Referencing Values of Other Fields

Quite often there is a need to reuse (or reference) some other values already defined and used for some other fields. The v1 of this specification allowed referencing the external <enum> validValue-s only, while v2 of this specification extends such functionality to other fields as well. In general, when the other field is referenced its defaultValue is taken, unless inner value is referenced, such as validValue of the <enum> field or special value of the <int> field.

Referencing Values Defined in <enum>-s

Any specified <validValue> can be referenced by other numeric fields (not only <enum>) when specifying numeric value of some property. To reference it, the <enum> name must be specified followed by a . (dot) and name of the chosen <validValue>

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <enum name="SomeEnumField" type="uint8" defaultValue="Val2">
            <validValue name="Val1" val="0" />
            <validValue name="Val2" val="5" />
            <validValue name="Val3" val="10"/>
        </enum>

        <int name="SomeIntField" type="uint32" defaultValue="SomeEnumField.Val2" />
    </fields>
</schema>

In the example above the defaultValue of the SomeIntField will be 5.

When <enum> is referenced by its name, its defaultValue is taken.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <enum name="SomeEnumField" type="uint8" defaultValue="Val2">
            <validValue name="Val1" val="0" />
            <validValue name="Val2" val="5" />
            <validValue name="Val3" val="10"/>
        </enum>

        <int name="SomeIntField" type="uint32" defaultValue="SomeEnumField.Val3">
            <special name="S1" val="SomeEnumField" />
            <special name="S2" val="SomeEnumField.Val1" />
        </int>
    </fields>
</schema>

In the example above the defaultValue of the SomeIntField is 10, the value of the S1 special is 5 (equals to defaultValue of SomeEnumField), and value of the S2 special is 0.

Floating point fields can also reference values defined in <enum> fields.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <enum name="SomeEnumField" type="uint8" defaultValue="Val2">
            <validValue name="Val1" val="0" />
            <validValue name="Val2" val="5" />
            <validValue name="Val3" val="10"/>
        </enum>

        <float name="SomeFloatField" type="double" defaultValue="SomeEnumField.Val1">
            <special name="S1" val="SomeEnumField.Val2" />
        </float>
    </fields>
</schema>

Referencing Values Defined in <int>-s

Similar to <enum> the inner value of <int> field can be referenced by other numeric fields.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <int name="SomeIntField" type="uint32" defaultValue="10">
            <special name="S1" val="0" />
            <special name="S2" val="5" />
        </int>

        <enum name="SomeEnumField" type="uint8" defaultValue="SomeIntField.S1">
            <validValue name="Val1" val="SomeIntField.S2" />
            <validValue name="Val2" val="SomeIntField" />
        </enum>

        <float name="SomeFloatField" type="double" defaultValue="SomeIntField.S2">
            <special name="S1" val="SomeIntField" />
        </float>
    </fields>
</schema>

In the example above defaultValue of SomeEnumField is 0, validValue Val1 equals to 5, and validValue Val2 equals to 10.

Also the defaultValue of SomeFloatField is 5.0, while value of its S1 special is 10.0.

Referencing Values Defined in <set>-s

The defaultValue property of any element of the <set> field can also be referenced.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <set name="SomeSetField" length="1" defaultValue="false">
            <bit name="B0" idx="0" defaultValue="true" />
            <bin name="B1" idx="1" />
        </set>

        <set name="SomeOtherSetField" length="1" defaultValue="SomeSetField" reservedValue="SomeSetField.B0">
            <bit name="B5" idx="5" defaultValue="SomeSetField.B1" />
            ...
        </set>
    </fields>
</schema>

In the example above the defaultValue of SomeOtherSetField is false (same as defaultValue of SomeSetField), the reservedValue of SetOtherField is true (same as defaultValue of SomeSetField.B0), and the defaultValue of SomeOtherSetField.B5" is false (same as defaultValue of SomeSetField.B1*).

Other numeric fields, such as <enum>, <int>, and <float> can also reference boolean values of <set>, which will result in numeric values been either 0 or 1.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <set name="SomeSetField" length="1" defaultValue="false">
            <bit name="B0" idx="0" defaultValue="true" />
            <bin name="B1" idx="1" />
        </set>

        <float name="SomeFloatField" type="float" defaultValue="SomeSetField.B0" />
    </fields>
</schema>

The definition above will result in defaultValue of SomeFloatField to be 1.0.

Referencing Values Defined in <float>-s

Similar to <int> it is possible to reference <float> values used in defaultValue property and/or as <special> value.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <float name="SomeFloatField" type="double" defaultValue="nan">
            <special name="S1" val="inf" />
        </float>

        <float name="SomeOtherFloatField" type="double" defaultValue="SomeFloatField.S1">
            <special name="S1" val="SomeFloatField" />
        </float>

    </fields>
</schema>

In the example above defaultValue of SomeOtherFloatField is inf, while value of SomeOtherFloatField.S1 special is nan.

Referencing Values Defined in <string>-s

When referencing values of <string> fields there is a need to differentiate between a reference to external field and a genuine string value. To do so the ^ prefix was introduced. If a property value, that requires a string, starts with ^ it means external reference and error must be reported if referenced field is not found.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <string name="SomeString" defaultValue="hello" />
        <string name="SomeOtherString" defaultValue="^SomeString" />
    </fields>
</schema>

In the example above the defaultValue of SomeOtherString field is hello.

