitem>
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<para><type>ClutterFixedLayout</type> arranges actors
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at fixed positions on the stage. No alignment options are
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available, so you have to manually compute and manage the
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coordinates (or use <type>ClutterConstraints</type>) which
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will align actors how you want them.</para>
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</listitem>
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<listitem>
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<para><type>ClutterBinLayout</type> arranges actors in a
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depth-ordered stack on top of each other, aligned to the container.
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This is useful for arranging actors inside composites (e.g.
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creating a button widget from a <type>ClutterTexture</type>
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with a <type>ClutterText</type> on top of it).</para>
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</listitem>
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<listitem>
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<para><type>ClutterBoxLayout</type> arranges actors in a
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single horizontal row or vertical column. This type of layout is
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common in UI elements like toolbars and menus.</para>
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</listitem>
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<listitem>
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<para><type>ClutterFlowLayout</type> arranges actors
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in reflowing columns and rows. If the container's allocation
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changes, the child actors are rearranged to fit inside its
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new allocation. This can be useful for arranging actors
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where you're not sure how many there might be; or where
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new ones are going to be added into the UI, perhaps displacing
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others. An example might be a photo viewer or an
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RSS feed display.</para>
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</listitem>
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</itemizedlist>
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</section>
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<section id="layouts-introduction-layout-properties">
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<title>Layout properties</title>
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<para>How actors are sized and positioned inside a container
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associated with a layout manager depends on two things:</para>
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<orderedlist>
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<listitem>
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<formalpara>
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<title>Properties which apply to all actors added to the layout</title>
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<para>There will be one setting at the layout level which can't
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be overridden per actor. This includes properties like spacing
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between rows and columns, whether the layout is homogenous
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(each actor gets the same allocation), etc.</para>
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</formalpara>
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</listitem>
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<listitem>
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<formalpara>
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<title>Properties for each actor added to the layout</title>
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<para>These are properties of the relationship between the
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layout, the container associated with the layout, and the
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children of the container. Each layout/container/actor
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combination can have different settings for each of these
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properties.</para>
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</formalpara>
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</listitem>
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</orderedlist>
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<para>Each layout manager implementation supports a subset of the
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following layout properties; different managers may have different
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names or functions for setting them, but the functionality remains
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the same. Individual recipes give more details about which
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properties can be set for each layout manager implementation.</para>
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<itemizedlist>
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<listitem>
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<formalpara>
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<title>Alignment</title>
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<para>How an actor aligns to the container's axes, e.g.
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aligned to the container's left, right, or center. For some
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layouts (like <type>ClutterBinLayout</type>) alignment
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is also used to set expand and fill properties.</para>
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</formalpara>
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</listitem>
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<listitem>
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<formalpara>
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<title>Horizontal/vertical orientation</title>
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<para>Whether actors are arranged in a horizontal row or
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vertical column.</para>
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</formalpara>
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</listitem>
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<listitem>
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<formalpara>
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<title>Homogenous rows and columns</title>
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<para>Grid-like layouts (e.g. <type>ClutterFlowLayout</type>)
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can be configured to have uniform rows and/or columns,
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expanding to fit the largest actor they contain.</para>
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</formalpara>
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</listitem>
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<listitem>
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<formalpara>
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<title>Row height and column width</title>
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<para>Grid-like layouts arranged in rows and columns
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can be configured with maximum and minimum row height and
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column width.</para>
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</formalpara>
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</listitem>
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<listitem>
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<formalpara>
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<title>Row and column spacing</title>
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<para>Grid-like layouts enable you to define a space (in pixels)
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between rows and columns.</para>
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</formalpara>
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</listitem>
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<listitem>
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<formalpara>
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<title>Expand</title>
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<para>Some layouts can be configured to minimize their size request
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to fit the actors they contain (<emphasis>expand is FALSE</emphasis>);
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or to increase the allocation of actors they contain so
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that all available space in the layout is used
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(<emphasis>expand is TRUE</emphasis>). In the latter case, you'd
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also need to set a size for the container associated with
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the layout, otherwise the container will just fit itself to the
