feat(stepper): add CdkStepHeader directive and fix CdkStepper error on init (#10614)

* Adds the `CdkStepHeader` directive which is the equivalent of `MatStepHeader` and can be used by consumers when building custom steppers.
* Fixes an error that was being thrown on init when using a `cdkStepper` directly.

Fixes #10611.

src/cdk/stepper/public-api.ts

@@ -10,3 +10,4 @@ export * from './stepper';
 export * from './step-label';
 export * from './stepper-button';
 export * from './stepper-module';
+export * from './step-header';

src/cdk/stepper/step-header.ts

@@ -0,0 +1,26 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {Directive, ElementRef} from '@angular/core';
+import {FocusableOption} from '@angular/cdk/a11y';
+
+
+@Directive({
+  selector: '[cdkStepHeader]',
+  host: {
+    'role': 'tab',
+  },
+})
+export class CdkStepHeader implements FocusableOption {
+  constructor(protected _elementRef: ElementRef<HTMLElement>) {}
+
+  /** Focuses the step header. */
+  focus() {
+    this._elementRef.nativeElement.focus();
+  }
+}

src/cdk/stepper/stepper-module.ts

@@ -11,11 +11,26 @@ import {CdkStepper, CdkStep} from './stepper';
 import {CommonModule} from '@angular/common';
 import {CdkStepLabel} from './step-label';
 import {CdkStepperNext, CdkStepperPrevious} from './stepper-button';
+import {CdkStepHeader} from './step-header';
 import {BidiModule} from '@angular/cdk/bidi';
 
 @NgModule({
   imports: [BidiModule, CommonModule],
-  exports: [CdkStep, CdkStepper, CdkStepLabel, CdkStepperNext, CdkStepperPrevious],
-  declarations: [CdkStep, CdkStepper, CdkStepLabel, CdkStepperNext, CdkStepperPrevious]
+  exports: [
+    CdkStep,
+    CdkStepper,
+    CdkStepHeader,
+    CdkStepLabel,
+    CdkStepperNext,
+    CdkStepperPrevious,
+  ],
+  declarations: [
+    CdkStep,
+    CdkStepper,
+    CdkStepHeader,
+    CdkStepLabel,
+    CdkStepperNext,
+    CdkStepperPrevious,
+  ]
 })
 export class CdkStepperModule {}

src/cdk/stepper/stepper.ts

@@ -38,6 +38,7 @@ import {AbstractControl} from '@angular/forms';
 import {CdkStepLabel} from './step-label';
 import {Observable, Subject, of as obaservableOf} from 'rxjs';
 import {startWith, takeUntil} from 'rxjs/operators';
+import {CdkStepHeader} from './step-header';
 
 /** Used to generate unique ID for each stepper component. */
 let nextId = 0;
@@ -242,8 +243,12 @@ export class CdkStepper implements AfterViewInit, OnDestroy {
   /** The list of step components that the stepper is holding. */
   @ContentChildren(CdkStep) _steps: QueryList<CdkStep>;
 
-  /** The list of step headers of the steps in the stepper. */
-  _stepHeader: QueryList<FocusableOption>;
+  /**
+   * The list of step headers of the steps in the stepper.
+   * @deprecated Type to be changed to `QueryList<CdkStepHeader>`.
+   * @breaking-change 8.0.0
+   */
+  @ContentChildren(CdkStepHeader) _stepHeader: QueryList<FocusableOption>;
 
   /** Whether the validity of previous steps should be checked or not. */
   @Input()
@@ -302,7 +307,10 @@ export class CdkStepper implements AfterViewInit, OnDestroy {
   }
 
   ngAfterViewInit() {
-    this._keyManager = new FocusKeyManager(this._stepHeader)
+    // Note that while the step headers are content children by default, any components that
+    // extend this one might have them as view chidren. We initialize the keyboard handling in
+    // AfterViewInit so we're guaranteed for both view and content children to be defined.
+    this._keyManager = new FocusKeyManager<FocusableOption>(this._stepHeader)
       .withWrap()
       .withVerticalOrientation(this._orientation === 'vertical');
 

src/lib/stepper/step-header.ts

@@ -21,7 +21,8 @@ import {Subscription} from 'rxjs';
 import {MatStepLabel} from './step-label';
 import {MatStepperIntl} from './stepper-intl';
 import {MatStepperIconContext} from './stepper-icon';
-import {StepState} from '@angular/cdk/stepper';
+import {CdkStepHeader, StepState} from '@angular/cdk/stepper';
+
 
 @Component({
   moduleId: module.id,
@@ -35,7 +36,7 @@ import {StepState} from '@angular/cdk/stepper';
   encapsulation: ViewEncapsulation.None,
   changeDetection: ChangeDetectionStrategy.OnPush,
 })
-export class MatStepHeader implements OnDestroy {
+export class MatStepHeader extends CdkStepHeader implements OnDestroy {
   private _intlSubscription: Subscription;
 
   /** State of the given step. */
@@ -65,15 +66,16 @@ export class MatStepHeader implements OnDestroy {
   constructor(
     public _intl: MatStepperIntl,
     private _focusMonitor: FocusMonitor,
-    private _element: ElementRef<HTMLElement>,
+    _elementRef: ElementRef<HTMLElement>,
     changeDetectorRef: ChangeDetectorRef) {
-    _focusMonitor.monitor(_element, true);
+    super(_elementRef);
+    _focusMonitor.monitor(_elementRef, true);
     this._intlSubscription = _intl.changes.subscribe(() => changeDetectorRef.markForCheck());
   }
 
   ngOnDestroy() {
     this._intlSubscription.unsubscribe();
-    this._focusMonitor.stopMonitoring(this._element);
+    this._focusMonitor.stopMonitoring(this._elementRef);
   }
 
   /** Returns string label of given step if it is a text label. */
@@ -88,7 +90,7 @@ export class MatStepHeader implements OnDestroy {
 
   /** Returns the host HTML element. */
   _getHostElement() {
-    return this._element.nativeElement;
+    return this._elementRef.nativeElement;
   }
 
   /** Template context variables that are exposed to the `matStepperIcon` instances. */
@@ -99,8 +101,4 @@ export class MatStepHeader implements OnDestroy {
       optional: this.optional
     };
   }
-
-  focus() {
-    this._getHostElement().focus();
-  }
 }