feat(material/icon): SEO friendly ligature icons

The new way to use ligature icons, via the `fontIcon` attribute, allows to hide
the font name from search engine results. Otherwise, the font name, which was
never intended to be read by any end-users, would appear in the middle of legit
sentences in search results, thus making the search result very confusing to read.

New recommended usage is:

```diff
- <mat-icon>home</mat-icon>
+ <mat-icon fontIcon="home"></mat-icon>
```

To also enable this for custom font, include the special `mat-ligature-font` class
when registering the font alias. So like this:

```ts
iconRegistry.registerFontClassAlias('f1', 'font1 mat-ligature-font');
```

And use like this:

```html
<mat-icon fontSet="f1" fontIcon="home"></mat-icon>
```

Fixes #23195
Fixes #23183
Fixes google/material-design-icons#498

src/components-examples/material/icon/icon-harness/icon-harness-example.spec.ts

@@ -32,24 +32,24 @@ describe('IconHarnessExample', () => {
 
   it('should load all icon harnesses', async () => {
     const icons = await loader.getAllHarnesses(MatIconHarness);
-    expect(icons.length).toBe(3);
+    expect(icons.length).toBe(4);
   });
 
   it('should get the name of an icon', async () => {
     const icons = await loader.getAllHarnesses(MatIconHarness);
     const names = await parallel(() => icons.map(icon => icon.getName()));
-    expect(names).toEqual(['fontIcon', 'svgIcon', 'ligature_icon']);
+    expect(names).toEqual(['fontIcon', 'svgIcon', 'ligature_icon', 'ligature_icon_by_attribute']);
   });
 
   it('should get the namespace of an icon', async () => {
     const icons = await loader.getAllHarnesses(MatIconHarness);
     const namespaces = await parallel(() => icons.map(icon => icon.getNamespace()));
-    expect(namespaces).toEqual(['fontIcons', 'svgIcons', null]);
+    expect(namespaces).toEqual(['fontIcons', 'svgIcons', null, null]);
   });
 
   it('should get whether an icon is inline', async () => {
     const icons = await loader.getAllHarnesses(MatIconHarness);
     const inlineStates = await parallel(() => icons.map(icon => icon.isInline()));
-    expect(inlineStates).toEqual([false, false, true]);
+    expect(inlineStates).toEqual([false, false, true, false]);
   });
 });

src/components-examples/material/icon/icon-overview/icon-overview-example.html

@@ -1 +1 @@
-<mat-icon aria-hidden="false" aria-label="Example home icon">home</mat-icon>
+<mat-icon aria-hidden="false" aria-label="Example home icon" fontIcon="home"></mat-icon>

src/dev-app/icon/icon-demo.html

@@ -38,7 +38,12 @@
   </p>
 
   <p>
-    Ligature from Material Icons font:
+    Ligature from Material Icons font by attribute:
+    <mat-icon fontIcon="home"></mat-icon>
+  </p>
+
+  <p>
+    Ligature from Material Icons font by content:
     <mat-icon>home</mat-icon>
   </p>
 

src/material/icon/icon-registry.ts

@@ -13,12 +13,12 @@ import {
   Inject,
   Injectable,
   InjectionToken,
+  OnDestroy,
   Optional,
   SecurityContext,
   SkipSelf,
-  OnDestroy,
 } from '@angular/core';
-import {DomSanitizer, SafeResourceUrl, SafeHtml} from '@angular/platform-browser';
+import {DomSanitizer, SafeHtml, SafeResourceUrl} from '@angular/platform-browser';
 import {forkJoin, Observable, of as observableOf, throwError as observableThrow} from 'rxjs';
 import {catchError, finalize, map, share, tap} from 'rxjs/operators';
 import {TrustedHTML, trustedHTMLFromString} from './trusted-types';
@@ -149,7 +149,7 @@ export class MatIconRegistry implements OnDestroy {
    * specified. The default 'material-icons' value assumes that the material icon font has been
    * loaded as described at http://google.github.io/material-design-icons/#icon-font-for-the-web
    */
-  private _defaultFontSetClass = ['material-icons'];
+  private _defaultFontSetClass = ['material-icons', 'mat-ligature-font'];
 
   constructor(
     @Optional() private _httpClient: HttpClient,
@@ -281,15 +281,28 @@ export class MatIconRegistry implements OnDestroy {
   }
 
