mutter/doc/cookbook/script.xml

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<chapter id="script"
         xmlns:xi="http://www.w3.org/2003/XInclude">
  <title>Script</title>

  <epigraph>
    <attribution>Alfred Hitchcock</attribution>
    <para>When an actor comes to me and wants to discuss his character,
    I say, "It's in the script". If he says, "But what's my motivation?",
    I say, "Your salary".</para>
  </epigraph>

  <section id="script-introduction">
    <title>Introduction</title>

    <para>User interfaces can become difficult to maintain when
    described entirely in code: declarations of UI
    elements become entwined with procedural code for
    handling interactions. This can make refactoring tough, as
    you have to find the right place in the code to modify the UI
    ("Where did I set the color of that rectangle?") and make sure
    your UI modifications don't break any behaviour.</para>

    <para>Many frameworks separate presentation from programming
    logic, making it easier to change the appearance of the UI
    without affecting its behaviour (and vice versa). For example,
    in web development you can use HTML and CSS to define
    presentation, and JavaScript to implement application logic.</para>

    <para><type>ClutterScript</type> enables a similar separation:
    you can define the UI declaratively using
    <ulink href="http://www.json.org/">JSON</ulink>, load
    the UI from the JSON, then handle interactions with it through Clutter code
    (in C, Python, Vala or some other language). This has several
    benefits, including:</para>

    <itemizedlist>
      <listitem>
        <para>Separation of UI element declarations from control logic
        (see above).</para>
      </listitem>
      <listitem>
        <para>More concise code: typically, describing a UI in JSON
        requires far fewer characters than the equivalent procedural
        code (at least, once you have more than three or four actors in
        your application).</para>
      </listitem>
      <listitem>
        <para>If you write your JSON in external files, you can make the
        structure of the UI evident in the layout of the file. For
        example, child elements can be indented within the parent
        element. This can make identifying relationships between
        elements simpler and less error-prone.</para>
      </listitem>
      <listitem>
        <para>Creating and configuring some objects (e.g. animations,
        layouts) can be much simpler in JSON.</para>
      </listitem>
      <listitem>
        <para>Less compilation (if you're using a compiled language):
        because you can change the UI by editing external JSON files,
        you can make changes to it without needing to recompile
        the whole application.</para>
      </listitem>
    </itemizedlist>

    <para>The following sections are intended
    to give an overview of  how <type>ClutterScript</type> works, and
    how to use it in an application. The recipes in this chapter
    then provide more detail about particular aspects of
    <type>ClutterScript</type>, such as how to connect signals to handlers,
    how to merge multiple JSON definitions in a single script, etc.
    There is also a lot of useful information in the <type>ClutterScript</type>
    API reference.</para>

    <section>
      <title>Basic principles of <type>ClutterScript</type></title>

      <para>Clutter is built on top of
      <ulink href="http://library.gnome.org/devel/gobject/">GObject</ulink>,
      an object system for C. <type>ClutterScript</type>
      provides a way to create instances of GObjects and
      set their properties. For example:</para>

      <example>
        <title>Example UI definition in JSON for use with
        <type>ClutterScript</type></title>

        <programlistingco>
          <programlisting>
[   <co id="script-ui-introduction-json-list-bracket" />
  {   <co id="script-ui-introduction-json-object-bracket" />
    "id" : "stage",   <co id="script-ui-introduction-json-id" />
    "type" : "ClutterStage",   <co id="script-ui-introduction-json-type" />
    "is-default" : true,   <co id="script-ui-introduction-json-default-stage" />
    "width" : 400,
    "height" : 400,
    "color" : "#333355ff",   <co id="script-ui-introduction-json-color-html" />
    "children" : [ "box" ]   <co id="script-ui-introduction-json-child-by-id" />
  },

  {
    "id" : "box",
    "type" : "ClutterBox",
    "width" : 400,
    "height" : 400,

    "layout-manager" : {   <co id="script-ui-introduction-json-no-id" />
      "type" : "ClutterBinLayout",
      "x-align" : "CLUTTER_BIN_ALIGNMENT_CENTER",
      "y-align" : "CLUTTER_BIN_ALIGNMENT_CENTER"
    },

    "children" : [   <co id="script-ui-introduction-json-child-by-embedding" />
      {
        "id" : "rectangle",
        "type" : "ClutterRectangle",
        "width" : 200,
        "height" : 200,
        "color" : "red"   <co id="script-ui-introduction-json-color-word" />
      }
    ]
  }
]
          </programlisting>

          <note>
            <para>N.B. The numbers in brackets in the example further
            explain the JSON structure, and are not part of the UI
            definition.</para>
          </note>

