fix(@angular/build): rebuild library when transitive stylesheet dependencies change in watch mode

Track transitive stylesheet resource references per containing component and stylesheet file in `BuilderState.addFileReferences`, and expand modified files via `BuilderState.expandChangedFiles` during watch mode rebuilds.

Previously, modifying a Sass partial imported by an inline component stylesheet did not invalidate the containing component file or reset the Sass worker pool cache.

Author: alan-agius4

packages/angular/build/src/builders/library/builder-state.ts

@@ -6,6 +6,7 @@
  * found in the LICENSE file at https://angular.dev/license
  */
 
+import { toPosixPath } from '../../utils/path';
 import { ExecutionResult } from './execution-result';
 import type {
   BundleResult,
@@ -15,10 +16,12 @@ import type {
 } from './types';
 
 /**
- * Encapsulates incremental compilation state, watched compilation file tracking,
- * and execution results across watch-mode rebuilds.
+ * Encapsulates incremental compilation state, transitive resource reference tracking,
+ * watched compilation file tracking, and execution results across watch-mode rebuilds.
  */
 export class BuilderState {
+  readonly #referencingFiles = new Map<string, Set<string>>();
+  readonly #referencedByFile = new Map<string, Set<string>>();
   readonly #watchedCompilationFiles = new Set<string>();
   readonly #pendingCompilationFiles = new Set<string>();
 
@@ -116,6 +119,82 @@ export class BuilderState {
     return this.#executionResult;
   }
 
+  /**
+   * Records transitive resource dependencies (such as Sass partials) for a containing
+   * component or stylesheet file, pruning any previously recorded references for that
+   * containing file and registering the current references in `watchedCompilationFiles`.
+   *
+   * @param containingFile Path of the referencing component or stylesheet file.
+   * @param referencedFiles Paths of the referenced files.
+   */
+  addFileReferences(containingFile: string, referencedFiles: Iterable<string>): void {
+    const posixContainingFile = toPosixPath(containingFile);
+
+    let forwardReferences = this.#referencedByFile.get(posixContainingFile);
+    if (forwardReferences) {
+      for (const prevRef of forwardReferences) {
+        const referencing = this.#referencingFiles.get(prevRef);
+        if (referencing) {
+          referencing.delete(posixContainingFile);
+          if (referencing.size === 0) {
+            this.#referencingFiles.delete(prevRef);
+          }
+        }
+      }
+      forwardReferences.clear();
+    }
+
+    for (const file of referencedFiles) {
+      const posixFile = toPosixPath(file);
+      this.#watchedCompilationFiles.add(posixFile);
+      if (posixFile === posixContainingFile) {
+        continue;
+      }
+
+      if (!forwardReferences) {
+        forwardReferences = new Set();
+        this.#referencedByFile.set(posixContainingFile, forwardReferences);
+      }
+      forwardReferences.add(posixFile);
+
+      let referencing = this.#referencingFiles.get(posixFile);
+      if (!referencing) {
+        referencing = new Set();
+        this.#referencingFiles.set(posixFile, referencing);
+      }
+      referencing.add(posixContainingFile);
+    }
+
+    if (forwardReferences?.size === 0) {
+      this.#referencedByFile.delete(posixContainingFile);
+    }
+  }
+
+  /**
+   * Expands a set of changed files with all transitively referencing files
+   * (e.g., partial `.scss` -> component `.scss` -> component `.ts`).
+   *
+   * @param files Initial changed file paths.
+   * @returns Set of POSIX paths including all transitively referencing files.
+   */
+  expandChangedFiles(files: Iterable<string>): Set<string> {
+    const expanded = new Set<string>();
+    for (const file of files) {
+      expanded.add(toPosixPath(file));
+    }
+
+    for (const file of expanded) {
+      const referencing = this.#referencingFiles.get(file);
+      if (referencing) {
+        for (const refFile of referencing) {
+          expanded.add(refFile);
+        }
+      }
+    }
+
+    return expanded;
+  }
+
   /**
    * Determines whether compilation and bundling need to run for the current build iteration.
    * Also records incoming modified files so they are retained if the iteration aborts early.
@@ -233,6 +312,8 @@ export class BuilderState {
     this.pendingChangedEsmFiles.clear();
     this.pendingChangedDtsFiles.clear();
     this.#pendingCompilationFiles.clear();
+    this.#referencingFiles.clear();
+    this.#referencedByFile.clear();
     this.#watchedCompilationFiles.clear();
   }
 }

packages/angular/build/src/builders/library/builder-state_spec.ts

@@ -70,6 +70,44 @@ describe('BuilderState', () => {
     });
   });
 
