Add automatic global style injection

README.md

@@ -69,6 +69,63 @@ FullName.propTypes = {
 register(FullName, 'full-name');
 ```
 
+### API
+
+### register(Component, tagName, props?, options?)
+
+Returns a Custom Element.
+
+#### Component
+
+Type: `object`
+
+#### tagName
+
+Type: `string`
+
+#### props?
+
+Type: `Array<string>`
+
+#### options
+
+Type: `boolean | object`
+Default: `false`
+
+#### options.shadow
+
+Type: `boolean`
+Default: `false`
+
+Attaches a Shadow DOM instance to your Custom Element. Remember that global styles won't affect the Custom Element unless you inject (see `options.injectGlobalStyles`) or somehow import them from the component itself.
+
+#### options.injectGlobalStyles
+
+Type: `boolean | object`
+Default: `false`
+
+Injects current and future style and link elements related to styling into your Custom Element. Only works if `shadow: true`.
+
+##### options.injectGlobalStyles.target
+
+Type: `DOMNode`
+Default: `document.head`
+
+Where to look for styles to get added. Most 3rd party tooling loads style tags into document.head.
+
+##### options.injectGlobalStyles.selector
+
+Type: `(querySelector) string`
+Default: `'style, link[rel="stylesheet"], link[rel="preload"][as="style"]'`
+
+What types of elements to inject to your Custom Element's Shadow DOM.
+
+##### options.injectGlobalStyles.filter
+
+Type: `Function<boolean>`
+Default: `undefined`
+
+Allows you to filter what elements get added to the Custom Element more granularly. Gets executed on `addedNodes` by the MutationObserver, as well as the initial styles present on the page.
 
 ## Related
 

src/index.js

@@ -6,6 +6,29 @@ export default function register(Component, tagName, propNames, options) {
 		inst._vdomComponent = Component;
 		inst._root =
 			options && options.shadow ? inst.attachShadow({ mode: 'open' }) : inst;
+
+		if (options && options.shadow && options.injectGlobalStyles) {
+			const defaults = {
+				target: document.head,
+				selector:
+					'style, link[rel="stylesheet"], link[rel="preload"][as="style"]',
+				filter: undefined,
+				observeOptions: { childList: true, subtree: true },
+			};
+
+			this.styleObserver = beginInjectingGlobalStyles(
+				inst.shadowRoot,
+				/* eslint-disable indent */
+				options.injectGlobalStyles === true
+					? defaults
+					: {
+							...defaults,
+							...options.injectGlobalStyles,
+					  }
+				/* eslint-enable indent */
+			);
+		}
+
 		return inst;
 	}
 	PreactElement.prototype = Object.create(HTMLElement.prototype);
@@ -62,6 +85,62 @@ function ContextProvider(props) {
 	return cloneElement(children, rest);
 }
 
+export function cloneElementsToShadowRoot(shadowRoot, elements) {
+	elements.forEach((el) => shadowRoot.appendChild(el.cloneNode(true)));
+}
+
+export function getAllStyles(target, selector, filter) {
+	const elements = Array.prototype.slice.call(
+		target.querySelectorAll(selector)
+	);
+
+	return filter ? elements.filter(filter) : elements;
+}
+
+export const beginInjectingGlobalStyles = (
+	shadowRootRef,
+	injectGlobalStyles
+) => {
+	cloneElementsToShadowRoot(
+		shadowRootRef,
+		getAllStyles(
+			injectGlobalStyles.target,
+			injectGlobalStyles.selector,
+			injectGlobalStyles.filter
+		)
+	);
+
+	return observeStyleChanges(
+		(elements) => {
+			cloneElementsToShadowRoot(shadowRootRef, elements);
+		},
+		injectGlobalStyles.target,
+		injectGlobalStyles.selector,
+		injectGlobalStyles.filter,
+		injectGlobalStyles.observeOptions
+	);
+};
+
+export function observeStyleChanges(
+	callback,
+	target,
+	selector,
+	filter,
+	observeOptions
+) {
+	return new MutationObserver((mutations, observer) => {
+		mutations.forEach((mutation) => {
+			const matchedElements = Array.prototype.slice
+				.call(mutation.addedNodes)
+				.filter((node) => node.matches && node.matches(selector));
+
+			if (matchedElements.length > 0) {
+				callback(filter ? matchedElements.filter(filter) : matchedElements);
+			}
+		});
+	}).observe(target, observeOptions);
+}
+
 function connectedCallback() {
 	// Obtain a reference to the previous context by pinging the nearest
 	// higher up node that was rendered with Preact. If one Preact component
@@ -99,6 +178,10 @@ function attributeChangedCallback(name, oldValue, newValue) {
 
 function disconnectedCallback() {
 	render((this._vdom = null), this._root);
+
+	if (this.styleObserver) {
+		this.styleObserver.disconnect();
