override default behavior of langium folding provider, so the first line of a block remains visible

Author: lstreckeisen

src/language/ContextMapperDslModule.ts

@@ -14,6 +14,7 @@ import { SemanticTokenProviderRegistry } from './semantictokens/SemanticTokenPro
 import { ContextMapperDslValidationRegistry } from './validation/ContextMapperDslValidationRegistry.js'
 import { ContextMapperValidationProviderRegistry } from './validation/ContextMapperValidationProviderRegistry.js'
 import { ContextMapperDslScopeProvider } from './references/ContextMapperDslScopeProvider.js'
+import { ContextMapperDslFoldingRangeProvider } from './folding/ContextMapperDslFoldingRageProvider.js'
 
 /**
  * Declaration of custom services - add your own service classes here.
@@ -47,7 +48,8 @@ export const ContextMapperDslModule: Module<ContextMapperDslServices, ModuleType
     ValidationRegistry: (services) => new ContextMapperDslValidationRegistry(services)
   },
   lsp: {
-    SemanticTokenProvider: (services) => new ContextMapperDslSemanticTokenProvider(services, semanticTokenProviderRegistry)
+    SemanticTokenProvider: (services) => new ContextMapperDslSemanticTokenProvider(services, semanticTokenProviderRegistry),
+    FoldingRangeProvider: (services) => new ContextMapperDslFoldingRangeProvider(services)
   },
   references: {
     ScopeProvider: (services) => new ContextMapperDslScopeProvider(services)

src/language/folding/ContextMapperDslFoldingRageProvider.ts

@@ -0,0 +1,25 @@
+import { DefaultFoldingRangeProvider } from 'langium/lsp'
+import { CstNode, LangiumDocument } from 'langium'
+import { FoldingRange } from 'vscode-languageserver-types'
+
+export class ContextMapperDslFoldingRangeProvider extends DefaultFoldingRangeProvider {
+  // modified version of DefaultFoldingRangeProvider#toFoldingRange
+  protected override toFoldingRange (document: LangiumDocument, node: CstNode, kind?: string): FoldingRange | undefined {
+    const range = node.range
+    let start = range.start
+    let end = range.end
+
+    // Don't generate foldings for nodes that are less than 3 lines
+    if (end.line - start.line < 2) {
+      return undefined
+    }
+
+    if (!this.includeLastFoldingLine(node, kind)) { // checks if block is parenthesis/bracket/brace block
+      // To prevent something like '...}' in the editor when a code block is collapsed, we want to still show the first line of a code block
+      start = document.textDocument.positionAt(document.textDocument.offsetAt({ line: start.line + 1, character: 0 }) - 1)
+      // As we don't want to hide the end token like 'if { ... --> } <--', we simply select the end of the previous line as the end position
+      end = document.textDocument.positionAt(document.textDocument.offsetAt({ line: end.line, character: 0 }) - 1)
+    }
+    return FoldingRange.create(start.line, end.line, start.character, end.character, kind)
+  }
+}