callbacks.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ --> 

<chapter xml:id="yaml.callbacks" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <title>Callbacks</title>

 <section xml:id="yaml.callbacks.parse">
  <title>Parse callbacks</title>
  <para>
   Parse <type>callable</type>s are invoked by
   <function>yaml_parse</function>, <function>yaml_parse_file</function> or
   <function>yaml_parse_url</function> functions when a registered YAML tag is
   encountered. The callback is passed the tagged entity's value, the tag, and
   flags indicating the scalar entity style. The callback must return the data
   that the YAML parser should emit for this entity.
  </para>
  <example>
   <title>Parse callback example</title>
   <programlisting role="php">
<![CDATA[
<?php
/**
 * Parsing callback for yaml tag.
 * @param mixed $value Data from yaml file
 * @param string $tag Tag that triggered callback
 * @param int $flags Scalar entity style (see YAML_*_SCALAR_STYLE)
 * @return mixed Value that YAML parser should emit for the given value
 */
function tag_callback ($value, $tag, $flags) {
  var_dump(func_get_args()); // debugging
  return "Hello {$value}";
}

$yaml = <<<YAML
greeting: !example/hello World
YAML;

$result = yaml_parse($yaml, 0, $ndocs, array(
    '!example/hello' => 'tag_callback',
  ));

var_dump($result);
?>
]]>
   </programlisting>
   &example.outputs.similar;
   <screen>
<![CDATA[
array(3) {
  [0]=>
  string(5) "World"
  [1]=>
  string(14) "!example/hello"
  [2]=>
  int(1)
}
array(1) {
  ["greeting"]=>
  string(11) "Hello World"
}
]]>
   </screen>
  </example>
 </section>

 <section xml:id="yaml.callbacks.emit">
  <title>Emit callbacks</title>
  <para>
   Emit callbacks are invoked when an instance of a registered class is
   emitted by <function>yaml_emit</function> or
   <function>yaml_emit_file</function>. The callback is passed the object to
   be emitted. The callback must return an array having two keys:
   &quot;<literal>tag</literal>&quot; and &quot;<literal>data</literal>&quot;.
   The value associated with the &quot;<literal>tag</literal>&quot; key must
   be a string to be used as the YAML tag in the output. The value associated
   with the &quot;<literal>data</literal>&quot; key will be encoded as YAML
   and emitted in place of the intercepted object.
  </para>
  <example>
   <title>Emit callback example</title>
   <programlisting role="php">
<![CDATA[
<?php
class EmitExample {
  public $data;    // data may be in any pecl/yaml suitable type

  public function __construct ($d) {
    $this->data = $d;
  }

  /**
   * Yaml emit callback function, referred on yaml_emit call by class name.
   *
   * Expected to return an array with 2 values:
   *   - 'tag': custom tag for this serialization
   *   - 'data': value to convert to yaml (array, string, bool, number)
   *
   * @param object $obj Object to be emitted
   * @return array Tag and surrogate data to emit
   */
  public static function yamlEmit (EmitExample $obj) {
    return array(
      'tag' => '!example/emit',
      'data' => $obj->data,
    );
  }
}

$emit_callbacks = array(
  'EmitExample' => array('EmitExample', 'yamlEmit')
);

$t = new EmitExample(array('a','b','c'));
$yaml = yaml_emit(
  array(
    'example' => $t,
  ),
  YAML_ANY_ENCODING,
  YAML_ANY_BREAK,
  $emit_callbacks
);
var_dump($yaml);
?>
]]>
   </programlisting>
   &example.outputs.similar;
   <screen>
<![CDATA[
string(43) "---
example: !example/emit
- a
- b
- c
...
"
]]>
   </screen>
  </example>
 </section>

</chapter>

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