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cvs commit: xml-fop/src/documentation/content/design/alt.design/properties getInitialValue.html
pbwest 2003/01/20 06:55:02
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getInitialValue.html
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<h1>Generating Initial Values</h1>
<p>
<font size="-2">by Peter B. West</font>
</p>
<ul class="minitoc">
<li>
<a href="#N10014">Introduction</a>
</li>
<li>
<a href="#N10021">
Generating & Accessing Initial Values
</a>
<ul class="minitoc">
<li>
<a href="#N10062">Properties without
getInitialValue()</a>
</li>
</ul>
</li>
</ul>
<a name="N10014"></a>
<h3>Introduction</h3>
<p>
The <a href="simple-properties.html">previous section</a>
discussed the common data elements in the simplest examples of
property classes. This section discusses more complex classes
whose facilities are accessed only through various methods.
</p>
<a name="N10021"></a>
<h3>
Generating & Accessing Initial Values
</h3>
<p>
<a href= "javascript:parent.displayCode( 'AutoRestore.html' )"
><span class="codefrag"
>org.apache.fop.fo.properties.AutoRestore</span></a> is an
example of the next most complex property class. In
addition to all of the common static fields, these classes
have initial data value types which require the dynamic
generation of a PropertyValue instance.
</p>
<p>
The method <a href= "javascript:parent.displayCode(
'AutoRestore.html#getInitialValue' )" ><span
class="codefrag">PropertyValue getInitialValue(int)</span></a>
returns an instance of PropertyValue of the appropriate
subclass containing the initial value for this property. Like
the static data fields, this value is, in turn, stored in the
array of initial values maintained in the <a href=
"javascript:parent.displayCode(
'PropertyConsts.html#PropertyConstsClass' )" ><span
class="codefrag">PropertyConsts</span></a> singleton <a href=
"javascript:parent.displayCode( 'PropertyConsts.html#pconsts'
)" ><span class="codefrag">pconsts</span></a>.` As with the
fields, the first invocation of the method <a href=
"javascript:parent.displayCode(
'PropertyConsts.html#setupProperty' )" ><span
class="codefrag">setupProperty</span></a> on the property
instantiates the singleton instance of the class, and stores
that instance in the in the <a href=
"javascript:parent.displayCode(
'PropertyConsts.html#properties' )" ><span
class="codefrag">Property[] properties</span></a> array of <a
href= "javascript:parent.displayCode(
'PropertyConsts.html#pconsts' )" ><span
class="codefrag">pconsts</span></a>.
</p>
<p>
Unlike the static data fields, however, the initial value is
not immediately generated. It is generated by a call to <a
href= "javascript:parent.displayCode(
'PropertyConsts.html#getInitialValue' )" ><span
class="codefrag">PropertyValue getInitialValue(int)</span></a>
in <a href= "javascript:parent.displayCode(
'PropertyConsts.html#pconsts' )" ><span
class="codefrag">pconsts</span></a>. This call, in turn,
locates the relevant instance of the particular property class
in the <a href= "javascript:parent.displayCode(
'PropertyConsts.html#properties' )" ><span class= "codefrag"
>properties</span> array of <span class= "codefrag"
>PropertyConsts</span></a>, and invokes the <span class=
"codefrag" >getInitialValue()</span> of that instance. A
side-effect of this call is to store the initial value in <a
href= "javascript:parent.displayCode(
'PropertyConsts.html#initialValues' )" ><span
class="codefrag">PropertyValue[] initialValues</span></a>.
</p>
<a name="N10062"></a>
<h4>Properties without
getInitialValue()</h4>
<p>
What about property classes which have no <span
class="codefrag">getInitialValue()</span> method? The
simplest classes, e.g. <span
class="codefrag">Character</span>, fall into this category.
As <a href="classes-overview.html#property-classes">noted
previously</a>, all of the property classes extend <span
class="codefrag">org.apache.fop.fo.properties.Property</span>.
<a href= "javascript:parent.displayCode(
'Property.html#PropertyClass' )" ><span
class="codefrag">Property</span></a> provides a base <a href=
"javascript:parent.displayCode(
'Property.html#getInitialValue' )" ><span
class="codefrag">PropertyValue getInitialValue(int)</span></a>
method to which the simple classes fall back. Note that it is
only valid for <span class="codefrag">NOTYPE_IT</span>, <span
class="codefrag">AUTO_IT</span>, <span
class="codefrag">NONE_IT</span> and <span
class="codefrag">AURAL_IT</span> initial value types, so all
classes which have any other initial value type must override
this method.
</p>
<p>
<strong>Previous:</strong> <a href = "simple-properties.html"
>Simple property classes</a>
</p>
<p>
<strong>Next:</strong> <a href= "enumerated-values.html"
>Enumerated values</a>
</p>
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