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actors inside it.</para>
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</formalpara>
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</listitem>
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<listitem>
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<formalpara>
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<title>Fill</title>
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<para>This property only has an effect when
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<emphasis>expand</emphasis> is on. The <emphasis>fill</emphasis>
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setting controls whether actors are resized to fill their
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allocation (<emphasis>fill is TRUE</emphasis>); or if the
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space around the actor is increased (<emphasis>fill is
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FALSE</emphasis>).</para>
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</formalpara>
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</listitem>
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<listitem>
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<formalpara>
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<title>Pack at start/end</title>
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<para>This controls whether actors at prepended or appended
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to the layout.</para>
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<itemizedlist>
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<listitem>
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<para>If the orientation is vertical, prepended
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actors are added to the top of the layout and appended
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actors to the bottom.</para>
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</listitem>
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<listitem>
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<para>If the orientation is horizontal, prepended
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actors are added at the left of the layout and appended actors
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on the right.</para>
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</listitem>
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</itemizedlist>
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</formalpara>
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</listitem>
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</itemizedlist>
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<section>
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<title>Setting layout properties</title>
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<para>Layout properties can be set in one or more of the following ways
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(depending on the type of property and the layout manager):</para>
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<orderedlist>
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<listitem>
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<para>By setting a default value for the property on the
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layout manager (e.g. using
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<function>clutter_bin_layout_set_alignment()</function>,
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<function>clutter_box_layout_set_expand()</function>). Any
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actor added to the layout gets this value for the property,
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unless it is overridden for that actor.</para>
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</listitem>
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<listitem>
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<para>When adding an actor to a <type>ClutterBox</type> container
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using <function>clutter_box_pack()</function>, you can set
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properties on the actor which you're adding.</para>
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</listitem>
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<listitem>
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<para>When adding an actor to a layout you can use a function
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which enables setting properties simultaneously (e.g.
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<function>clutter_box_layout_pack()</function>,
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<function>clutter_bin_layout_add()</function>).</para>
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</listitem>
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<listitem>
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<para>By using
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<function>clutter_layout_manager_child_set()</function> on
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the child of a layout.</para>
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</listitem>
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</orderedlist>
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</section>
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</section>
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<section id="layouts-introduction-not-using-layout-managers">
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<title>Not using layout managers</title>
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<para>It is perfectly possible to arrange <type>ClutterActors</type>
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without using layout managers; however, you may have to do
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more of your own calculations about actor sizes and positions.</para>
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<para>There are two (not mutually-exclusive) approaches you can
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take to do this, described below.</para>
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<section>
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<title>Manual positioning and alignment</title>
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<para>This basically means using the <type>ClutterActor</type>
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bounding box mechanism (see the <type>ClutterActor</type>
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documentation for details) to set actor sizes and positions.
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This is the approach you will see in a lot of older Clutter
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code (written before layout managers were available).</para>
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<para>This approach is simplest where the UI is relatively static
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and is composed of a few known actors. It will work in larger,
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more complex scenarios, but in those sorts of cases it is better
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to make use of layout managers and constraints (see below) instead.</para>
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</section>
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<section>
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<title>Using <type>ClutterConstraint</type></title>
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<para>Constraints provide mechanisms for:</para>
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<itemizedlist>
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<listitem>
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<para>Aligning actors with each other
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(<type>ClutterAlignConstraint</type>). For example, you
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can align the top, bottom or center of one actor with the
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top, bottom or center of another (on the <code>y</code>
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axis). Similarly, you can align one actor to another
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on the <code>x</code> axis.</para>
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</listitem>
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<listitem>
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<para>Binding properties of one actor to those of
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another. For example, you could ensure that two actors
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always remain the same width; or you could specify
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that two actors always have the same <code>x</code>
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coordinate. In both these cases and others, you can
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specify that the properties should be the same, or the same
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+/- some offset.</para>
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</listitem>
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</itemizedlist>
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<note>
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<para><type>ClutterConstraints</type> can be used in combination
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with some layout managers, but you need to be careful that
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constraints don't fight with the layout manager policies.
