Native cssstylesheet

src/lib/css-tag.ts

@@ -12,7 +12,8 @@ found at http://polymer.github.io/PATENTS.txt
 /**
  * Whether the current browser supports `adoptedStyleSheets`.
  */
-export const supportsAdoptingStyleSheets =
+export const supportsAdoptingStyleSheets = (window.ShadowRoot) &&
+    (window.ShadyCSS === undefined || window.ShadyCSS.nativeShadow) &&
     ('adoptedStyleSheets' in Document.prototype) &&
     ('replace' in CSSStyleSheet.prototype);
 
@@ -28,15 +29,16 @@ export class CSSResult {
       throw new Error(
           'CSSResult is not constructable. Use `unsafeCSS` or `css` instead.');
     }
+
     this.cssText = cssText;
   }
 
   // Note, this is a getter so that it's lazy. In practice, this means
   // stylesheets are not created until the first element instance is made.
   get styleSheet(): CSSStyleSheet|null {
     if (this._styleSheet === undefined) {
-      // Note, if `adoptedStyleSheets` is supported then we assume CSSStyleSheet
-      // is constructable.
+      // Note, if `supportsAdoptingStyleSheets` is true then we assume
+      // CSSStyleSheet is constructable.
       if (supportsAdoptingStyleSheets) {
         this._styleSheet = new CSSStyleSheet();
         this._styleSheet.replaceSync(this.cssText);

src/lit-element.ts

@@ -18,7 +18,7 @@ import {PropertyValues, UpdatingElement} from './lib/updating-element.js';
 export * from './lib/updating-element.js';
 export * from './lib/decorators.js';
 export {html, svg, TemplateResult, SVGTemplateResult} from 'lit-html/lit-html.js';
-import {supportsAdoptingStyleSheets, CSSResult} from './lib/css-tag.js';
+import {supportsAdoptingStyleSheets, CSSResult, unsafeCSS} from './lib/css-tag.js';
 export * from './lib/css-tag.js';
 
 declare global {
@@ -33,7 +33,10 @@ declare global {
 (window['litElementVersions'] || (window['litElementVersions'] = []))
     .push('2.3.1');
 
-export interface CSSResultArray extends Array<CSSResult|CSSResultArray> {}
+export type CSSResultOrNative = CSSResult|CSSStyleSheet;
+
+export interface CSSResultArray extends
+    Array<CSSResultOrNative|CSSResultArray> {}
 
 /**
  * Sentinal value used to avoid calling lit-html's render function when
@@ -80,19 +83,19 @@ export class LitElement extends UpdatingElement {
 
   /**
    * Array of styles to apply to the element. The styles should be defined
-   * using the [[`css`]] tag function.
+   * using the [[`css`]] tag function or via constructible stylesheets.
    */
-  static styles?: CSSResult|CSSResultArray;
+  static styles?: CSSResultOrNative|CSSResultArray;
 
-  private static _styles: CSSResult[]|undefined;
+  private static _styles: Array<CSSResultOrNative|CSSResult>|undefined;
 
   /**
    * Return the array of styles to apply to the element.
    * Override this method to integrate into a style management system.
    *
    * @nocollapse
    */
-  static getStyles(): CSSResult|CSSResultArray|undefined {
+  static getStyles(): CSSResultOrNative|CSSResultArray|undefined {
     return this.styles;
   }
 
