Xtext Code Completion For Qualified Names

A prefix matcher is responsible for filtering candidates in code completion based on what the user has already entered. Considering qualified names, the user may want to start writing the simple name rather than the fully qualified name (even if the qualified name is necessary at that position.) The FQNPrefixMatcher already does that (which was pointed out to me by Sebastian Zarnekow immediately after publishing the original version of this blog post). However, that implementation fails if the simple name starts with a lower case letter. That is for foo.bar.Baz everything is as it should be, but for foo.bar.baz the prefix b eliminates the candidate (althoug baz starts with b).

This is caused by the FQNPrefixMatcher using the “wrong” LastSegmentFinder when doing no customisation. For foo.bar.Baz it returns Baz as expected, but for foo.bar.baz it returns null. If you don’t want that (or if you want to do any other fancy matching) you could bind your own LastSegmentFinder; something like

public class VerySimpleLastSegmentFinder implements LastSegmentFinder {

	public String getLastSegment(String fqn, char delimiter) {
		try{
			int index = fqn.lastIndexOf(delimiter)+1;
			return fqn.substring(index);
		}catch(Exception e){
		}
		return fqn;
	}
}