fix(stepper): update state when steps change (#8398)

src/cdk/stepper/stepper.ts

@@ -28,12 +28,14 @@ import {
   ChangeDetectionStrategy,
   ChangeDetectorRef,
   OnChanges,
+  OnDestroy
 } from '@angular/core';
 import {LEFT_ARROW, RIGHT_ARROW, ENTER, SPACE} from '@angular/cdk/keycodes';
 import {CdkStepLabel} from './step-label';
 import {coerceBooleanProperty} from '@angular/cdk/coercion';
 import {AbstractControl} from '@angular/forms';
 import {Direction, Directionality} from '@angular/cdk/bidi';
+import {Subject} from 'rxjs/Subject';
 
 /** Used to generate unique ID for each stepper component. */
 let nextId = 0;
@@ -132,7 +134,10 @@ export class CdkStep implements OnChanges {
   selector: '[cdkStepper]',
   exportAs: 'cdkStepper',
 })
-export class CdkStepper {
+export class CdkStepper implements OnDestroy {
+  /** Emits when the component is destroyed. */
+  protected _destroyed = new Subject<void>();
+
   /** The list of step components that the stepper is holding. */
   @ContentChildren(CdkStep) _steps: QueryList<CdkStep>;
 
@@ -186,6 +191,11 @@ export class CdkStepper {
     this._groupId = nextId++;
   }
 
+  ngOnDestroy() {
+    this._destroyed.next();
+    this._destroyed.complete();
+  }
+
   /** Selects and focuses the next step in list. */
   next(): void {
     this.selectedIndex = Math.min(this._selectedIndex + 1, this._steps.length - 1);

src/lib/stepper/stepper.ts

@@ -9,6 +9,7 @@
 import {animate, state, style, transition, trigger} from '@angular/animations';
 import {CdkStep, CdkStepper} from '@angular/cdk/stepper';
 import {
+  AfterContentInit,
   Component,
   ContentChild,
   ContentChildren,
@@ -26,6 +27,7 @@ import {FormControl, FormGroupDirective, NgForm} from '@angular/forms';
 import {ErrorStateMatcher} from '@angular/material/core';
 import {MatStepHeader} from './step-header';
 import {MatStepLabel} from './step-label';
+import {takeUntil} from 'rxjs/operators/takeUntil';
 
 /** Workaround for https://github.com/angular/angular/issues/17849 */
 export const _MatStep = CdkStep;
@@ -66,12 +68,17 @@ export class MatStep extends _MatStep implements ErrorStateMatcher {
 @Directive({
   selector: '[matStepper]'
 })
-export class MatStepper extends _MatStepper {
+export class MatStepper extends _MatStepper implements AfterContentInit {
   /** The list of step headers of the steps in the stepper. */
   @ViewChildren(MatStepHeader, {read: ElementRef}) _stepHeader: QueryList<ElementRef>;
 
   /** Steps that the stepper holds. */
   @ContentChildren(MatStep) _steps: QueryList<MatStep>;
+
+  ngAfterContentInit() {
+    // Mark the component for change detection whenever the content children query changes
+    this._steps.changes.pipe(takeUntil(this._destroyed)).subscribe(() => this._stateChanged());
+  }
 }
 
 @Component({