Add some sort of extension point in `CBORParser` for pluggable custom-tag support

[cowtowncoder]

(note: inspired by FasterXML/jackson-core#1428)

Since tag system in CBOR is extensible, users may well want to use custom tags.
Although there is a very bare-bones accessibility mechanism with CBORParser, basically 2 methods:

   public int getCurrentTag() {
        return _tagValues.getFirstTag();
    }

    public TagList getCurrentTags() {
        return _tagValues;
    }

it would be good to have some sort of mechanism (callback(s)?) to allow some automation of handling -- to give low-level access to CBORParser state upon encountering tags (either ones not supported by Jackson CBOR module, or maybe all).

I don't know exactly how that should look like so help would be appreciated.

[honhimW]

@cowtowncoder

I have currently succeeded in using jackson-cbor to handle serialization/deserialization with custom tags. I'm not sure if my implementation is sufficient, but it works exactly as I expected🎉.

From my experience, during serialization, I can continue using the JsonSerializer interface, so the main issues are concentrated in the Parser part.

• As I mentioned in another issue(​​Question:​​ Delegating a parser/generator like ParserMinimalBase or ParserBase jackson-core#1428), ParserMinimalBase#_updateToken(JsonToken) require reflection;
• For composite structures, they are converted to TreeNode first and then further transformed into the target type, and a context is needed to store the result. In such, I can reuse most of the functionality without having to process each token individually;
• If possible, use JsonDeserializer for deserialization, but it seems unlikely?
• Perhaps a Visitor-like approach could be added for callbacks, such as how to handle when a certain tag is encountered?

Note: My approach may not be optimal, but I hope it can be helpful to you.
Refs:
Code
Curtom Tags

[fbacchella]

No one started thinking about it ? CBOR without handling tags is quite useless.

[fbacchella]

I don’t have big hope for this, the code is not plugable and it’s a lot of work.
So I wrote a custom parser based on Jackson: https://github.com/fbacchella/LogHub/blob/master/loghub-cbor/src/main/java/loghub/cbor/CborParser.java.
I also implemented tags handling as a java Service, see https://github.com/fbacchella/LogHub/blob/master/loghub-cbor/src/main/java/loghub/cbor/CborTagHandlerService.java.
It’s also used also by a generic serializer, https://github.com/fbacchella/LogHub/blob/master/loghub-cbor/src/main/java/loghub/cbor/CborSerializer.java.
Please feel free to steal the code.