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Unpredictable results could ensue.</para>
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</note>
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</section>
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</section>
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</section>
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<section id="layouts-stacking">
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<title>Stacking actors on top of each other</title>
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<section>
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<title>Problem</title>
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<para>You want to lay out several actors so that they are in
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layers on top of each other (e.g. to create a button widget
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composed from a rectangle with text on top of it).</para>
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</section>
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<section id="layouts-stacking-solution">
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<title>Solution</title>
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<para>The most flexible approach is to use a <type>ClutterBinLayout</type>
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associated with a <type>ClutterBox</type>:</para>
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<informalexample>
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<programlisting>
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<![CDATA[
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/* define some colors */
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const ClutterColor background_color = { 0xaa, 0x99, 0x00, 0xff };
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const ClutterColor text_color = { 0xff, 0xff, 0xff, 0xff };
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ClutterLayoutManager *layout;
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ClutterActor *box;
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ClutterActor *background;
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ClutterActor *text;
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/*
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* create a layout, setting the default x and y alignment;
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* actors fill the whole allocation of the layout's container
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* by default
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*/
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layout = clutter_bin_layout_new (CLUTTER_BIN_ALIGNMENT_FILL,
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CLUTTER_BIN_ALIGNMENT_FILL);
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/* create the box whose children the layout will manage */
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box = clutter_box_new (layout);
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/*
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* fill doesn't have much effect here
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* unless the container has height and/or width
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*/
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clutter_actor_set_size (box, 100, 30);
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/*
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* background for the button; could equally be a texture
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* with an image loaded into it or any other ClutterActor
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*/
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background = clutter_rectangle_new_with_color (&background_color);
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/*
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* add the background to the container;
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* as it should use the default alignment, it can be added
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* direct to the container, rather than via the layout
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*/
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clutter_container_add_actor (CLUTTER_CONTAINER (box), background);
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/* text for the button */
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text = clutter_text_new_full ("Sans 15px", "Click me", &text_color);
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/*
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* the text requires a different alignment from the background
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* (centered on the box)
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* so we add it via the layout so the default
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* alignment can be overridden
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*/
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clutter_bin_layout_add (CLUTTER_BIN_LAYOUT (layout),
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text,
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CLUTTER_BIN_ALIGNMENT_CENTER,
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CLUTTER_BIN_ALIGNMENT_CENTER);
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/*
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* ensure the actors are arranged in the correct depth order;
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* in this case, the text is on top
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* (NB this is not strictly necesary here as text is added after
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* background)
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*/
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clutter_actor_raise_top (text);
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]]>
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</programlisting>
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</informalexample>
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</section>
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<section>
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<title>Discussion</title>
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<para>This section covers some other aspects of using a
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<type>ClutterBinLayout</type>.</para>
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<section>
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<title>Setting and changing alignment</title>
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<para>Alignment is the only
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<link linkend="layouts-introduction-layout-properties">layout
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property</link> available for <type>ClutterBinLayout</type>. Each
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actor can have a different setting for its alignment in one or both
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of the <code>x</code> or <code>y</code> axes. However, as shown in the
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solution above, alignment can also be used to expand an actor to
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fill the container (<constant>CLUTTER_BIN_ALIGNMENT_FILL</constant>)
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in one or both axes.</para>
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<para>Setting alignment does not have any effect if the container
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is the same size as all of the actors inside it: in this case,
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every alignment produces the same layout. But if the container
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associated with the layout is larger than the actor being aligned,
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alignment will have an effect; see
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<link linkend="layouts-stacking-size-requisitioning">this
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section</link> for more details.</para>
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<para>Changing an actor's alignment after it has been added
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to a <type>ClutterBinLayout</type> may make the actor "jump"
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(without animation) to a new position and/or change its size.
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The exception is changing from some other alignment to
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<constant>CLUTTER_BIN_ALIGNMENT_FIXED</constant>:
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in this case, the actor will retain the position and size it
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had before its alignment was fixed.</para>
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</section>
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<section id="layouts-stacking-size-requisitioning">
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<title>Size requisitioning</title>
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<para>A container with a <type>ClutterBinLayout</type> will by
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default request the width of the widest actor in it, and the
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height of the tallest. If you add actors smaller than those
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dimensions, they will be aligned inside the container according
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to the layout's policies. Here's an example where a
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<type>ClutterBinLayout</type> requests a size to encompass the
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tallest (light grey rectangle) and widest (dark grey rectangle)
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actors inside it, with other actors aligned within
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those bounds:</para>
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<screenshot>
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<mediaobject>
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<imageobject>
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<imagedata format="PNG"
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fileref="images/layouts-stacking-diff-actor-sizes.png" />
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</imageobject>
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<alt>
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<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>
|
|
|
|
</chapter>
|