mutter-performance-source/doc/cookbook/layouts.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>

  <section id="layouts-box">
    <title>Arranging actors in a single row or column</title>

    <section>
      <title>Problem</title>

      <para>You want to layout several actors in a single row
      or column.</para>

      <para>Example use cases:</para>

      <itemizedlist>
        <listitem>
          <para>Creating an application menu.</para>
        </listitem>
        <listitem>
          <para>Showing message subject lines as a list in an
          email client.</para>
        </listitem>
      </itemizedlist>
    </section>

    <section>
      <title>Solution</title>

      <para>Create a <type>ClutterBox</type> with a
      <type>ClutterBoxLayout</type> as its layout manager.</para>

      <para>A <type>ClutterBoxLayout</type> can hold a single row or
      column of <type>ClutterActors</type>, and has configurable spacing,
      actor alignments, and expand and fill options.</para>

      <para>The code fragment below is excerpted from the
      <link linkend="layouts-box-example-1">full example</link>. It
      demonstrates how to lay out three rectangles in a vertical
      column. A different approach is used to set the <varname>x-fill</varname>
      property on each rectangle, so each fills the horizontal space in
      the layout (each rectangle is 100 pixels wide, while the
      box they are inside is 200 pixels wide).</para>

      <informalexample>
        <programlisting>
<![CDATA[
/* create a ClutterBoxLayout */
box_layout = clutter_box_layout_new ();

/* configure it to lay out actors vertically */
clutter_box_layout_set_vertical (CLUTTER_BOX_LAYOUT (box_layout), TRUE);

/* put 5px of spacing between actors */
clutter_box_layout_set_spacing (CLUTTER_BOX_LAYOUT (box_layout), 5);

/* actors are packed into this box; we set its width, but
 * allow its height to be determined by the children it contains
 */
box = clutter_box_new (box_layout);
clutter_box_set_color (CLUTTER_BOX (box), &box_color);
clutter_actor_set_position (box, 100, 50);
clutter_actor_set_width (box, 200);

/* pack an actor into the layout and set all layout properties on it
 * at the same time
 */
yellow = clutter_rectangle_new_with_color (&yellow_color);
clutter_actor_set_size (yellow, 100, 100);

clutter_box_layout_pack (CLUTTER_BOX_LAYOUT (box_layout),
                         yellow,
                         FALSE,                         /* expand */
                         TRUE,                          /* x-fill */
                         FALSE,                         /* y-fill */
                         CLUTTER_BOX_ALIGNMENT_START,   /* x-align */
                         CLUTTER_BOX_ALIGNMENT_START);  /* y-align */

/* pack an actor into the box and set layout properties at the
 * same time; note this is more concise if you mostly want to
 * use the default properties for the layout
 */
red = clutter_rectangle_new_with_color (&red_color);
clutter_actor_set_size (red, 100, 100);

clutter_box_pack (CLUTTER_BOX (box),
                  red,
                  "x-fill", TRUE,
                  NULL);

/* add an actor to the box as a container and set layout properties
 * afterwards; the latter is useful if you want to change properties on
 * actors already inside a layout, but note that you have to
 * pass the function both the layout AND the container
 */
blue = clutter_rectangle_new_with_color (&blue_color);
clutter_actor_set_size (blue, 100, 100);

clutter_container_add_actor (CLUTTER_CONTAINER (box), blue);

clutter_layout_manager_child_set (box_layout,
                                  CLUTTER_CONTAINER (box),
                                  blue,
                                  "x-fill", TRUE,
                                  NULL);

/* put the box on the stage */
clutter_container_add_actor (CLUTTER_CONTAINER (stage), box);
]]>
        </programlisting>
      </informalexample>

      <para>The result looks like this:</para>

      <screenshot>
        <mediaobject>
          <imageobject>
            <imagedata format="PNG"
                       fileref="images/layouts-box.png" />
          </imageobject>
          <alt>
            <para>A simple vertical <type>ClutterBoxLayout</type></para>
          </alt>
        </mediaobject>
      </screenshot>

    </section>

    <section>
      <title>Discussion</title>

      <para><type>ClutterBoxLayout</type> is not a reflowing layout:
      that is, if the layout's container changes size, the actors inside
      aren't automatically repositioned to occupy or find new positions
      in its available area. If you want that behaviour, use
      <type>ClutterFlowLayout</type> instead.</para>

