Add collectStylesheets that allows to pick stylesheets from provide…

…d urls. (#17752)

Feature (utils): Add a `collectStylesheets` helper function to retrieve stylesheets from the provided URLs.

packages/ckeditor5-utils/src/collectstylesheets.ts

@@ -0,0 +1,66 @@
+/**
+ * @license Copyright (c) 2003-2025, CKSource Holding sp. z o.o. All rights reserved.
+ * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-licensing-options
+ */
+
+/**
+ * @module utils/collectstylesheets
+ */
+
+/**
+ * A helper function for getting concatenated CSS rules from external stylesheets.
+ *
+ * @param stylesheets An array of stylesheet paths delivered by the user through the plugin configuration.
+ */
+export default async function collectStylesheets( stylesheets?: Array<string> ): Promise<string> {
+	if ( !stylesheets ) {
+		return '';
+	}
+
+	const results = await Promise.all(
+		stylesheets.map( async stylesheet => {
+			if ( stylesheet === 'EDITOR_STYLES' ) {
+				return getEditorStyles();
+			}
+
+			const response = await window.fetch( stylesheet );
+
+			return response.text();
+		} )
+	);
+
+	return results.join( ' ' ).trim();
+}
+
+/**
+ * A helper function for getting the basic editor content styles for the `.ck-content` class
+ * and all CSS variables defined in the document.
+ */
+function getEditorStyles(): string {
+	const editorStyles = [];
+	const editorCSSVariables = [];
+
+	for ( const styleSheet of Array.from( document.styleSheets ) ) {
+		const ownerNode = styleSheet.ownerNode as Element;
+
+		if ( ownerNode.hasAttribute( 'data-cke' ) ) {
+			for ( const rule of Array.from( styleSheet.cssRules ) ) {
+				if ( rule.cssText.indexOf( '.ck-content' ) !== -1 ) {
+					editorStyles.push( rule.cssText );
+				} else if ( rule.cssText.indexOf( ':root' ) !== -1 ) {
+					editorCSSVariables.push( rule.cssText );
+				}
+			}
+		}
+	}
+
+	if ( !editorStyles.length ) {
+		console.warn(
+			'The editor stylesheet could not be found in the document. ' +
+			'Check your webpack config - style-loader should use data-cke=true attribute for the editor stylesheet.'
+		);
+	}
+
+	// We want to trim the returned value in case of `[ "", "", ... ]`.
+	return [ ...editorCSSVariables, ...editorStyles ].join( ' ' ).trim();
+}

packages/ckeditor5-utils/src/index.ts

@@ -94,6 +94,7 @@ export { default as delay, type DelayedFunc } from './delay.js';
 export { default as wait } from './wait.js';
 export { default as parseBase64EncodedObject } from './parsebase64encodedobject.js';
 export { default as crc32, type CRCData } from './crc32.js';
+export { default as collectStylesheets } from './collectstylesheets.js';
 
 export * from './unicode.js';
 

packages/ckeditor5-utils/tests/collectstylesheets.js

@@ -0,0 +1,109 @@
+/**
+ * @license Copyright (c) 2003-2025, CKSource Holding sp. z o.o. All rights reserved.
+ * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-licensing-options
+ */
+
+/* global document, window, console, Response */
+
+import collectStylesheets from '../src/collectstylesheets.js';
+
+describe( 'collectStylesheets', () => {
+	let styleSheetsMock;
+
+	beforeEach( () => {
+		styleSheetsMock = [
+			{
+				ownerNode: {
+					hasAttribute: name => name === 'data-cke'
+				},
+				cssRules: [
+					{ cssText: ':root { --variable1: white; }' },
+					{ cssText: '.ck-content { color: black }' },
+					{ cssText: '.some-styles { color: red }' }
+				]
+			},
+			{
+				ownerNode: {
+					hasAttribute: name => name === 'data-cke'
+				},
+				cssRules: [
+					{ cssText: ':root { --variable2: blue; }' },
+					{ cssText: '.ck-content { background: white }' }
+				]
+			},
+			{
+				ownerNode: {
+					hasAttribute: () => false
+				},
+				cssRules: [
+					{ cssText: 'h2 { color: black }' }
+				]
+			}
+		];
+
+		sinon.stub( document, 'styleSheets' ).get( () => styleSheetsMock );
+	} );
+
+	afterEach( () => {
+		sinon.restore();
+	} );
+
+	it( 'should not return any styles if no paths to stylesheets provided', async () => {
+		expect( await collectStylesheets( undefined ) ).to.equal( '' );
+	} );
+
+	it( 'should log into the console when ".ck-content" styles are missing', async () => {
+		styleSheetsMock = [ {
+			ownerNode: {
+				hasAttribute: name => name === 'data-cke'
+			},
+			cssRules: [
+				{ cssText: ':root { --variable: white; }' }
+			]
+		} ];
+
+		const consoleSpy = sinon.stub( console, 'warn' );
+
+		await collectStylesheets( [ 'EDITOR_STYLES' ] );
+
+		sinon.assert.calledOnce( consoleSpy );
+	} );
+
+	it( 'should get ".ck-content" styles when "EDITOR_STYLES" token is provided', async () => {
+		const consoleSpy = sinon.stub( console, 'warn' );
+
+		const styles = await collectStylesheets( [ './foo.css', 'EDITOR_STYLES' ] );
+
+		sinon.assert.notCalled( consoleSpy );
+
+		expect( styles.length > 0 ).to.be.true;
+		expect( styles.indexOf( '.ck-content' ) !== -1 ).to.be.true;
+	} );
+
+	it( 'should get styles from multiple stylesheets with data-cke attribute', async () => {
+		const styles = await collectStylesheets( [ 'EDITOR_STYLES' ] );
+
+		expect( styles ).to.include( ':root { --variable1: white; }' );
+		expect( styles ).to.include( ':root { --variable2: blue; }' );
+		expect( styles ).to.include( '.ck-content { color: black }' );
+		expect( styles ).to.include( '.ck-content { background: white }' );
+	} );
+
+	it( 'should collect all :root styles from stylesheets with data-cke attribute', async () => {
+		const styles = await collectStylesheets( [ 'EDITOR_STYLES' ] );
+
+		expect( styles ).to.include( '--variable1: white' );
+		expect( styles ).to.include( '--variable2: blue' );
+	} );
+
+	it( 'should fetch stylesheets from the provided paths and return concat result', async () => {
+		sinon
+			.stub( window, 'fetch' )
+			.onFirstCall().resolves( new Response( '.foo { color: green; }' ) )
+			.onSecondCall().resolves( new Response( '.bar { color: red; }' ) );
+
+		const styles = await collectStylesheets( [ './foo.css', './bar.css' ] );
+
+		expect( styles ).to.equal( '.foo { color: green; } .bar { color: red; }' );
+	} );
+} );