Updating external models in Langium correctly

[bjthehun]

I have integrated an external model into Langium as described here, and so far I've succeeded in extracting all references I need for the user-facing language.
However, the user may update the external model, so I need to update the references in Langium automatically as well. This doesn't work properly for me right now.

Whenever my language server receives an update in the external model, I reconstruct the references like this:

let ws = services.sharedServices.workspace;
const referenceAST = extractReferencesToAST(externalModel)

let referencesDocument = 
  ws.LangiumDocumentFactory.fromModel(referenceAST, pathToRefs);
await ws.DocumentBuilder.build(
  [referencesDocument], 
  {validationChecks: 'all'}
);

if (ws.LangiumDocuments.hasDocument(pathToRefs)) {
  ws.LangiumDocuments.deleteDocument(pathToRefs);
}    
ws.LangiumDocuments.addDocument(referencesDocument);

However, with this code, I miss the last update, as the newly calculated references don't show up until I receive the next update. So what am I doing wrong?

[msujew]

Hey @bjthehun,

I assume the way you update the document is incorrect. The document builder probably takes the old document instead of the updated one, which would explain why it's always lagging one update cycle behind. The following should work better:

let ws = services.sharedServices.workspace;
const referenceAST = extractReferencesToAST(externalModel)

let referencesDocument = 
  ws.LangiumDocumentFactory.fromModel(referenceAST, pathToRefs);
if (ws.LangiumDocuments.hasDocument(pathToRefs)) {
  ws.LangiumDocuments.deleteDocument(pathToRefs);
}    
ws.LangiumDocuments.addDocument(referencesDocument);
await ws.DocumentBuilder.update(
  [referencesDocument.uri], 
  []
);

[bjthehun]

Unfortunately, I instead get this error:

Error: Method not implemented.
      at EmptyFileSystemProvider.readFileSync (node_modules/langium/src/workspace/file-system-provider.ts:45:15)
      at DefaultLangiumDocumentFactory.getContentFromFileSystem (node_modules/langium/src/workspace/documents.ts:246:40)
      at DefaultLangiumDocumentFactory.update (node_modules/langium/src/workspace/documents.ts:222:67)
      at ~/project/node_modules/langium/src/workspace/document-builder.ts:137:41
      at DefaultDocumentBuilder.runCancelable (node_modules/langium/src/workspace/document-builder.ts:166:19)
      at async DefaultDocumentBuilder.buildDocuments (node_modules/langium/src/workspace/document-builder.ts:136:9)
      at async DefaultDocumentBuilder.update (node_modules/langium/src/workspace/document-builder.ts:102:9)
...

What I forgot to mention is I hold the external model in memory (pathToRefs starts with memory://...), and therefore, I'm using EmptyFileSystem as FileSystemProvider.

[bjthehun]

From what I can see, Langium tries to parse my model references again, even though I create an AST without parsing, following this suggestion.

But even when I want to parse the external model as text document, update in DefaultLangiumDocumentFactory ignores the textDocument property of the document it gets as input. Is that intended?

[msujew]

Right, I forgot that update will actually discard the AST and reattempt parsing the model. Note that it doesn't ignore the textDocument, but will attempt to get the latest text from the TextDocuments service which only works in case the document is actually opened in an editor. You can try continue using DocumentBuilder.build though. That should likely work.

[bjthehun]

I've figured out that build and update work in combination for me:

let ws = services.sharedServices.workspace;
const referenceAST = extractReferencesToAST(externalModel)

let referencesDocument = 
    ws.LangiumDocumentFactory.fromModel(referenceAST, pathToRefs);

if (ws.LangiumDocuments.hasDocument(pathToRefs)) {
    ws.DocumentBuilder.update([], [pathToRefs]);
} 
await ws.DocumentBuilder.build(
    [referencesDocument], 
    {validationChecks: 'all'}
);
ws.LangiumDocuments.addDocument(referencesDocument);

So once I tell DocumentBuilder i want to remove the existing external model, the "update" succeeds.

Thanks for the tip with build.