      <para>If you want the container to be resizable, but find that
      resizing the container obscures its child actors, you could put
      the container inside a scrollable area. Then the container's actors
      can be scrolled to if they go out of sight.
      <link linkend="events-mouse-scroll">This recipe</link> explains
      how to make a container scrollable.</para>

      <section>
        <title>Layout properties</title>

        <para><type>ClutterBoxLayout</type> is very flexible, with
        several properties which influence the appearance of the
        layout and its children. As with other layouts, these properties
        are either applicable to the layout itself, or to individual
        children of the layout.</para>

        <para>As most of these properties are documented in the API reference,
        they aren't covered in much detail here. However, below is a brief
        overview of the properties available, as well as details of properties
        particular to <type>ClutterBoxLayout</type>.</para>

        <para>The main issue you may face when applying these properties
        is understanding how they interact. As this is harder to describe
        than to show, you can run the
        <link linkend="layouts-box-example-3">example</link>
        below to toggle and tweak various properties and see
        how they affect the layout's appearance.</para>

        <note>
          <para>The "toggle and tweak"
          <link linkend="layouts-box-example-3">example</link> sets
          child properties (fill, alignment, expand) on all children
          of the layout when those properties are changed. If you want
          to see the effect of setting these to different values for
          <emphasis>each</emphasis> child, you will have to experiment
          yourself.</para>
        </note>

        <section>
          <title><type>ClutterBoxLayout</type> properties</title>

          <para><type>ClutterBoxLayout</type> has the following properties
          which affect the appearance of all children inside the container.</para>

          <note>
            <para>Animation properties are covered separately
            <link linkend="layouts-box-animating-layout-changes">later</link>.
            </para>
          </note>

          <itemizedlist>
              <listitem>
                <formalpara>
                  <title><varname>vertical</varname>; set with
                  <function>clutter_box_layout_set_vertical()</function></title>

                  <para>Set to <constant>TRUE</constant> to lay out
                  child actors in a column; if <constant>FALSE</constant>
                  (the default), actors are laid out horizontally.</para>
                </formalpara>
              </listitem>

              <listitem>
                <formalpara>
                  <title><varname>homogeneous</varname>; set with
                  <function>clutter_box_layout_set_homogeneous()</function></title>

                  <para>Set to <constant>TRUE</constant> to allocate all
                  child actors the same amount of space in the row or column
                  (depending on the setting for <varname>vertical</varname>).
                  This overrides per-actor <varname>expand</varname> settings
                  and preferred sizes for child actors. The default value for
                  this property is <constant>FALSE</constant>.</para>
                </formalpara>
              </listitem>

              <listitem>
                <formalpara>
                  <title><varname>spacing</varname>; set with
                  <function>clutter_box_layout_set_spacing()</function></title>

                  <para>Sets the number of pixels to place between actors
                  in the layout.</para>
                </formalpara>

                <para>Note that if you increase spacing too much, actors
                may go outside the edges of the layout's container (if
                the container has a fixed size).</para>
              </listitem>

              <listitem>
                <formalpara>
                  <title><varname>pack-start</varname>; set with
                  <function>clutter_box_layout_set_pack_start()</function></title>

                  <para>Set <varname>pack-start</varname> to
                  <constant>TRUE</constant> to configure the layout to
                  prepend actors to the row or column; the default is
                  <constant>FALSE</constant>, meaning that actors are
                  appended to the row or column when added.</para>
                </formalpara>

                <para>Changing this property on a layout which already has
                actors in it will reverse the order of those actors, as
                well as changing how new actors are added to the layout.</para>
              </listitem>
            </itemizedlist>

          </section>

          <section>
            <title>Child properties</title>

            <para>These properties apply to individual children within
            the layout's container. Each child can have different values
            for each of these properties.</para>

            <para>To set properties, you can use
            <function>clutter_box_layout_pack()</function> or
            <function>clutter_box_pack()</function> (if using a
            <type>ClutterBox</type>) while packing actors into the layout.
            You can also set properties later using
            <function>clutter_layout_manager_child_set()</function>
            etc. See the <link linkend="layouts-introduction">layouts
            introduction</link> for more details.</para>