+  describe('file reference tracking', () => {
+    it('should expand transitive multi-hop references and ignore self-references', () => {
+      const state = new BuilderState();
+      const compTs = '/workspace/projects/my-lib/src/lib/comp.ts';
+      const compScss = '/workspace/projects/my-lib/src/lib/comp.scss';
+      const varsScss = '/workspace/projects/my-lib/src/lib/_vars.scss';
+
+      state.addFileReferences(compTs, [compTs, compScss, varsScss]);
+      state.addFileReferences(compScss, [compScss, varsScss]);
+
+      expect(state.watchedCompilationFiles.has(compTs)).toBeTrue();
+      expect(state.watchedCompilationFiles.has(compScss)).toBeTrue();
+      expect(state.watchedCompilationFiles.has(varsScss)).toBeTrue();
+
+      const expanded = state.expandChangedFiles([varsScss]);
+      expect(expanded).toEqual(new Set([varsScss, compTs, compScss]));
+
+      // Subsequent expandChangedFiles calls preserve mappings when not yet overwritten
+      const expandedAgain = state.expandChangedFiles([varsScss]);
+      expect(expandedAgain).toEqual(new Set([varsScss, compTs, compScss]));
+    });
+
+    it('should prune stale references when a containing file removes an import', () => {
+      const state = new BuilderState();
+      const compTs = '/workspace/projects/my-lib/src/lib/comp.ts';
+      const oldPartial = '/workspace/projects/my-lib/src/lib/_old.scss';
+      const newPartial = '/workspace/projects/my-lib/src/lib/_new.scss';
+
+      state.addFileReferences(compTs, [compTs, oldPartial]);
+      expect(state.expandChangedFiles([oldPartial])).toEqual(new Set([oldPartial, compTs]));
+
+      // Re-record references for compTs without oldPartial
+      state.addFileReferences(compTs, [compTs, newPartial]);
+      expect(state.expandChangedFiles([oldPartial])).toEqual(new Set([oldPartial]));
+      expect(state.expandChangedFiles([newPartial])).toEqual(new Set([newPartial, compTs]));
+    });
+  });
+
   describe('shouldCompileEntryPoints & getCompilationModifiedFiles', () => {
     it('should preserve pending compilation files across aborted iterations', () => {
       const state = new BuilderState({

packages/angular/build/src/builders/library/builder.ts

@@ -261,29 +261,18 @@ async function* runWatchLoop(
       console.clear();
     }
 
-    const changedFiles = new Set<string>();
-    let hasStyleChanges = false;
-    let hasSassChanges = false;
-    for (const file of changes.all) {
-      const posixFile = toPosixPath(file);
-      changedFiles.add(posixFile);
-      if (/\.(?:scss|sass)$/i.test(posixFile)) {
-        hasStyleChanges = true;
-        hasSassChanges = true;
-      } else if (/\.(?:less|css)$/i.test(posixFile)) {
-        hasStyleChanges = true;
-      }
-    }
+    const rawChangedFiles = changes.all.map(toPosixPath);
+    const invalidatedStyles = stylesheetBundler.invalidate(rawChangedFiles);
+    const changedFiles = buildState.expandChangedFiles(
+      invalidatedStyles
+        ? [...rawChangedFiles, ...invalidatedStyles.map(toPosixPath)]
+        : rawChangedFiles,
+    );
 
-    if (hasStyleChanges) {
-      if (hasSassChanges) {
+    for (const posixFile of changedFiles) {
+      if (/\.(?:scss|sass)$/i.test(posixFile)) {
         resetSassWorkerPoolCaches();
-      }
-      const invalidatedStyles = stylesheetBundler.invalidate(changedFiles);
-      if (invalidatedStyles) {
-        for (const styleFile of invalidatedStyles) {
-          changedFiles.add(toPosixPath(styleFile));
-        }
+        break;
       }
     }
 

packages/angular/build/src/builders/library/pipeline/compilation.ts

@@ -26,7 +26,7 @@ const EMITTED_EXTENSIONS = ['.js', '.mjs', '.cjs', '.d.ts', '.d.mts', '.d.cts'];
  *
  * @param options The normalized library builder options with POSIX paths.
  * @param stylesheetBundler The component stylesheet bundler instance.
- * @param buildState The build state tracking compilation caches and diagnostics.
+ * @param buildState The build state tracking compilation caches, file references, and diagnostics.
  * @param modifiedFiles Optional set of modified POSIX file paths for incremental compilation.
  * @returns A promise resolving to the compilation output containing emitted files and cache state.
  */
@@ -68,7 +68,6 @@ export async function compileLibrary(
       await compilationInstance.update?.(effectiveModifiedFiles);
     }
 
