<module name="XML::Schema::Attribute">

  <about>
    The XML::Schema::Attribute module implements a class of objects
    that represent XML Schema attributes.
  </about>

  <synopsis>
<perl>
use XML::Schema::Attribute;

# list of arguments
my $attr = XML::Schema::Attribute->new( name => 'foo', type => 'string' )
    || die XML::Schema::Attribute->error();

# hash ref of arguments
my $attr = XML::Schema::Attribute->new( {
    name      => 'bar', 
    type      => XML::Schema::Type::string->new(),
    scope     => $complex_type,
    default   => 20,
    namespace => 'http://tt2.org/XML/Example.xml',
} ) || die XML::Schema::Attribute->error();
</perl>
  </synopsis>

  <description>
    <p>
    This module implements an object class for representing XML
    attributes with XML Schema.  An attribute is, of course, a
    '<xcode>name="value"</xcode>' pair within the starting tag of an
    XML element.  In the following example, the attributes are named
    '<xcode>id</xcode>' and '<xcode>name</xcode>'.
    </p>

<markup>
<user id="lwall" name="Larry Wall">
    ...
</user>
</markup>
  
    <p>
    Attribute are defined as having a specific type which determines
    what the acceptable values can be.  These types are implemented by
    various objects derived from the
    <module>XML::Schema::Type::Simple</module> module.  There are over
    30 inbuilt simple type, defined in the
    <module>XML::Schema::Type::Builtin</module> module, and which
    include things like string, integer, float, time, date, and so on.
    Furthermore, you can extend these basic types to create your own
    custom simple types by applying additional <module
    class="XML::Schema::Facet">validation facets</module>. 
    </p>

    <p>
    An attribute can also define default and fixed values for itself.
    A fixed constraint specifies a value that the attribute must have,
    if present in a particular instance document.  A default value can 
    be specified for attributes that are missing.
    </p>

    <p>
    An attribute can be defined within a particular scope.  Usually,
    it is within the definition of a <module
    class="XML::Schema::Type::Complex">complex type</module>, but can 
    also exist within an <module
    class="XML::Schema::Attribute::Group">attribute group</module>.
    The attribute communicates with its enclosing scope to resolve
    its type name (e.g. 'string') with a type object.  This allows 
    types to effectively be used before they are defined, and also 
    for attributes to be relocated for use in different scope (e.g.
    as part of an attribute group) and to resolve the correct type
    in each location.
    </p>
    
  </description>

  <methods>
    <method id="new">
      <p>
        Constructor method called to create a new attribute object.  A
        list of '<perlcode>key =&gt; value</perlcode>' pairs can be
        specified as command line arguments, or alternately, a hash
        reference can be passed which contains these configuration
        values.  The method returns a newly instantiated object on
        success.  On error it returns undef and sets an internal error
        message which can be retrieved by calling <method
        class="XML::Schema::Base">error()</method> as a class method.

<perl>
# list of options
my $attr = XML::Schema::Attribute->new( name => 'foo', type => 'string' )
    || die XML::Schema::Attribute->error();

# hash ref of options
my $attr = XML::Schema::Attribute->new( {
    name => 'foo', 
    type => 'string',
} ) || die XML::Schema::Attribute->error();
</perl>
      </p>

      <p>
        The following configuration options may be specifed:
      </p>

      <config>
        <item key="name" value="'myattr'">
	    The name of the attribute.  In the XML fragment <xcode><foo bar="baz"/></xcode>
            the attribute name is '<xcode>bar</xcode>'.  This item is mandatory.
        </item>

        <item key="type">
            <value>'typename'</value>
            <value><module class="XML::Schema::Type::Simple">$type</module></value>
	    The simple type for this attribute specified either by name or as 
	    as reference to an <module>XML::Schema::Type::Simple</module> object
	    or subclass thereof.  This item is also mandatory.  The remaining items
	    are optional.
        </item>

        <item key="constraint">
            <value>[ fixed => 3.14 ]</value> 
            <value>[ default => 43 ]</value>
            This option can be used to specify a fixed
            constraint for the attribute, asserting that any specified
            instance value matches the fixed value.  The second use
            shows how a default value can be specified which is used
            when no specific instance value is provided.  Note that
            '<perlcode>constrain</perlcode>' is a valid alias for
            '<perlcode>constraint</perlcode>'.
        </item>

        <item key="fixed" value="3.14">
	    An alternate way to specify a fixed constraint for an attribute.
	</item>

        <item key="default" value="42">
	    An alternate way to specify a default value for an attribute.
	</item>

        <item key="scope">
	    <value><module class="XML::Schema::Scope">$scope</module></value>
	    This configuration item can be used to bind the attribute to a 
            particular lexical scope.  The attribute <method>type()</method>
            method is then able to retrieve type objects from type names via
            the scope reference.
	</item>

