feat(tab-nav-bar): allow setting tabindex for links

* For navigation bars, navigation through the arrow keys is not supported. Since every element needs to be tabbable for accessibility reasons, the tabindex should be changeable.
* Also uses the tabindex mixin to reduce payload size.

Author: devversion

src/lib/tabs/tab-nav-bar/tab-nav-bar.spec.ts

@@ -4,7 +4,7 @@ import {By} from '@angular/platform-browser';
 import {dispatchFakeEvent, dispatchMouseEvent} from '@angular/cdk/testing';
 import {Direction, Directionality} from '@angular/cdk/bidi';
 import {Subject} from 'rxjs/Subject';
-import {MatTabNav, MatTabsModule, MatTabLink} from '../index';
+import {MatTabLink, MatTabNav, MatTabsModule} from '../index';
 
 
 describe('MatTabNavBar', () => {
@@ -17,6 +17,8 @@ describe('MatTabNavBar', () => {
       declarations: [
         SimpleTabNavBarTestApp,
         TabLinkWithNgIf,
+        TabLinkWithTabIndexBinding,
+        TabLinkWithNativeTabindexAttr,
       ],
       providers: [
         {provide: Directionality, useFactory: () => ({
@@ -119,9 +121,7 @@ describe('MatTabNavBar', () => {
       fixture.componentInstance.disabled = true;
       fixture.detectChanges();
 
-      expect(tabLinkElements.every(tabLink => {
-        return tabLink.getAttribute('tabIndex') === null;
-      }))
+      expect(tabLinkElements.every(tabLink => tabLink.tabIndex === -1))
         .toBe(true, 'Expected element to no longer be keyboard focusable if disabled.');
     });
 
@@ -212,6 +212,30 @@ describe('MatTabNavBar', () => {
     expect(link.querySelector('.mat-ripple-element'))
       .toBeFalsy('Expected no ripple to be created when ripple target is destroyed.');
   });
+
+  it('should support the native tabindex attribute', () => {
+      const fixture = TestBed.createComponent(TabLinkWithNativeTabindexAttr);
+    fixture.detectChanges();
+
+    const tabLink = fixture.debugElement.query(By.directive(MatTabLink)).injector.get(MatTabLink);
+
+    expect(tabLink.tabIndex)
+      .toBe(5, 'Expected the tabIndex to be set from the native tabindex attribute.');
+  });
+
+  it('should support binding to the tabIndex', () => {
+    const fixture = TestBed.createComponent(TabLinkWithTabIndexBinding);
+    fixture.detectChanges();
+
+    const tabLink = fixture.debugElement.query(By.directive(MatTabLink)).injector.get(MatTabLink);
+
+    expect(tabLink.tabIndex).toBe(0, 'Expected the tabIndex to be set to 0 by default.');
+
+    fixture.componentInstance.tabIndex = 3;
+    fixture.detectChanges();
+
+    expect(tabLink.tabIndex).toBe(3, 'Expected the tabIndex to be have been set to 3.');
+  });
 });
 
 @Component({
@@ -249,3 +273,23 @@ class SimpleTabNavBarTestApp {
 class TabLinkWithNgIf {
   isDestroyed = false;
 }
+
+@Component({
+  template: `
+    <nav mat-tab-nav-bar>
+      <a mat-tab-link [tabIndex]="tabIndex">TabIndex Link</a>
+    </nav>
+  `
+})
+class TabLinkWithTabIndexBinding {
+  tabIndex = 0;
+}
+
+@Component({
+  template: `
+    <nav mat-tab-nav-bar>
+      <a mat-tab-link tabindex="5">Link</a>
+    </nav>
+  `
+})
+class TabLinkWithNativeTabindexAttr {}

src/lib/tabs/tab-nav-bar/tab-nav-bar.ts

@@ -13,6 +13,7 @@ import {takeUntil} from '@angular/cdk/rxjs';
 import {ViewportRuler} from '@angular/cdk/scrolling';
 import {
   AfterContentInit,
+  Attribute,
   ChangeDetectionStrategy,
   ChangeDetectorRef,
   Component,
@@ -34,11 +35,13 @@ import {
   CanColor,
   CanDisable,
   CanDisableRipple,
+  HasTabIndex,
   MAT_RIPPLE_GLOBAL_OPTIONS,
   MatRipple,
   mixinColor,
   mixinDisabled,
   mixinDisableRipple,
+  mixinTabIndex,
   RippleGlobalOptions,
   ThemePalette,
 } from '@angular/material/core';
@@ -171,15 +174,15 @@ export class MatTabNav extends _MatTabNavMixinBase implements AfterContentInit,
 
 // Boilerplate for applying mixins to MatTabLink.
 export class MatTabLinkBase {}
-export const _MatTabLinkMixinBase = mixinDisabled(MatTabLinkBase);
+export const _MatTabLinkMixinBase = mixinTabIndex(mixinDisabled(MatTabLinkBase));
 
 /**
  * Link inside of a `mat-tab-nav-bar`.
  */
 @Directive({
   selector: '[mat-tab-link], [matTabLink]',
   exportAs: 'matTabLink',
-  inputs: ['disabled'],
+  inputs: ['disabled', 'tabIndex'],
   host: {
     'class': 'mat-tab-link',
     '[attr.aria-disabled]': 'disabled.toString()',
@@ -188,7 +191,9 @@ export const _MatTabLinkMixinBase = mixinDisabled(MatTabLinkBase);
     '[class.mat-tab-label-active]': 'active',
   }
 })
-export class MatTabLink extends _MatTabLinkMixinBase implements OnDestroy, CanDisable {
+export class MatTabLink extends _MatTabLinkMixinBase
+    implements OnDestroy, CanDisable, HasTabIndex {
+
   /** Whether the tab link is active or not. */
   private _isActive: boolean = false;
 
@@ -216,21 +221,19 @@ export class MatTabLink extends _MatTabLinkMixinBase implements OnDestroy, CanDi
     this._tabLinkRipple._updateRippleRenderer();
   }
 
-  /** @docs-private */
-  get tabIndex(): number | null {
-    return this.disabled ? null : 0;
-  }
-
   constructor(private _tabNavBar: MatTabNav,
               private _elementRef: ElementRef,
               ngZone: NgZone,
               platform: Platform,
-              @Optional() @Inject(MAT_RIPPLE_GLOBAL_OPTIONS) globalOptions: RippleGlobalOptions) {
+              @Optional() @Inject(MAT_RIPPLE_GLOBAL_OPTIONS) globalOptions: RippleGlobalOptions,
+              @Attribute('tabindex') tabIndex: string) {
     super();
 
     // Manually create a ripple instance that uses the tab link element as trigger element.
     // Notice that the lifecycle hooks for the ripple config won't be called anymore.
     this._tabLinkRipple = new MatRipple(_elementRef, ngZone, platform, globalOptions);
+
+    this.tabIndex = parseInt(tabIndex) || 0;
   }
 
   ngOnDestroy() {