   /**
-   * Defines an alias for a CSS class name to be used for icon fonts. Creating an matIcon
+   * Defines an alias for CSS class names to be used for icon fonts. Creating an matIcon
    * component with the alias as the fontSet input will cause the class name to be applied
    * to the `<mat-icon>` element.
    *
+   * If the registered font is a ligature font, then don't forget to also include the special
+   * class `mat-ligature-font` to allow the usage via attribute. So register like this:
+   *
+   * ```ts
+   * iconRegistry.registerFontClassAlias('f1', 'font1 mat-ligature-font');
+   * ```
+   *
+   * And use like this:
+   *
+   * ```html
+   * <mat-icon fontSet="f1" fontIcon="home"></mat-icon>
+   * ```
+   *
    * @param alias Alias for the font.
-   * @param className Class name override to be used instead of the alias.
+   * @param classNames Class names override to be used instead of the alias.
    */
-  registerFontClassAlias(alias: string, className: string = alias): this {
-    this._fontCssClassesByAlias.set(alias, className);
+  registerFontClassAlias(alias: string, classNames: string = alias): this {
+    this._fontCssClassesByAlias.set(alias, classNames);
     return this;
   }
 

src/material/icon/icon.scss

@@ -21,6 +21,10 @@ $size: 24px !default;
     line-height: inherit;
     width: inherit;
   }
+
+  &.mat-ligature-font[fontIcon]::before {
+    content: attr(fontIcon);
+  }
 }
 
 // Icons that will be mirrored in RTL.

src/material/icon/icon.spec.ts

@@ -69,6 +69,7 @@ describe('MatIcon', () => {
       declarations: [
         IconWithColor,
         IconWithLigature,
+        IconWithLigatureByAttribute,
         IconWithCustomFontCss,
         IconFromSvgName,
         IconWithAriaHiddenFalse,
@@ -125,6 +126,7 @@ describe('MatIcon', () => {
     fixture.detectChanges();
     expect(sortedClassNames(matIconElement)).toEqual([
       'mat-icon',
+      'mat-ligature-font',
       'mat-primary',
       'material-icons',
       'notranslate',
@@ -143,6 +145,7 @@ describe('MatIcon', () => {
     expect(sortedClassNames(matIconElement)).toEqual([
       'mat-icon',
       'mat-icon-no-color',
+      'mat-ligature-font',
       'material-icons',
       'notranslate',
     ]);
@@ -179,7 +182,7 @@ describe('MatIcon', () => {
   });
 
   describe('Ligature icons', () => {
-    it('should add material-icons class by default', () => {
+    it('should add material-icons and mat-ligature-font class by default', () => {
       const fixture = TestBed.createComponent(IconWithLigature);
 
       const testComponent = fixture.componentInstance;
@@ -189,13 +192,14 @@ describe('MatIcon', () => {
       expect(sortedClassNames(matIconElement)).toEqual([
         'mat-icon',
         'mat-icon-no-color',
+        'mat-ligature-font',
         'material-icons',
         'notranslate',
       ]);
     });
 
     it('should use alternate icon font if set', () => {
-      iconRegistry.setDefaultFontSetClass('myfont');
+      iconRegistry.setDefaultFontSetClass('myfont', 'mat-ligature-font');
 
       const fixture = TestBed.createComponent(IconWithLigature);
 
@@ -206,6 +210,7 @@ describe('MatIcon', () => {
       expect(sortedClassNames(matIconElement)).toEqual([
         'mat-icon',
         'mat-icon-no-color',
+        'mat-ligature-font',
         'myfont',
         'notranslate',
       ]);
@@ -223,7 +228,7 @@ describe('MatIcon', () => {
     });
 
     it('should be able to provide multiple alternate icon set classes', () => {
-      iconRegistry.setDefaultFontSetClass('myfont', 'myfont-48x48');
+      iconRegistry.setDefaultFontSetClass('myfont', 'mat-ligature-font', 'myfont-48x48');
 
       let fixture = TestBed.createComponent(IconWithLigature);
 