+	}
 }
 
 /**

src/index.test.jsx

@@ -230,4 +230,163 @@ describe('web components', () => {
 		});
 		assert.equal(getShadowHTML(), '<p>Active theme: sunny</p>');
 	});
+
+	function Thing() {
+		return <span>Hello world!</span>;
+	}
+
+	const styleElem = document.createElement('style');
+	styleElem.innerHTML = 'span { color: red; }';
+	document.head.appendChild(styleElem);
+
+	registerElement(Thing, 'x-thing', undefined, {
+		shadow: true,
+		injectGlobalStyles: true,
+	});
+
+	describe('Global style injections from document.head', () => {
+		it('injects style-tags', () => {
+			const el = document.createElement('x-thing');
+
+			root.appendChild(el);
+
+			assert.equal(
+				document.querySelector('x-thing').shadowRoot.innerHTML,
+				'<span>Hello world!</span><style>span { color: red; }</style>'
+			);
+
+			const computedStyle = window
+				.getComputedStyle(
+					document.querySelector('x-thing').shadowRoot.querySelector('span'),
+					null
+				)
+				.getPropertyValue('color');
+
+			assert.equal(computedStyle, 'rgb(255, 0, 0)');
+
+			// assert.equal(
+			// 	root.innerHTML,
+			// 	'<x-thing><span>Hello world!</span></x-thing>'
+			// );
+			styleElem.parentElement.removeChild(styleElem);
+		});
+
+		it('injects link-tags of rel="stylesheet"', async () => {
+			const blob = new Blob([], { type: 'text/css' });
+
+			let linkElementLoaded;
+
+			let deferred;
+			let promise = new Promise((resolve) => {
+				deferred = resolve;
+			});
+
+			const linkElem = document.createElement('link');
+			linkElem.rel = 'stylesheet';
+			linkElem.href = window.URL.createObjectURL(blob);
+			linkElem.onload = () => {
+				linkElementLoaded = true;
+				deferred();
+			};
+
+			document.head.appendChild(linkElem);
+
+			const el = document.createElement('x-thing');
+
+			root.appendChild(el);
+
+			assert.match(
+				document.querySelector('x-thing').shadowRoot.innerHTML,
+				new RegExp(
+					`<span>Hello world!</span><link rel="stylesheet" href="blob:http://.*?/[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{12}">`
+				)
+			);
+
+			await promise;
+			assert.isTrue(linkElementLoaded);
+
+			linkElem.parentElement.removeChild(linkElem);
+		});
+
+		it('injects link-tags of rel="preload"', async () => {
+			const blob = new Blob([], { type: 'text/css' });
+
+			let linkElementLoaded;
+
+			let deferred;
+			let promise = new Promise((resolve) => {
+				deferred = resolve;
+			});
+
+			const linkElem = document.createElement('link');
+			linkElem.rel = 'preload';
+			linkElem.as = 'style';
+			linkElem.href = window.URL.createObjectURL(blob);
+			linkElem.onload = () => {
+				linkElementLoaded = true;
+				deferred();
+			};
+
+			document.head.appendChild(linkElem);
+
+			const el = document.createElement('x-thing');
+
+			root.appendChild(el);
+
+			assert.match(
+				document.querySelector('x-thing').shadowRoot.innerHTML,
+				new RegExp(
+					`<span>Hello world!</span><link rel="preload" as="style" href="blob:http://.*?/[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{12}">`
+				)
+			);
+
+			await promise;
+			assert.isTrue(linkElementLoaded);
+
+			linkElem.parentElement.removeChild(linkElem);
+		});
+
+		it('injects style-tags that is added after custom element is loaded', async () => {
+			const el = document.createElement('x-thing');
+
+			root.appendChild(el);
+
+			assert.equal(
+				document.querySelector('x-thing').shadowRoot.innerHTML,
+				'<span>Hello world!</span>'
+			);
+
+			const computedStyle = window
+				.getComputedStyle(
+					document.querySelector('x-thing').shadowRoot.querySelector('span'),
+					null
+				)
+				.getPropertyValue('color');
+
+			assert.equal(computedStyle, 'rgb(0, 0, 0)');
+
+			const styleElem = document.createElement('style');
+			styleElem.innerHTML = 'span { color: red; }';
+
+			// wait for the element to be added
+			await new Promise((resolve) => {
+				new MutationObserver((mutations, observer) => {
+					resolve();
+					observer.disconnect();
+				}).observe(document.querySelector('x-thing').shadowRoot, {
+					childList: true,
+					subtree: true,
+				});
+
+				document.head.appendChild(styleElem);
+			});
+
+			assert.equal(
+				document.querySelector('x-thing').shadowRoot.innerHTML,
+				'<span>Hello world!</span><style>span { color: red; }</style>'
+			);
+
+			styleElem.parentElement.removeChild(styleElem);
+		});
+	});
 });