@@ -109,31 +112,48 @@ export class LitElement extends UpdatingElement {
     // This should be addressed when a browser ships constructable
     // stylesheets.
     const userStyles = this.getStyles();
-    if (userStyles === undefined) {
-      this._styles = [];
-    } else if (Array.isArray(userStyles)) {
+
+    if (Array.isArray(userStyles)) {
       // De-duplicate styles preserving the _last_ instance in the set.
       // This is a performance optimization to avoid duplicated styles that can
       // occur especially when composing via subclassing.
       // The last item is kept to try to preserve the cascade order with the
       // assumption that it's most important that last added styles override
       // previous styles.
-      const addStyles =
-          (styles: CSSResultArray, set: Set<CSSResult>): Set<CSSResult> =>
-              styles.reduceRight(
-                  (set: Set<CSSResult>, s) =>
-                      // Note: On IE set.add() does not return the set
-                  Array.isArray(s) ? addStyles(s, set) : (set.add(s), set),
-                  set);
+      const addStyles = (styles: CSSResultArray, set: Set<CSSResultOrNative>):
+          Set<CSSResultOrNative> => styles.reduceRight(
+              (set: Set<CSSResultOrNative>, s) =>
+                  // Note: On IE set.add() does not return the set
+              Array.isArray(s) ? addStyles(s, set) : (set.add(s), set),
+              set);
       // Array.from does not work on Set in IE, otherwise return
       // Array.from(addStyles(userStyles, new Set<CSSResult>())).reverse()
-      const set = addStyles(userStyles, new Set<CSSResult>());
-      const styles: CSSResult[] = [];
+      const set = addStyles(userStyles, new Set<CSSResultOrNative>());
+      const styles: CSSResultOrNative[] = [];
       set.forEach((v) => styles.unshift(v));
       this._styles = styles;
     } else {
-      this._styles = [userStyles];
+      this._styles = userStyles === undefined ? [] : [userStyles];
     }
+
+    // Ensure that there are no invalid CSSStyleSheet instances here. They are
+    // invalid in two conditions.
+    // (1) the sheet is non-constructible (`sheet` of a HTMLStyleElement), but
+    //     this is impossible to check except via .replaceSync or use
+    // (2) the ShadyCSS polyfill is enabled (:. supportsAdoptingStyleSheets is
+    //     false)
+    this._styles = this._styles.map((s) => {
+      if (s instanceof CSSStyleSheet && !supportsAdoptingStyleSheets) {
+        // Flatten the cssText from the passed constructible stylesheet (or
+        // undetectable non-constructible stylesheet). The user might have
+        // expected to update their stylesheets over time, but the alternative
+        // is a crash.
+        const cssText = Array.prototype.slice.call(s.cssRules)
+            .reduce((css, rule) => css + rule.cssText, '');
+        return unsafeCSS(cssText);
+      }
+      return s;
+    });
   }
 
   private _needsShimAdoptedStyleSheets?: boolean;
@@ -189,15 +209,15 @@ export class LitElement extends UpdatingElement {
     }
     // There are three separate cases here based on Shadow DOM support.
     // (1) shadowRoot polyfilled: use ShadyCSS
-    // (2) shadowRoot.adoptedStyleSheets available: use it.
+    // (2) shadowRoot.adoptedStyleSheets available: use it
     // (3) shadowRoot.adoptedStyleSheets polyfilled: append styles after
     // rendering
     if (window.ShadyCSS !== undefined && !window.ShadyCSS.nativeShadow) {
       window.ShadyCSS.ScopingShim!.prepareAdoptedCssText(
           styles.map((s) => s.cssText), this.localName);
     } else if (supportsAdoptingStyleSheets) {
       (this.renderRoot as ShadowRoot).adoptedStyleSheets =
-          styles.map((s) => s.styleSheet!);
+          styles.map((s) => s instanceof CSSStyleSheet ? s : s.styleSheet!);
     } else {
       // This must be done after rendering so the actual style insertion is done
       // in `update`.