          <calloutlist>
            <callout arearefs="script-ui-introduction-json-list-bracket">
              <para>All the objects defined for the UI sit inside a JSON
              list structure, marked with square brackets.</para>
            </callout>
            <callout arearefs="script-ui-introduction-json-object-bracket">
              <para>A pair of braces surrounds each object definition;
              inside the braces, key-value pairs set properties on the
              object. See the
              <link linkend="script-introduction-data-types">section on
              datatypes</link> for more about the acceptable values.</para>
            </callout>
            <callout arearefs="script-ui-introduction-json-id
            script-ui-introduction-json-no-id">
              <para>An <varname>id</varname> is required for objects which
              are referred to elsewhere in the JSON or which need to be
              accessible from code (see
              <link linkend="script-ui">this recipe</link> for the basics of
              using object IDs from code).</para>
              <para>In cases where an object doesn't need to be accessible
              from code and is not referenced elsewhere in the JSON file,
              the <varname>id</varname> can be omitted.</para>
            </callout>
            <callout arearefs="script-ui-introduction-json-type">
              <para>The <varname>type</varname> key is mandatory, and
              specifies the type of the object; usually this will be
              one of the Clutter object types.</para>
            </callout>
            <callout arearefs="script-ui-introduction-json-default-stage">
              <para><varname>is-default</varname> is a special stage-only
              property, which tells Clutter that the <type>ClutterStage</type>
              instance used should be the default stage.</para>
            </callout>
            <callout arearefs="script-ui-introduction-json-color-html
            script-ui-introduction-json-color-word">
              <para>Colors can be set using hexadecimal color code strings,
              as used in HTML and CSS; or by using color words. The
              range of acceptable values is as for the
              <function>pango_color_from_string()</function> function.</para>
            </callout>
            <callout arearefs="script-ui-introduction-json-child-by-id
            script-ui-introduction-json-child-by-embedding">
              <para>Children can be associated with a parent through
              the <varname>children</varname> property. Children are
              either added to the <varname>children</varname> list by ID;
              or by directly embedding the child JSON object as an element
              within the list. The two can be mixed in a single
              list of <varname>children</varname>.</para>
            </callout>
          </calloutlist>

        </programlistingco>
      </example>

      <para>Once you grasp that Clutter objects are GObjects, and you
      are setting their properties, you can work out what is "scriptable"
      by referring to the <emphasis>Properties</emphasis> sections
      of the API reference for each Clutter type. Any of the properties
      described there can be set using <type>ClutterScript</type>.</para>

      <para>Having said this, there are some special properties which
      aren't obvious, but which can be set via JSON;
      <emphasis>layout properties</emphasis> are one example. These aren't
      listed as properties of <type>ClutterActor</type> but can be set
      as part of a <type>ClutterActor</type> object definition
      (using the <code>layout::&lt;property name&gt;</code>
      syntax for the key). Some of these are covered in recipes later in
      this chapter.</para>

    </section>

    <section id="script-introduction-data-types">
      <title>Data types</title>

      <para><type>ClutterScript</type> uses the standard JSON format.
      It is very important that you respect the data type of the property
      you are setting, ensuring that you use the right JSON data type.
      You may get unexpected results or errors if you try to set a property
      using the wrong data type: for example, setting a property
      to an integer <type>number</type> in the JSON, when the Clutter property
      is expecting a <type>gfloat</type>, may cause errors.</para>

      <para>To assist in using the right data types in your JSON
      definitions, the table below shows how Clutter and GLib data
      types map to JSON:</para>

      <informaltable>
        <thead>
          <tr>
            <th>C data type (Clutter/GLib)</th>
            <th>Maps to JSON</th>
            <th>Example (C =&gt; JSON)</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td>floating point number (gfloat, gdouble)</td>
            <td>number (int frac, int exp, int frac exp)</td>
            <td>
              <para><code>1.0</code> =&gt; <code>1.0</code></para>
              <para><code>1e-1</code> =&gt; <code>1e-1</code></para>
              <para><code>1E-1</code> =&gt; <code>1E-1</code></para>
              <para><code>0.1E-1</code> =&gt; <code>0.1E-1</code></para>
            </td>
          </tr>
          <tr>
            <td>integer (guint8, gint)</td>
            <td>number (int)</td>
            <td>
              <para><code>1</code> =&gt; <code>1</code></para>
              <para><code>0x00</code> =&gt; <code>0</code> (no hex in JSON)</para>
              <para><code>01</code> =&gt; <code>1</code> (no octal in JSON)</para>
            </td>
          </tr>
          <tr>
            <td>gboolean</td>
            <td>true/false</td>
            <td>
              <para><code>TRUE</code> =&gt; <code>true</code></para>
              <para><code>FALSE</code> =&gt; <code>false</code></para>
            </td>
          </tr>
          <tr>
            <td>gchar</td>
            <td>string</td>
            <td><code>"hello world"</code> =&gt; <code>"hello world"</code></td>
          </tr>
          <tr>
            <td>enum (e.g. Clutter constants)</td>
            <td>string</td>
            <td><code>CLUTTER_ALIGN_X_AXIS</code> =&gt;
            <code>"CLUTTER_ALIGN_X_AXIS"</code></td>
          </tr>
          <tr>
            <td>ClutterColor</td>
            <td>color string</td>
            <td>
              <code>clutter_color_new (255, 0, 0, 255)</code> =&gt;
              <code>"red"</code> or <code>"#f00f"</code> or
              <code>"#ff0000ff"</code>; alternatively,
              <code>"#f00"</code> or <code>"#ff0000"</code>
              (implicitly sets alpha value to 255)
            </td>
          </tr>
          <tr>
            <td>ClutterActor (or other Clutter type)</td>
            <td>object</td>
            <td>
              <code>clutter_rectangle_new ()</code> =&gt;
              <code>{ "type" : "ClutterRectangle" }</code>
            </td>
          </tr>
          <tr>
            <td>Property which takes a list or array of values</td>
            <td>array of objects and/or IDs</td>
            <td>
              <code>clutter_container_add_actor (stage, rectangle)</code> =&gt;
              <programlisting>
{
  "id" : "stage",
  "type" : "ClutterStage",
  ...,