@@ -234,6 +239,62 @@ describe('MatIcon', () => {
       expect(sortedClassNames(matIconElement)).toEqual([
         'mat-icon',
         'mat-icon-no-color',
+        'mat-ligature-font',
+        'myfont',
+        'myfont-48x48',
+        'notranslate',
+      ]);
+    });
+  });
+
+  describe('Ligature icons by attribute', () => {
+    it('should add material-icons and mat-ligature-font class by default', () => {
+      const fixture = TestBed.createComponent(IconWithLigatureByAttribute);
+
+      const testComponent = fixture.componentInstance;
+      const matIconElement = fixture.debugElement.nativeElement.querySelector('mat-icon');
+      testComponent.iconName = 'home';
+      fixture.detectChanges();
+      expect(sortedClassNames(matIconElement)).toEqual([
+        'mat-icon',
+        'mat-icon-no-color',
+        'mat-ligature-font',
+        'material-icons',
+        'notranslate',
+      ]);
+    });
+
+    it('should use alternate icon font if set', () => {
+      iconRegistry.setDefaultFontSetClass('myfont', 'mat-ligature-font');
+
+      const fixture = TestBed.createComponent(IconWithLigatureByAttribute);
+
+      const testComponent = fixture.componentInstance;
+      const matIconElement = fixture.debugElement.nativeElement.querySelector('mat-icon');
+      testComponent.iconName = 'home';
+      fixture.detectChanges();
+      expect(sortedClassNames(matIconElement)).toEqual([
+        'mat-icon',
+        'mat-icon-no-color',
+        'mat-ligature-font',
+        'myfont',
+        'notranslate',
+      ]);
+    });
+
+    it('should be able to provide multiple alternate icon set classes', () => {
+      iconRegistry.setDefaultFontSetClass('myfont', 'mat-ligature-font', 'myfont-48x48');
+
+      let fixture = TestBed.createComponent(IconWithLigatureByAttribute);
+
+      const testComponent = fixture.componentInstance;
+      const matIconElement = fixture.debugElement.nativeElement.querySelector('mat-icon');
+      testComponent.iconName = 'home';
+      fixture.detectChanges();
+      expect(sortedClassNames(matIconElement)).toEqual([
+        'mat-icon',
+        'mat-icon-no-color',
+        'mat-ligature-font',
         'myfont',
         'myfont-48x48',
         'notranslate',
@@ -1042,17 +1103,18 @@ describe('MatIcon', () => {
     it('should handle values with extraneous spaces being passed in to `fontIcon`', () => {
       const fixture = TestBed.createComponent(IconWithCustomFontCss);
       const matIconElement = fixture.debugElement.nativeElement.querySelector('mat-icon');
+      fixture.componentInstance.fontSet = 'f1';
 
       expect(() => {
         fixture.componentInstance.fontIcon = 'font icon';
         fixture.detectChanges();
       }).not.toThrow();
 
       expect(sortedClassNames(matIconElement)).toEqual([
+        'f1',
         'font',
         'mat-icon',
         'mat-icon-no-color',
-        'material-icons',
         'notranslate',
       ]);
 
@@ -1063,9 +1125,9 @@ describe('MatIcon', () => {
 
       expect(sortedClassNames(matIconElement)).toEqual([
         'changed',
+        'f1',
         'mat-icon',
         'mat-icon-no-color',
-        'material-icons',
         'notranslate',
       ]);
     });
@@ -1311,6 +1373,11 @@ class IconWithLigature {
   iconName = '';
 }
 
+@Component({template: `<mat-icon [fontIcon]="iconName"></mat-icon>`})
+class IconWithLigatureByAttribute {
+  iconName = '';
+}
+
 @Component({template: `<mat-icon [color]="iconColor">{{iconName}}</mat-icon>`})
 class IconWithColor {
   iconName = '';

src/material/icon/icon.ts

@@ -114,13 +114,18 @@ const funcIriPattern = /^url\(['"]?#(.*?)['"]?\)$/;
  *     `<mat-icon svgIcon="left-arrow"></mat-icon>
  *     <mat-icon svgIcon="animals:cat"></mat-icon>`
  *
- * - Use a font ligature as an icon by putting the ligature text in the content of the `<mat-icon>`
- *   component. By default the Material icons font is used as described at
+ * - Use a font ligature as an icon by putting the ligature text in the `fontIcon` attribute or the
+ *   content of the `<mat-icon>` component. If you register a custom font class, don't forget to also
+ *   include the special class `mat-ligature-font`. It is recommended to use the attribute alternative
+ *   to prevent the ligature text to be selectable and to appear in search engine results.
+ *   By default, the Material icons font is used as described at
  *   http://google.github.io/material-design-icons/#icon-font-for-the-web. You can specify an
  *   alternate font by setting the fontSet input to either the CSS class to apply to use the
  *   desired font, or to an alias previously registered with MatIconRegistry.registerFontClassAlias.
  *   Examples:
- *     `<mat-icon>home</mat-icon>
+ *     `<mat-icon fontIcon="home"></mat-icon>
+ *     <mat-icon>home</mat-icon>
+ *     <mat-icon fontSet="myfont" fontIcon="sun"></mat-icon>
  *     <mat-icon fontSet="myfont">sun</mat-icon>`
  *
  * - Specify a font glyph to be included via CSS rules by setting the fontSet input to specify the
@@ -359,15 +364,18 @@ export class MatIcon extends _MatIconBase implements OnInit, AfterViewChecked, C
     const elem: HTMLElement = this._elementRef.nativeElement;
     const fontSetClasses = (
       this.fontSet
-        ? [this._iconRegistry.classNameForFontAlias(this.fontSet)]
+        ? this._iconRegistry.classNameForFontAlias(this.fontSet).split(/ +/)
         : this._iconRegistry.getDefaultFontSetClass()
     ).filter(className => className.length > 0);
 