If there is a need to define a genuine string value that starts with ^ character, then there is a need to escape it with \.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <string name="SomeOtherString" defaultValue="\^SomeString" />
    </fields>
</schema>

In the example above the defaultValue of SomeOtherString field is ^SomeString.

The question may arise what if a genuine value string needs to start with \^. In this case just add additional \ at the front.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <string name="SomeOtherString" defaultValue="\\^SomeString" />
    </fields>
</schema>

In the example above the defaultValue of SomeOtherString field is \^SomeString.

The bottom line: any prefix sequence of \ followed by the ^ will result in drop of one \ in the final string value. In case there is any other character used in the middle, the string value remains as is.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <string name="String1" defaultValue="\\SomeString" />
        <string name="String2" defaultValue="\.\^SomeString" />
    </fields>
</schema>

In the example above the defaultValue of String1 field is \SomeString because there is no ^ character after \ and the defaultValue of String2 field is .\^SomeString because the sequence of \ is interrupted by ..

NOTE, that string referenced can be useful when <enum> field is used to specify numeric message IDs.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <string name="Msg1Name" defaultValue="Message 1" />
        <string name="Msg2Name" defaultValue="Message 2" />

        <enum name="MsgId" type="uint8" semanticType="messageId">
            <validValue name="Msg1" val="1" displayName="^Msg1Name" />
            <validValue name="Msg2" val="2" displayName="^Msg2Name" />
        </enum>

        <message name="Msg1" id="MsgId.Msg1" displayName="^Msg1Name">
            ...
        </message>

        <message name="Msg2" id="MsgId.Msg2" displayName="^Msg2Name">
            ...
        </message>
    </fields>
</schema>

In the example above the displayName property of a message is expected to be the same as displayName property of appropriate ≶validValue> of MsgId enum. Referencing common value insures that the change to the name (if happens) propagates to appropriate fields.

Referencing Values Defined in <data>-s

When referencing values of <data> fields there is also a need to differentiate between a reference to external field and a genuine data value. For example the string abcd can be interpreted as valid field name as well as valid hexadecimal bytes. As the result there is also a need to use ^ prefix (just like with <string> values) to indicate external reference.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <data name="SomeData" defaultValue="12 34 56" />
        <string name="SomeOtherData" defaultValue="^SomeData" />
    </fields>
</schema>

The defaultValue of SomeOtherData will be 0x12 0x34 0x56.

Referencing Values via <ref>-s

The <ref> field is there to create an alias to other field. The CommsDSL allows retrieving value for the <ref> field as if it was retrieved from the referenced field.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <int name="SomeInt" type="uint8" defaultValue="1">
            <special name="S1" val="5" />
        </int>
        <ref name="SomeRef" field="SomeInt" />
        <int name="SomeOtherInt" type="uint18" defaultValue="SomeRef.S1" />
    </fields>
</schema>

In the example above the defaultValue of SomeOtherInt is 5, same as the value of SomeInt.S1.

Referencing Values in Namespaces

In case referenced field resides in a namespace, add it to the reference string as well. The same referencing rules apply.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <ns name="ns1"
        <fields>
            <enum name="SomeEnumField" type="uint8">
                <validValue name="Val1" val="0" />
                <validValue name="Val2" val="5" />
                <validValue name="Val3" val="10"/>
            </enum>

            <int name="SomeIntField" type="uint32" defaultValue="ns1.SomeEnumField.Val2" />
        </fields>
    </ns>
</schema>

Referencing Values in <bitfield>-s or <bundle>-s

The CommsDSL also allows referencing values from member fields of a <bitfield> or a <bundle>.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <ns name="ns1"
        <fields>
            <bundle name="SomeBundle">
                <enum name="SomeEnumMember" type="uint8">
                    <validValue name="Val1" val="0" />
                    <validValue name="Val2" val="5" />
                    <validValue name="Val3" val="10"/>
                </enum>
                <int name="SomeIntMember" type="uint8" />
            </bundle>            

            <int name="SomeIntField" type="uint32" defaultValue="ns1.SomeBundle.SomeEnumField.Val2" />
        </fields>
    </ns>
</schema>

In the example above the defaultValue of SomeIntField is 5.

Referencing Values in <optional>-s;

There are two forms of <optional> fields. One references external field, another defines it as a member.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <int name="SomeInt" type="uint8" defaultValue="1">
            <special name="S1" val="5" />
        </int>
        <optional name="Opt1" field="SomeInt" />
        <optional name="Opt2">
            <int name="SomeOptInt" type="uint8" defaultValue="1">
                <special name="S1" val="5" />
            </int>
        </optional>
    </fields>
</schema>

In case the <optional> field references external field it can NOT be used for value reference. The one that defines optional field internally as a child element, can.

<?xml version="1.0" encoding="UTF-8"?>
<schema ...>
    <fields>
        <optional name="Opt2">
            <int name="SomeOptInt" type="uint8" defaultValue="1">
                <special name="S1" val="5" />
            </int>
        </optional>
        <int name="SomeOtherInt" type="int16" defaultValue="Opt2.SomeOptInt.S1" />
    </fields>
</schema>

NOTE that there is a need to reference internal member field by name.

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