src/test/lit-element_styling_test.ts

@@ -792,10 +792,16 @@ suite('Static get styles', () => {
 
   test('element class only gathers styles once', async () => {
     const base = generateElementName();
-    let styleCounter = 0;
+    let getStylesCounter = 0;
+    let stylesCounter = 0;
     customElements.define(base, class extends LitElement {
+      static getStyles() {
+        getStylesCounter++;
+        return super.getStyles();
+      }
+
       static get styles() {
-        styleCounter++;
+        stylesCounter++;
         return css`:host {
           border: 10px solid black;
         }`;
@@ -821,7 +827,8 @@ suite('Static get styles', () => {
         '10px',
         'el2 styled correctly');
     assert.equal(
-        styleCounter, 1, 'styles property should only be accessed once');
+        stylesCounter, 1, 'styles property should only be accessed once');
+    assert.equal(getStylesCounter, 1, 'getStyles() should be called once');
   });
 
   test(
@@ -890,6 +897,88 @@ suite('Static get styles', () => {
 
         document.body.removeChild(element);
       });
+
+  const testAdoptedStyleSheets =
+      (window.ShadowRoot) &&
+      ('replace' in CSSStyleSheet.prototype);
+  (testAdoptedStyleSheets ? test : test.skip)(
+      'Can return CSSStyleSheet where adoptedStyleSheets are natively supported',
+      async () => {
+        const sheet = new CSSStyleSheet();
+        sheet.replaceSync('div { border: 4px solid red; }');
+        const normal = css`span { border: 4px solid blue; }`;
+
+        const base = generateElementName();
+        customElements.define(base, class extends LitElement {
+          static styles = [sheet, normal];
+
+          render() {
+            return htmlWithStyles`<div></div><span></span>`;
+          }
+        });
+
+        const el = document.createElement(base);
+        container.appendChild(el);
+        await (el as LitElement).updateComplete;
+        const div = el.shadowRoot!.querySelector('div')!;
+        assert.equal(
+            getComputedStyle(div).getPropertyValue('border-top-width').trim(),
+            '4px');
+
+        const span = el.shadowRoot!.querySelector('span')!;
+        assert.equal(
+            getComputedStyle(span).getPropertyValue('border-top-width').trim(),
+            '4px');
+
+        // When the WC polyfills are included, calling .replaceSync is a noop to
+        // our styles as they're already flattened (so expect 4px). Otherwise,
+        // look for the updated value.
+        const usesAdoptedStyleSheet = (window.ShadyCSS === undefined || window.ShadyCSS.nativeShadow);
+        const expectedValue = usesAdoptedStyleSheet ? '2px' : '4px';
+        sheet.replaceSync('div { border: 2px solid red; }');
+
+        assert.equal(
+            getComputedStyle(div).getPropertyValue('border-top-width').trim(),
+            expectedValue);
+      });
+
+  // Test that when ShadyCSS is enabled (while still having native support for
+  // adoptedStyleSheets), we can return a CSSStyleSheet that will be flattened
+  // and play nice with others.
+  const testShadyCSSWithAdoptedStyleSheetSupport =
+      (window.ShadowRoot) &&
+      ('replace' in CSSStyleSheet.prototype) &&
+      (window.ShadyCSS !== undefined && !window.ShadyCSS.nativeShadow);
+  (testShadyCSSWithAdoptedStyleSheetSupport ? test : test.skip)(
+      'CSSStyleSheet is flattened where ShadyCSS is enabled yet adoptedStyleSheets are supported',
+      async () => {
+        const sheet = new CSSStyleSheet();
+        sheet.replaceSync('div { border: 4px solid red; }');
+
+        const base = generateElementName();
+        customElements.define(base, class extends LitElement {
+          static styles = sheet;
+
+          render() {
+            return htmlWithStyles`<div></div>`;
+          }
+        });
+
+        const el = document.createElement(base);
+        container.appendChild(el);
+        await (el as LitElement).updateComplete;
+
+        const div = el.shadowRoot!.querySelector('div')!;
+        assert.equal(
+            getComputedStyle(div).getPropertyValue('border-top-width').trim(),
+            '4px');
+
+        // CSSStyleSheet update should fail, as the styles will be flattened.
+        sheet.replaceSync('div { border: 2px solid red; }');
+        assert.equal(
+            getComputedStyle(div).getPropertyValue('border-top-width').trim(),
+            '4px', 'CSS should not reflect CSSStyleSheet as it was flattened');
+      });
 });
 
 suite('ShadyDOM', () => {