  "children" : [
    {
      "id" : "rectangle",
      "type" : "ClutterRectangle",
      ...
    }
  ]
}
              </programlisting>
            </td>
          </tr>
          <tr>
            <td><code>NULL</code></td>
            <td><code>null</code></td>
            <td>-</td>
          </tr>
        </tbody>
      </informaltable>

    </section>

  </section>

  <section id="script-ui">
    <title>Defining a user interface with JSON</title>

    <section>
      <title>Problem</title>

      <para>You want to create a user interface as quickly as
      possible; you also need to change it easily as requirements shift.</para>

      <para>This need can arise when:</para>

      <itemizedlist>
        <listitem>
          <para>you are prototyping a user interface, and you need to
          quickly test new ideas.</para>
        </listitem>
        <listitem>
          <para>the user interface you are building is likely to contain many
          elements and relationships between them.</para>
        </listitem>
      </itemizedlist>
    </section>

    <section>
      <title>Solution</title>

      <para>Define the user interface in an external JSON file. Then
      create a <type>ClutterScript</type> object and load the JSON
      into it from the file.</para>

      <para>This keeps the UI definition separate from the application
      logic and makes it easier to manage.</para>

      <note>
        <para>See <link linkend="script-introduction">the introduction</link>
        for the reasons why <type>ClutterScript</type> is a good solution,
        and for an overview of how JSON definitions work.</para>
      </note>

      <para>Here's an example JSON definition to put in the file:</para>

      <informalexample>
        <programlisting>
<xi:include href="examples/script-ui.json" parse="text">
  <xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
        </programlisting>
      </informalexample>

      <para>In the application, load the JSON from the file with
      <function>clutter_script_load_from_file()</function>. (You can
      also load JSON from a string (<type>gchar*</type>) with
      <function>clutter_script_load_from_data()</function>.)</para>

      <para>Then retrieve objects by ID to use them in your code:</para>

      <example>
        <title>Loading JSON from a file and retrieving objects
        defined by it</title>
        <programlisting>
<xi:include href="examples/script-ui.c" parse="text">
  <xi:fallback>a code sample should be here... but isn't</xi:fallback>
</xi:include>
        </programlisting>
      </example>

      <para>Although we only retrieved the stage in the example above,
      <function>clutter_script_get_objects()</function> can
      retrieve multiple objects with a single call:</para>

      <informalexample>
        <programlisting>
<![CDATA[
ClutterScript *script;
script = clutter_script_new ();

/* ...load JSON file etc. */

ClutterStage *stage;
ClutterActor *actor1;
ClutterActor *actor2;

/* use a NULL-terminated argument list of id,variable pairs */
clutter_script_get_objects (script,
                            "stage", &stage,
                            "actor1", &actor1,
                            "actor2", &actor2,
                            NULL);
]]>
        </programlisting>
      </informalexample>

      <para>You can also use <function>clutter_script_get_object()</function>
      to retrieve a single object, though you may have to cast
      it to the right type before use; for example:</para>

      <informalexample>
        <programlisting>
ClutterStage *stage = CLUTTER_STAGE (clutter_script_get_object (script, "stage));
        </programlisting>
      </informalexample>

    </section>

    <section>
      <title>Discussion</title>

      <para>In the sample code, the stage is part of the JSON definition.
      However, it doesn't have to be: it is possible to create the
      stage in application code; then load more components from one
      or more JSON definitions and attach them to the stage you
      constructed in code.</para>

      <para>However, keeping most of the user interface definition
      in external JSON files makes it easier to change
      the UI without having to touch any code. If you have some user
      interface elements constructed in code and some in JSON, it can
      make refactoring more difficult.</para>

    </section>

  </section>

</chapter>