-    const allReferencedFiles = new Set<string>();
     const stylesheetWarnings: PartialMessage[] = [];
     const stylesheetErrors: PartialMessage[] = [];
     const failedFiles = new Set<string>();
@@ -84,7 +83,13 @@ export async function compileLibrary(
           ? await stylesheetBundler.bundleFile(stylesheetFile)
           : await stylesheetBundler.bundleInline(data, containingFile);
 
-        result.referencedFiles?.forEach((f) => allReferencedFiles.add(toPosixPath(f)));
+        if (result.referencedFiles) {
+          buildState.addFileReferences(containingFile, result.referencedFiles);
+          if (stylesheetFile) {
+            buildState.addFileReferences(stylesheetFile, result.referencedFiles);
+          }
+        }
+
         if (result.warnings.length > 0) {
           stylesheetWarnings.push(...result.warnings);
         }
@@ -133,14 +138,11 @@ export async function compileLibrary(
     const dtsFiles = cached?.dtsFiles ?? new Map<string, string>();
     const changedEsmFiles = new Set<string>();
     const changedDtsFiles = new Set<string>();
-
-    for (const ref of referencedFiles) {
-      allReferencedFiles.add(toPosixPath(ref));
-    }
+    const posixReferencedFiles = new Set(referencedFiles.map(toPosixPath));
 
     if (cached && effectiveModifiedFiles) {
       for (const modifiedFile of effectiveModifiedFiles) {
-        if (allReferencedFiles.has(modifiedFile)) {
+        if (posixReferencedFiles.has(modifiedFile)) {
           continue;
         }
 
@@ -183,7 +185,7 @@ export async function compileLibrary(
       dtsFiles,
       changedEsmFiles,
       changedDtsFiles,
-      referencedFiles: allReferencedFiles,
+      referencedFiles: posixReferencedFiles,
       cache: {
         compilationInstance,
         esmFiles,

packages/angular/build/src/builders/library/tests/behavior/watch_spec.ts

@@ -8,6 +8,7 @@
 
 import { rm } from 'node:fs/promises';
 import { executeLibraryBuilder } from '../../builder';
+import { InlineStyleLanguage } from '../../schema';
 import { BASE_OPTIONS, LIBRARY_BUILDER_INFO, describeLibraryBuilder } from '../setup';
 
 describeLibraryBuilder(executeLibraryBuilder, LIBRARY_BUILDER_INFO, (harness) => {
@@ -601,5 +602,46 @@ describeLibraryBuilder(executeLibraryBuilder, LIBRARY_BUILDER_INFO, (harness) =>
         },
       ]);
     });
+
+    it('should rebuild when a Sass partial imported by an inline component stylesheet is modified in watch mode', async () => {
+      await harness.writeFiles({
+        'projects/lib/src/lib/_vars.scss': '$primary-color: rgb(10, 20, 30);',
+        'projects/lib/src/lib/lib.component.ts': `
+          import { Component } from '@angular/core';
+
+          @Component({
+            selector: 'lib-inline-sass',
+            template: '<span>Inline Sass</span>',
+            styles: ['@use "./vars"; :host { color: vars.$primary-color; }'],
+          })
+          export class LibComponent {}
+        `,
+      });
+
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        inlineStyleLanguage: InlineStyleLanguage.Scss,
+        watch: true,
+      });
+
+      await harness.executeWithCases([
+        async ({ result }) => {
+          expect(result?.success).toBeTrue();
+          const content = harness.readFile('dist/lib/fesm2022/lib.mjs');
+          expect(content).toMatch(/color:\s*(?:#0a141e|rgb\(10,\s*20,\s*30\))/);
+
+          // Modify the Sass partial referenced by the inline component style
+          await harness.writeFile(
+            'projects/lib/src/lib/_vars.scss',
+            '$primary-color: rgb(200, 100, 50);',
+          );
+        },
+        ({ result }) => {
+          expect(result?.success).toBeTrue();
+          const content = harness.readFile('dist/lib/fesm2022/lib.mjs');
+          expect(content).toMatch(/color:\s*(?:#c86432|rgb\(200,\s*100,\s*50\))/);
+        },
+      ]);
+    });
   });
 });