Add isTransform(), isEmpty() and isPresent() to ChannelTransformation

[jimtng]

This is needed by the user of this class, e.g. mqtt binding, to know whether to convert the command to StringType when transformations were involved, or continue using the native type.

Example use cases

https://github.com/openhab/openhab-addons/blob/0d6feb72aa23dcfce946e1367ea0ce8b8c03b330/bundles/org.openhab.binding.mqtt.generic/src/main/java/org/openhab/binding/mqtt/generic/ChannelState.java#L136

https://github.com/openhab/openhab-addons/blob/0d6feb72aa23dcfce946e1367ea0ce8b8c03b330/bundles/org.openhab.binding.mqtt.generic/src/main/java/org/openhab/binding/mqtt/generic/ChannelState.java#L340

isTransform() is needed by the modbus addon

[jimtng]

Modbus refactoring also requires this PR.

[openhab-bot]

This pull request has been mentioned on openHAB Community. There might be relevant details there:

https://community.openhab.org/t/syntax-for-binding-transformations/157861/1

[J-N-K]

I don't understand the need for .isEmpty and .isPresent. In case it is empty, the original value is returned, so what will break if we call .apply in every case?

[jimtng]

See https://github.com/openhab/openhab-addons/blob/0d6feb72aa23dcfce946e1367ea0ce8b8c03b330/bundles/org.openhab.binding.mqtt.generic/src/main/java/org/openhab/binding/mqtt/generic/ChannelState.java#L340

We need the ability to check if a transformation is at all defined. If it's defined, the command to be published will be converted to StringType, and subsequently formatBeforePublish will treat the data as a String data.

If it's not defined, then the original command type will be preserved (not converted to StringType) and the formatBeforePublish can apply type-specific formatting, e.g. special date format.

So that's why we need to be able to know whether transformation is defined.

I can instead check if config.transformationPatternOut is null or blank, and set the outgoingTransformation to null, and check it that way if you don't think we should add isEmpty/isPresent here.

However since ChannelTransformation already does a no-op (return input as output) when it has no transformations (i.e. empty), it's a lot more convenient to add isEmpty/isPresent to it than dealing with null checks around it.

[J-N-K]

IMO it would be fine to do this where needed, but I'm ok with adding it here. Please see my comments.

[J-N-K]

LGTM

[J-N-K]

I have triggered a new core snapshot, should be available in about 35 minutes.