Actual Parameter Updates in Both Original and Cross-Referencing Documents in Langium

[Subham7201]

DSL Definition for Call Expression and Actual Parameter

CallExpression infers Expression:  
    {infer CallExpression} (annotation=Annotation)? 'call' '(#'calledStatement=[MClass:QualifiedName]'#)' '{' parameters+=ActualParameter '}'  
;  

ActualParameter infers Parameter:  
    {infer ActualParameter} (annotation=Annotation)? refMember=[FieldMember:ID] '=' (value=ExpressionWrapper)?  
;  

Objective

• When a new actual parameter is added to a document, it should be reflected in that document as well as in all other documents that reference it.

Approach Taken

• We trigger an event handler from the frontend, which initiates an update call followed by a save call to the backend.

• In the update call, we attach a listener to the build phase of the Langium document, which triggers validation within our program.

• This validation filters all documents that reference the original document. We then iterate through each of these cross-referencing documents, identify those containing diagnostics, and add the newly introduced actual parameter to them.

• Once this process is complete, we attempt to send the URI of the original document to the frontend. The frontend uses this URI to trigger a save operation, ensuring that the actual parameter is added to the original document, thus completing the update in both the original and cross-referencing documents.

Issue Encountered

• Since the listener is attached to the Langium document’s build phase, all cross-referencing documents are updated within the same update call.

• However, because the listener execution modifies multiple documents, the original document's URI is not properly sent to the frontend.

• As a result, while the actual parameter is successfully added to all cross-referencing documents, the update to the original document does not occur.

• Since the validation has already run, the parameter is not processed again for the original document, leaving it unchanged.

• Possible Solution

What could be the possible solution for this problem?

[msujew]

Hey @Subham7201,

Since the listener is attached to the Langium document’s build phase, all cross-referencing documents are updated within the same update call.

You are probably using onDocumentPhase to determine the change, right? You can use onUpdate instead to just get the actually updated document. That should probably alleviate the issue.

[Subham7201]

Hey @msujew,

Thanks for the suggestion! I initially used onDocumentPhase, but based on your input, I tried switching to onUpdate. However, I ran into an issue:

onUpdate triggers before the document is rebuilt, whereas we need it to run after the document has been rebuilt. Additionally, the documentation states that listeners attached to onUpdate should not perform long-running tasks, and the refactoring we need to perform could take some time.

Given these constraints, do you have any suggestions on how we can ensure our logic runs after the document is fully rebuilt while still handling cross-references correctly?

[msujew]

You shoule be able to store the changed URIs from onUpdate somewhere and then use onDocumentPhase to work on the specific URIs/documents.

[spoenemann]

👍 That's how we do it in our Sprotty integration:

langium/packages/langium-sprotty/src/diagram-server-manager.ts

Lines 55 to 56 in c3afb24

	services.workspace.DocumentBuilder.onUpdate((changed, deleted) => this.documentsUpdated(changed, deleted));
	services.workspace.DocumentBuilder.onBuildPhase(DocumentState.Validated, (built, ct) => this.documentsBuilt(built, ct));