<value> Layer

The <value> layer represents extra values (such as protocol version and/or extra flags), that are applicable to all the messages, but delivered as part of transport framing instead of message payload.

<?xml version="1.0" encoding="UTF-8"?>
<schema endian="big" ...>
    <frame name="ProtocolFrame">
        <size name="Size">
            ...
        </size>
        <value name="Version" ...>
            <int name="VersionField" type="uint8" />
        </value>
        <id name="Id">
            ...  
        </id>
        <payload name="Data" />
    </frame>
</schema>

The <value> layer has all the common properties as well as extra properties and elements described below.

Interfaces

In most cases the received value must be reassigned to appropriate field of the <interface>. To specify the supported interfaces use interfaces property. The value of the property is comma separated list of reference names of the supported interfaces.

<?xml version="1.0" encoding="UTF-8"?>
<schema endian="big" ...>
    <interface name="Interface1">
        <int name="Version" type="uint8" semanticType="version" />
    </interface>

    <interface name="Interface2">
        <int name="Version" type="uint8" semanticType="version" />
        <set name="Flags> length="1">
            ...
        </set>
    </interface>


    <frame name="ProtocolFrame">
        <size name="Size">
            ...
        </size>
        <value name="Version" interfaces="Interface1, Interface2" ...>
            <int name="VersionField" type="uint8" />
        </value>
        <id name="Id">
            ...  
        </id>
        <payload name="Data" />
    </frame>
</schema>

In addition to specifying the interfaces themselves, there is a need to specify the field name in the interface(s), that will hold the processed value, using interfaceFieldName property.

<?xml version="1.0" encoding="UTF-8"?>
<schema endian="big" ...>
    <interface name="Interface1">
        <int name="Version" type="uint8" semanticType="version" />
    </interface>

    <interface name="Interface2">
        <int name="Version" type="uint8" semanticType="version" />
        <set name="Flags> length="1">
            ...
        </set>
    </interface>


    <frame name="ProtocolFrame">
        <size name="Size">
            ...
        </size>
        <value name="Version" interfaces="Interface1, Interface2">
            <interfaceFieldName value="Version" />
            <field>
                <int name="VersionField" type="uint8" />
            </field>
        </value>
        <id name="Id">
            ...  
        </id>
        <payload name="Data" />
    </frame>
</schema>

NOTE, that all the interfaces listed in the interfaces property value must have a field with the name specified as the interfaceFieldName value. Also the specified field's index (among other available fields of the <interface>) must be the same for all the listed interfaces.

Pseudo Value Layer

There are protocols that don't report protocol version in their transport framing. Instead, they use some special "connection" message that report protocol version. As the result, all subsequent messages must adhere to the reported version. The CommsDSL resolves this problem by defining pseudo <value> layer using pseudo property with boolean value. The field of the pseudo <value> layer does not get serialized. However, the code generator must allow external assignment of the required value to the private data members of the <value> layer's class.

<?xml version="1.0" encoding="UTF-8"?>
<schema endian="big" ...>
    <interface name="Interface1">
        <int name="Version" type="uint8" semanticType="version" />
    </interface>

    <message name="Connect" id="1">
        <int name="ProtocolVersion" type="uint8" />
    </message>

    <frame name="ProtocolFrame">
        <size name="Size">
            ...
        </size>
        <id name="Id">
            ...  
        </id>
        <value name="Version" interfaces="Interface1" pseudo="true">
            <interfaceFieldName value="Version" />
            <field>
                <int name="VersionField" type="uint8" />
            </field>
        </value>
        <payload name="Data" />
    </frame>
</schema>

The location of the pseudo <value> layer within the <frame> is not important. It is recommended to define it right before the <payload> layer.

As part of processing Connect message in the example above, the client code is expected to retrieve the value of the ProtocolVersion field and report it to the pseudo <value> layer within the processing frame. For all the subsequent messaged, the pseudo <value> layer is responsible to assign appropriate value (version in the example above) to the newly created message object before message payload gets read. This is because such values may have an influence on how the read operation is executed (existence of some fields may depend on the assigned value).

Use properties table for future references.

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