Separate parsing impl from setter in SearchParseElement

[synhershko]

Currently all SearchParseElement classes are implemented like this:

public class HighlighterParseElement implements SearchParseElement {
@OverRide
public void parse(XContentParser parser, SearchContext context) throws Exception {
// actual parsing logic
context.highlight(new SearchContextHighlight(fields));
}
}

For some (advanced) uses I want to be able to parse individual elements myself by relying on the actual core implementation, and to do that I need to use a hack like this:

                        HighlighterParseElement tmp = new HighlighterParseElement();
                        SearchContext ctx = new SearchContext(0, null, null, null, null, null, null, null);
                        tmp.parse(parser, ctx);
                        searchContextHighlight = ctx.highlight();

Just for the sake of parsing. It would be nice if the actual parsing logic could be separated from the call to the setter, something like this:

@Override
public void parse(XContentParser parser, SearchContext context) throws Exception {
    try {
        context.highlight(parseImpl(parser));
    }
    catch (IllegalArgumentException ex)
    {
        throw new SearchParseException(context, "Highlighter global preTags are set, but global postTags are not set");
    }
}

public static SearchContextHighlight parseImpl(XContentParser parser) throws Exception {
  // actual impl

}

[synhershko]

Hey ES people is it something you'd consider or should I close this one?

[s1monw]

@synhershko I think we should do this - do you wanna come up with a PR?

[synhershko]

@s1monw will do

[synhershko]

Hey @s1monw please see #6758

I added one commit changing only HighlighterParseElement for now, please review and if my approach makes sense (the re-throwing logic etc) I'll go ahead and apply this all over the place