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cookbook: Added first draft of script introduction

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New script chapter needs an introduction.

While writing the introduction, also slightly changed the
emphasis of the recipe (towards refactoring an existing
application to use ClutterScript) and incorporated example
code into documentation.
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Elliot Smith 2010-08-26 12:51:33 +01:00
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commit 0fc1c8503b
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doc/cookbook/script.xml
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@ -6,14 +6,181 @@
<title>Script</title>
<epigraph>
<attribution>who</attribution>
<para>what</para>
<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>intro</para>
<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 ("Where did I
set the color of that rectangle?") and make sure your edits
don't break any behaviour.</para>
<para>Other 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>; then load
the UI from the JSON and 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.</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) becomes much simpler in JSON.</para>
</listitem>
<listitem>
<para>Less compilation (if you're using a compiled language):
because you can edit 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 <type>ClutterScript</type> API reference describes many
of its useful features. However, the following sections are intended
to give an overview of how <type>ClutterScript</type> works, and
how to use it in an application.</para>
<para>The recipes in this chapter give more detail about
particular aspects of <type>ClutterScript</type>, such as
how to connect signals and how merge multiple JSON definitions
in a single script.</para>
<section>
<title>Basic principles of <type>ClutterScript</type></title>
<para>Clutter is built on top of <ulink href="???">GObject</ulink>,
an object system for C. As in other object systems, properties
are fundamental to GObject. <type>ClutterScript</type>
effectively provides a way to create instances of GObjects and
set their properties. For example:</para>
<informalexample>
<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,
"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>
</informalexample>
<note>
<para>N.B. The numbers in brackets in the example further
explain the JSON structure, and are not part of the UI
definition. See below for more details.</para>
</note>
<calloutlist>
<callout arearefs="script-ui-introduction-json-list-bracket">
<para>list bracket???</para>
</callout>
<callout arearefs="script-ui-introduction-json-object-bracket">
<para>object bracket???</para>
</callout>
<callout arearefs="script-ui-introduction-json-id">
<para>ID???</para>
</callout>
<callout arearefs="script-ui-introduction-json-type">
<para>type???</para>
</callout>
<callout arearefs="script-ui-introduction-json-color-html">
<para>color???</para>
</callout>
<callout arearefs="script-ui-introduction-json-child-by-id">
<para>referencing a child by ID???</para>
</callout>
<callout arearefs="script-ui-introduction-json-no-id">
<para>no ID for implicit objects???</para>
</callout>
<callout arearefs="script-ui-introduction-json-child-by-embedding">
<para>embedded child???</para>
</callout>
<callout arearefs="script-ui-introduction-json-color-word">
<para>color???</para>
</callout>
</calloutlist>
<para>Once you grasp that Clutter objects are GObjects, and you
are setting their properties, you can determine what is scriptable
by referring to the <emphasis>Properties</emphasis> sections
in the Clutter API reference. Any of the properties described
there can be set using <type>ClutterScript</type>.</para>
</section>
<section>
<title>Data types and JSON</title>
<para>The next important consideration is the data type of the
property.??? null, float, int, string, color</para>
<para>special properties which aren't obvious: layout::* ??? perhaps
leave to the appropriate recipe</para>
<para>properties with multiple values??? e.g. children, constraints</para>
<para>objects as property values??? referencing by ID??? objects without IDs???</para>
</section>
</section>
<section id="script-ui">
@ -22,22 +189,42 @@
<section>
<title>Problem</title>
<para>User interfaces are hard to maintain when the code
for creating elements and the code for interacting with
them is mixed together.</para>
<para>???</para>
</section>
<section>
<title>Solution</title>
<para>Use <type>ClutterScript</type> to define the UI
declaratively.</para>
<para>???</para>
</section>
<section>
<title>Discussion</title>
<para>...</para>
<para>???converting an existing program to ClutterScript</para>
</section>
<section>
<title>Full example</title>
<example id="script-ui-example-1">
<title>Description of a user interface in JSON</title>
<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>
</example>
<example id="script-ui-example-2">
<title>Loading a JSON user interface definition in Clutter</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>
</section>
</section>