Xtext 2.8+ formatter, formatting HiddenRegion with comment - eclipse

Question

I am using Xtext 2.9 formatter and I am trying to format hiddenRegion which contains comment. Here is part of my document region i am trying to format:
Columns: 1:offset 2:length 3:kind 4: text 5:grammarElement
Kind: H=IHiddenRegion S=ISemanticRegion B/E=IEObjectRegion
35 0 H
35 15 S ""xxx::a::b"" Refblock:namespace=Namespace
50 0 H
50 1 S "}" Refblock:RCBRACKET
E Refblock PackageHead:Block=Refblock path:PackageHead/Block=Package'xxx_constants'/head=Model/packages[0]
51 0 H
51 1 S ":" PackageHead:COLON
E PackageHead Package:head=PackageHead path:Package'xxx_constants'/head=Model/packages[0]
52 >>>H "\n " Whitespace:TerminalRule'WS'
"# asd" Comment:TerminalRule'SL_COMMENT'
15 "\n " Whitespace:TerminalRule'WS'<<<
B Error'ASSD' Package:expressions+=Expression path:Package'xxx_constants'/expressions[0]=Model/packages[0]
67 5 S "error" Error:'error'
72 1 H " " Whitespace:TerminalRule'WS'
and corresponding part of the grammar
Model:
{Model}
(packages+=Package)*;
Expression:
Error | Warning | Enum | Text;
Package:
{Package}
'package' name=Name head=PackageHead
(BEGIN
(imports+=Import)*
(expressions+=Expression)*
END)?;
Error:
{Error}
('error') name=ENAME parameter=Parameter COLON
(BEGIN
(expressions+=Estatement)+
END)?;
PackageHead:
Block=Refblock COLON;
Problem is that when i try prepend some characters before error keyword
for example
error.regionFor.keyword('error').prepend[setSpace("\n ")]
This indentation is prepended before the comment and not behind it. This results into improper formatting in case of single line comment before the 'error' keyword.
To provide more clarity, here is example code from my grammar and description of desired behavior:
package xxx_constants {namespace="xxx::a::b"}:
# asd
error ASSD {0}:
Hello {0,world}
This is expected result: (one space to the left)
package xxx_constants {namespace="xxx::a::b"}:
# asd
error ASSD {0}:
Hello {0,world}
and this is the actual result with prepend method
package xxx_constants {namespace="xxx::a::b"}:
# asd
error ASSD {0}:
Hello {0,world}
As the document structure says, the HiddenRegion is in this case is the statement:
# asd
error
How can i prepend my characters directly before the keyword 'error' and not before the comment? Thanks.

Answer 1

I assume you're creating an indentation-sensitive language, because you're explicitly calling BEGIN and END.
For indentation-sensitive language my answer is: You'll want to overwrite
org.eclipse.xtext.formatting2.internal.HiddenRegionReplacer.applyHiddenRegionFormatting(List<ITextReplacer>)
The methods append[] and prepend[] you're using are agnostic to comments and at a later time applyHiddenRegionFormatting() is called to decide how that formatting is weaved between comments.
To make Xtext use your own subclass of HiddenRegionReplacer, overwrite
org.eclipse.xtext.formatting2.AbstractFormatter2.createHiddenRegionReplacer(IHiddenRegion, IHiddenRegionFormatting)
For languages that do not do whitespace-sensitive lexing/parsing (that's the default) the answer is to not call setSpace() to create indentation or line-wraps.
Instead, do
pkg.interior[indent]
pkg.regionFor.keyword(":").append[newLine]
pkg.append[newLine]