            <itemizedlist>
              <listitem>
                <para><varname>x-fill</varname> and <varname>y-fill</varname>
                set whether an actor will fill its allocated horizontal
                or vertical space (respectively) within the layout. Setting
                these properties only has an effect where an actor is smaller
                (on the appropriate fill axes) than the layout's container.</para>

                <para>Note that the actor's actual size is not changed
                if it is set to fill: the reported width and height are
                unaffected.</para>
              </listitem>

              <listitem>
                <para><varname>expand</varname> sets whether an actor
                will be expanded inside the layout. If
                <varname>expand</varname> is <constant>TRUE</constant>
                and <varname>fill</varname> is <constant>TRUE</constant>
                for the orientation axis, the actor is resized to fill
                its whole allocation on that axis; if <varname>expand</varname>
                is <constant>TRUE</constant> but <varname>fill</varname>
                is <constant>FALSE</constant>, extra padding is added
                around the actor to fill the allocation.</para>
              </listitem>

              <listitem>
                <para><varname>x-align</varname> and <varname>y-align</varname>
                set how an actor is aligned within its allocation, in
                cases where it doesn't fill that allocation. In practical
                terms, these properties come into effect if a child is set
                to expand but <varname>fill</varname> is set to
                <constant>FALSE</constant> on the align axis.</para>

                <para>For example, if <varname>expand</varname> is
                <constant>TRUE</constant> but <varname>x-fill</varname>
                is <constant>FALSE</constant>, some padding is added
                around the actor to fill its allocation. In this case,
                the <varname>x-align</varname> property can be set to
                align the actor to the left, center or right of the
                allocation; any whitespace would be redistributed around
                the actor's new position after alignment.</para>
              </listitem>
            </itemizedlist>

            <para>These properties are only useful where you have
            actors of non-uniform sizes and/or a container which is
            either wider or taller (or both) than one or more of the
            child actors it contains.</para>

          </section>

      </section>

      <section id="layouts-box-animating-layout-changes">
        <title>Animating layout changes</title>

        <para>If actors are added to a layout, or if the layout's
        properties or its children's properties are changed, the
        appearance of the layout may also change. The
        <varname>use-animations</varname> property (set with
        <function>clutter_box_layout_set_use_animations()</function>)
        determines whether such changes to the layout are animated: if set
        to <constant>TRUE</constant>, any changes to actor
        allocations resulting from the changes (movements, resizings)
        are animated.</para>

        <para>If this property is <constant>FALSE</constant> (the default)
        changes to other properties or addition of new actors will
        cause actors to be laid out instantaneously. For example, if
        a new actor is prepended with animations on, the new actor is
        added to the layout and the other actors shifted to make room
        for it; if animations are off, child actors jump to their new
        positions at the same instant as the new actor is added.</para>

        <para>To change the appearance of the animations, you can use
        <function>clutter_box_layout_set_easing_mode()</function> and
        <function>clutter_box_layout_set_duration()</function> (see the
        <link linkend="animations-introduction">animations
        introduction</link> for more about easing and duration
        properties).</para>

        <para>The <link linkend="layouts-box-example-3">"toggle and tweak"
        example</link> uses animation for layout changes, and can give
        you some idea of what to expect in your own animated layouts.</para>

      </section>
    </section>

    <section>
      <title>Full examples</title>

      <example id="layouts-box-example-1">
        <title>Different approaches to setting child layout properties
        in a <type>ClutterBoxLayout</type></title>
        <programlisting>
<xi:include href="examples/layouts-box.c" parse="text">
  <xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
        </programlisting>
      </example>

      <example id="layouts-box-example-2">
        <title>A simple <type>ClutterBoxLayout</type> menu</title>
        <programlisting>
<xi:include href="examples/layouts-box-menu.c" parse="text">
  <xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
        </programlisting>
      </example>

      <example id="layouts-box-example-3">
        <title>A demonstrator for "toggling and tweaking" a
        <type>ClutterBoxLayout's</type> properties</title>
        <programlisting>
<xi:include href="examples/layouts-box-property-effects.c" parse="text">
  <xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
        </programlisting>
      </example>

    </section>

  </section>

</chapter>