=Accessors=
Say, you have several gigabytes of data in a file. The whole object doesn't need to be serialized at once. You can read and write the value partially using [[svn:foundation/databoard/trunk/org.simantics.databoard/src/org/simantics/databoard/accessor/|Accessor]] interface. The actual container can be a file, memory byte[]/ByteBuffer or a Java Object. The content is structured as tree using Databoard's type system. All but referable records are supported (=no recursion in accessors). 

'''[../javadoc/org/simantics/databoard/accessor.html|org.simantics.databoard.accessor] interfaces'''.
{| style="background-color: #e9e9e9; border: 1px solid #aaaaaa; "
| '''Class'''
| '''Description'''
|- style="background-color: #f9f9f9; " |
| [../javadoc/org/simantics/databoard/accessor/Accessor.html|Accessor]
| Base class for all data Accessors
|- style="background-color: #f9f9f9; " |
| [../javadoc/org/simantics/databoard/accessor/RecordAccessor.html|RecordAccessor]
| Record 
|- style="background-color: #f9f9f9; " |
| [../javadoc/org/simantics/databoard/accessor/ArrayAccessor.html|ArrayAccessor]
| Array - an ordered sequence of elements of one value.
|- style="background-color: #f9f9f9; " |
| [../javadoc/org/simantics/databoard/accessor/MapAccessor.html|MapAccessor]
| Map - an ''ordered'' map of keys to values. 
|- style="background-color: #f9f9f9; " |
| [../javadoc/org/simantics/databoard/accessor/UnionAccessor.html|UnionAccessor]
| Union
|- style="background-color: #f9f9f9; " |
| [../javadoc/org/simantics/databoard/accessor/BooleanAccessor.html|BooleanAccessor],[../javadoc/org/simantics/databoard/accessor/IntAccessor.html|IntAccessor],[../javadoc/org/simantics/databoard/accessor/LongAccessor.html|LongAccessor],[../javadoc/org/simantics/databoard/accessor/FloatAccessor.html|FloatAccessor],[../javadoc/org/simantics/databoard/accessor/DoubleAccessor.html|DoubleAccessor]
| Primitive and numeric Accessors
|- style="background-color: #f9f9f9; " |
| [../javadoc/org/simantics/databoard/accessor/StringAccessor.html|StringAccessor]
| String 
|- style="background-color: #f9f9f9; " |
| [../javadoc/org/simantics/databoard/accessor/OptionalAccessor.html|OptionalAccessor]
| Optional value
|- style="background-color: #f9f9f9; " |
| [../javadoc/org/simantics/databoard/accessor/VariantAccessor.html|VariantAccessor]
| Variant value
|}

[../javadoc/org/simantics/databoard/Accessors.html|Accessors] and [../javadoc/org/simantics/databoard/Files.html|Files] are facade classes that contains utilities for instantiating and handling Accessors.

<code>[../javadoc/org/simantics/databoard/accessor/binary/index.html|Binary Accessor]</code> is an access to a value in binary format (<tt>byte[]</tt> or <tt>ByteBuffer</tt>).

'''Example:''' Binary accessor
<pre class="code">
Datatype type = Datatypes.getDatatype( Rectangle2D.Double.class );
Binding binding = Bindings.getBinding( Rectangle2D.Double.class );
Serializer s = Binding.getSerializer( binding );

// Serialize rectangle
Rectangle2D rect = new Rectangle2D.Double(0,0, 10, 10);
byte[] data = s.serialize(rect);

// Open accessor to byte data and modify first field in the byte data
RecordAccessor ra = Accessors.getAccessor(data, type);
ra.setFieldValue(0, Bindings.DOUBLE, 5.0);

// Deserialize values from the byte data back to the rectangle object
s.deserialize(data, rect);
System.out.println(rect.getX());
</pre>

'''Example:''' File accessor, create
<pre class="code">
RecordType type = Datatypes.getDatatype( Rectangle2D.Double.class );
// Create a new file and initialize it with rectangle type, and open file accessor
FileRecordAccessor fa = Accessors.createFile( file, type );

// Write the first field (x)
fa.setFieldValue(0, Bindings.DOUBLE, 5.0);
fa.close();
</pre>

'''Example:''' File accessor, open
<pre class="code">
// Open an accessor to an existing binary file
FileVariantAccessor fa = Accessors.openAccessor(file);
RecordAccessor ra = fa.getContentAccessor();

// Read the first field (x)
Double x = (Double) ra.getFieldValue(0, Bindings.DOUBLE);
fa.close();
</pre>

'''Example:''' Java Accessor
<pre class="code">
Binding binding = Bindings.getBinding(Rectangle2D.Double.class);
Rectangle2D rect = new Rectangle2D.Double(0,0, 10, 10);

// Open accessor to rectangle
RecordAccessor ra = Accessors.getAccessor(binding, rect);

// Set rectangle's first field (x) to 5.0
ra.setFieldValue(0, Bindings.DOUBLE, 5.0);
System.out.println( rect.getX() );
</pre>


==Accessor Reference==
Accessors can be opened to a sub-nodes with AccessorReference or by calling getAccessor. AccessorReference is a string of instances, either accessor type specific of LabelReferences. 
<pre class="code">
    ChildReference ref = ChildReference.compile(
      new NameReference("node"),
      new ComponentReference()
    );
    Accessor child = accessor.getComponent( ref );

    ChildReference ref = ChildReference.compile(
       new LabelReference("node"),
       new LabelReference("v")
    );
    Accessor child = accessor.getComponent( ref );

    ChildReference ref = ChildReference.create("n-node/v");
    Accessor child = accessor.getComponent( ref );

    ChildReference ref = ChildReference.create("node/v");
    Accessor child = accessor.getComponent( ref );

    VariantAccessor va = recordAccessor.getFieldAccessor("node");
    Accessor child = va.getValueAccessor();
</pre>

==Listening mechanism==
Accessor offers a monitoring mechanism for the data model. 
There is an <code>[../javadoc/org/simantics/databoard/accessor/interestset/InterestSet.html|InterestSet]</code> that is a description of a sub-tree that is to be monitored of the data model.
<code>[../javadoc/org/simantics/databoard/accessor/event/Event.html|Events]</code> are objects that spawned on changes to the data model. Each event object is annotated with [../javadoc/org/simantics/databoard/accessor/reference/index.html|reference path] that is in relation to the node where the listener was placed.

Accessor Listeners use [EventThread Pattern] pattern.