     this._previousFontSetClass.forEach(className => elem.classList.remove(className));
     fontSetClasses.forEach(className => elem.classList.add(className));
     this._previousFontSetClass = fontSetClasses;
 
-    if (this.fontIcon !== this._previousFontIconClass) {
+    if (
+      this.fontIcon !== this._previousFontIconClass &&
+      !fontSetClasses.includes('mat-ligature-font')
+    ) {
       if (this._previousFontIconClass) {
         elem.classList.remove(this._previousFontIconClass);
       }

src/material/icon/testing/shared.spec.ts

@@ -37,7 +37,7 @@ export function runHarnessTests(
 
   it('should load all icon harnesses', async () => {
     const icons = await loader.getAllHarnesses(iconHarness);
-    expect(icons.length).toBe(3);
+    expect(icons.length).toBe(4);
   });
 
   it('should filter icon harnesses based on their type', async () => {
@@ -47,7 +47,7 @@ export function runHarnessTests(
     ]);
 
     expect(svgIcons.length).toBe(1);
-    expect(fontIcons.length).toBe(2);
+    expect(fontIcons.length).toBe(3);
   });
 
   it('should filter icon harnesses based on their name', async () => {
@@ -69,31 +69,31 @@ export function runHarnessTests(
 
     expect(regexFilterResults.length).toBe(1);
     expect(stringFilterResults.length).toBe(1);
-    expect(nullFilterResults.length).toBe(1);
+    expect(nullFilterResults.length).toBe(2);
   });
 
   it('should get the type of each icon', async () => {
     const icons = await loader.getAllHarnesses(iconHarness);
     const types = await parallel(() => icons.map(icon => icon.getType()));
-    expect(types).toEqual([IconType.FONT, IconType.SVG, IconType.FONT]);
+    expect(types).toEqual([IconType.FONT, IconType.SVG, IconType.FONT, IconType.FONT]);
   });
 
   it('should get the name of an icon', async () => {
     const icons = await loader.getAllHarnesses(iconHarness);
     const names = await parallel(() => icons.map(icon => icon.getName()));
-    expect(names).toEqual(['fontIcon', 'svgIcon', 'ligature_icon']);
+    expect(names).toEqual(['fontIcon', 'svgIcon', 'ligature_icon', 'ligature_icon_by_attribute']);
   });
 
   it('should get the namespace of an icon', async () => {
     const icons = await loader.getAllHarnesses(iconHarness);
     const namespaces = await parallel(() => icons.map(icon => icon.getNamespace()));
-    expect(namespaces).toEqual(['fontIcons', 'svgIcons', null]);
+    expect(namespaces).toEqual(['fontIcons', 'svgIcons', null, null]);
   });
 
   it('should get whether an icon is inline', async () => {
     const icons = await loader.getAllHarnesses(iconHarness);
     const inlineStates = await parallel(() => icons.map(icon => icon.isInline()));
-    expect(inlineStates).toEqual([false, false, true]);
+    expect(inlineStates).toEqual([false, false, true, false]);
   });
 }
 
@@ -102,6 +102,7 @@ export function runHarnessTests(
     <mat-icon fontSet="fontIcons" fontIcon="fontIcon"></mat-icon>
     <mat-icon svgIcon="svgIcons:svgIcon"></mat-icon>
     <mat-icon inline>ligature_icon</mat-icon>
+    <mat-icon fontIcon="ligature_icon_by_attribute"></mat-icon>
   `,
 })
 class IconHarnessTest {}