        <item key="namespace" value="'http://tt2.org/XML/xyz.xml'">
	    The optional XML namespace for the attribute.
        </item>

        <item key="annotation" value="'...interesting note...'">
	    An optional annotation for the attribute.  This is not yet
	    fully supported.
        </item>

      </config>
    </method>

    <method id="name">
      Returns the attribute name.
<perl>
my $name = $attribute->name();
</perl>

    </method>

    <method id="type">
      Returns the attribute type, as a reference to an object derived
      from <module>XML::Schema::Type::Simple</module>.  If the
      attribute type was specified as a name rather than a direct
      reference to a type then the method will first attempt to 
      fetch the type object matching that name.  It does this by
      delegation to its enclosing lexical scope which can be specified
      by the <cfgvar>scope</cfgvar> configuration item.
<perl>
my $typeobj = $attribute->type();
</perl>
    </method>

    <method id="namespace">
      <args></args>
      <args>$new_namespace</args>
      Returns the attribute namespace as optionally specified by the
      <cfgvar>namespace</cfgvar> configuration item.  Can also be 
      called with a single argument to set a new namespace value.
<perl>
my $ns = $attribute->namespace();

$attribute->namespace('http://tt2.org/XML/Example.xml');
</perl>
    </method>


    <method id="scope">
      <args></args>
      <args>$new_scope</args>

      Returns the current scope as specified in the
      <cfgvar>scope</cfgvar> configuation item.  Can also be called
      with a single argument to define a new scope for the attribute.
      This should be a reference to an object which inherits from the
      <class>XML::Schema::Scope</class> class.

<perl>
my $scope = $attribute->scope();

$attribute->scope($new_scope);
</perl>
    </method>

    <method id="constraint">
      <args></args>
      <args>default => $value</args>
      <args>fixed => $value</args>
      <p>
        Accessor method which can be used to get or set the current
        value constraints.  The value constraint is a pair consisting
        of a constraint type, which must be one of
        '<perlcode>fixed</perlcode>' or '<perlcode>default</perlcode>',
        and a value.  Returns the pair <perlcode>($type, $value)</perlcode>
        when called without any arguments.
      </p>
<perl>
my ($type, $value) = $attribute->constraint();
</perl>
      <p>
	When called with appropriate <perlcode>($type, $value)</perlcode>
        arguments, the method sets the current
	constraint by calling either the default() or fixed() methods.
      </p>
<perl>
$attribute->constraint( default => 42 );
$attribute->constraint( fixed   => 84 );

# equivalent to
$attribute->default(42);
$attribute->fixed(84);
</perl>

      <p>
      Note that <perlcode>constrain()</perlcode> is a valid alias for
      <perlcode>constraint()</perlcode>.
      </p>

<perl>
# equivalent
$attribute->constrain( fixed => 42 );
$attribute->constraint( fixed => 42 );
</perl>

    </method>

    <method id="default">
      <args></args>
      <args>$value</args>
      <p>
        Accessor method which can be used to get or set the current
        default value constraint.  When called with an argument, the
        method will force the current value constraint to be 'default'
        and set the value internally.
      </p>
<perl>
$attribute->default(42);
</perl>
      <p>
	Returns the current default value when called without any
	arguments.
      </p>
<perl>
my $default = $attribute->default();
</perl>
      <p>
	If the current value constraint is not defined or is set to a
	type other than 'default' (e.g. if a 'fixed' constraint has
	been specified) then the method returns undef and sets an
	internal error value which can be retrieved via the 
	<method class="XML::Schema::Base">error()</method>
	method.
      </p>
<perl>
my $default = $attribute->default();
die $attribute->error() unless defined $default;
</perl>
    </method>

    <method id="fixed">
      <args></args>
      <args>$value</args>
      <p>
	Accessor method which can be used to get or set the current
	fixed value constraint.  As per <method>default()</method>,
	this method can be called with or without an argument and in
	the latter case may return undef if a fixed constraint type
	isn't defined.
      </p>
<perl>
$attribute->fixed(84);
my $fixed = $attribute->fixed();
die $attribute->error() unless defined $fixed;
</perl>
    </method>


    <method id="instance">
      <args>$value</args>
      <p>
	This method attempts to create an instance of the attribute.
        The text value parsed from an instance document attribute is
        passed to the method as an argument, <perlcode>$value</perlcode>.
	This is then validated according to the underlying type of the 
        attribute and checked against any <cfgvar>default</cfgvar> or 
        <cfgvar>fixed</cfgvar> constraints.  If all is well then the 
        attribute is "activated", by calling any actions scheduled 
        against the type or attribute.  The method returns the result
	after activation or the regular post-validation result if no
	callbacks are scheduled.
      </p>
<perl>
my $val = $attribute->instance('hello world')
    || die $attribute->error();
</perl>
    </method>
  </methods>
</module>

