<list> Field

This field stores and abstracts away a list of other fields. The <list> field has all the common properties as well as extra properties and elements described below.

Element Field of the List

Every <list> must specify its element field. The element field can be defined as XML child of the <list> definition.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <list name="SomeListField">
            <int name="Element" type="uint32" />
        </list>
    </fields>
</schema>

If a list element contains multiple fields, they must be bundled as members of the <bundle> field.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <list name="SomeListField">
            <bundle name="Element">
                <int name="Mem1" type="uint32" />
                <int name="Mem2" type="int16" />
                <enum name="Mem3" type="uint16">
                    ...
                </enum>
            </bundle>
        </list>
    </fields>
</schema>

In case other properties of the <list> field are defined as child XML elements, then the element field definition needs to be wrapped by <element> XML child.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <list name="SomeListField">
            <displayName value="Some descriptive name" />
            <element>
                <bundle name="Element">
                    ...
                </bundle>
            </element>
        </list>
    </fields>
</schema>

The CommsDSL also allows reference of externally defined field to be an element using element property.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <bundle name="ExternallyDefinedElement">
            ...
        </bundle>

        <list name="SomeListField" element="ExternallyDefinedElement" />
    </fields>
</schema>

Fixed Count

If the defined list must contain predefined number of elements, use count property to provide the required information.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <list name="SomeListField" count="8">
            <int name="Element" type="uint32" />
        </list>
    </fields>
</schema>

Count Prefix

Most protocols prefix the variable length lists with number of elements that are going to follow, use countPrefix child XML element to specify a field that is going to be used as such prefix.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <list name="SomeListField">
            <element>
                <int name="Element" type="uint32" />
            </element>
            <countPrefix>
                <int name="CountPrefix" type="uint16" />
            </countPrefix>
        </list>
    </fields>
</schema>

In case the count prefix field is defined as external field, CommsDSL allows usage of countPrefix as property, value of which contains name of the referenced field.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <int name="ExternalCountPrefix" type="uint16" />

        <list name="SomeListField" countPrefix="ExternalCountPrefix">
            <int name="Element" type="uint32" />
        </list>
    </fields>
</schema>

The CommsDSL also supports detached count prefix, when there are several other fields in the <message> or in the <bundle> between the count field and the <list>.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <bundle name=SomeBundle">
            <int name="DataCountPrefixMember" type="uint16" />
            <int name="SomeOtherFieldMember" type="uint16" />
            <list name="SomeDataFieldMember" countPrefix="$DataCountPrefixMember">
                ...
            </list>
        </bundle>
    </fields>

    <message name="Msg1" id="1">
        <int name="DataCountPrefix" type="uint16" />
        <int name="SomeOtherField" type="uint16" />
        <list name="SomeListField" countPrefix="$DataCountPrefix">
            ...
        </list>
    </message>
</schema>

NOTE, the existence of $ prefix when specifying countPrefix value. It indicates that the referenced field is a sibling in the containing <message> or the <bundle> field.

The code generator is expected to take the existence of such detached prefix into account and generate correct code for various field operations (read, write, etc...).

Length Prefix

There are protocols that prefix a list with serialization length rather than number of elements. In this case use lengthPrefix instead of countPrefix. The allowed usage scenarios are exactly the same as described above in the Count Prefix section.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <list name="List1">
            <element>
                <int name="Element" type="uint32" />
            </element>
            <lengthPrefix>
                <int name="LengthPrefix" type="uint16" />
            </lengthPrefix>
        </list>

        <int name="ExternalLengthPrefix" type="uint16" />
        <list name="List2" lengthPrefix="ExternalLengthPrefix">
            <int name="Element" type="uint32" />
        </list>
    </fields>

    <message name="Msg1" id="1">
        <int name="DetachedLengthPrefix" type="uint16" />
        <int name="SomeOtherField" type="uint16" />
        <list name="List3" lengthPrefix="$DetachedLengthPrefix">
            ...
        </list>
    </message>
</schema>

NOTE, that count, countPrefix and lengthPrefix properties are mutually exclusive, i.e. cannot be used together.

Element Length Prefix

Some protocols prefix every element with its serialization length for the forward / backward compatibility of the protocol. If there is such need, use elemLengthPrefix to specify a field that will prefix every element of the list.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <list name="List1">
            <element>
                <bundle name="Element>
                    ...
                </bundle>
            </element>
            <countPrefix>
                <int name="CountPrefix" type="uint16" />
            </countPrefix>
            <elemLengthPrefix>
                <int name="ElemLengthPrefix" type="uint8" />
            </elemLengthPrefix>
        </list>

        <int name="ExternalElemLengthPrefix" type="uint8" />
        <list name="List2" count="16" elemLengthPrefix="ExternalElemLengthPrefix">
            <bundle name="Element>
                ...
            </bundle>
        </list>
    </fields>
</schema>

In case every list element has fixed length and protocol specification doesn't allow adding extra variable length fields to the element in the future, some protocols prefix only first element in the list with its serialization length. CommsDSL supports such lists with elemFixedLength property, that has boolean value.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <list name="SomeListField" elemFixedLength="true" count="8">
            <element>
                <bundle name="Element">
                    <int name="Mem1" type="uint32" />
                    <int name="Mem2" type="uint32" />
                    ...
                </bundle>
            </element>
            <elemLengthPrefix>
                <int name="ElemLengthPrefix" type="uint8" />
            </elemLenghtPrefix>
        </list>
    </fields>
</schema>

The code generator must report an error when element of such list (with elemFixedLength property set to true) has variable length.

Use properties table for future references.

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