feat(stepper): add the ability to reset a stepper

src/cdk/stepper/stepper.md

@@ -45,6 +45,10 @@ There are two button directives to support navigation between different steps:
 `CdkStepperNext` and `CdkStepperPrevious`. When placed inside of a step, these will automatically
 add click handlers to advance or rewind the workflow, respectively.
 
+### Resetting a stepper
+If you want to reset a stepper to its initial state, you can use the `reset` method. Note that
+resetting it will call `reset` on the underlying form control which clears the value.
+
 ### Keyboard interaction
 - <kbd>LEFT_ARROW</kbd>: Focuses the previous step header
 - <kbd>RIGHT_ARROW</kbd>: Focuses the next step header

src/cdk/stepper/stepper.ts

@@ -123,6 +123,16 @@ export class CdkStep implements OnChanges {
     this._stepper.selected = this;
   }
 
+  /** Resets the step to its initial state. Note that this includes resetting form data. */
+  reset(): void {
+    this.interacted = false;
+    this.completed = false;
+
+    if (this.stepControl) {
+      this.stepControl.reset();
+    }
+  }
+
   ngOnChanges() {
     // Since basically all inputs of the MatStep get proxied through the view down to the
     // underlying MatStepHeader, we have to make sure that change detection runs correctly.
@@ -208,6 +218,13 @@ export class CdkStepper implements OnDestroy {
     this.selectedIndex = Math.max(this._selectedIndex - 1, 0);
   }
 
+  /** Resets the stepper to its initial state. Note that this includes clearing form data. */
+  reset(): void {
+    this.selectedIndex = 0;
+    this._steps.forEach(step => step.reset());
+    this._stateChanged();
+  }
+
   /** Returns a unique id for each step label element. */
   _getStepLabelId(i: number): string {
     return `cdk-step-label-${this._groupId}-${i}`;

src/demo-app/stepper/stepper-demo.html

@@ -2,7 +2,7 @@
 
 <h3>Linear Vertical Stepper Demo using a single form</h3>
 <form [formGroup]="formGroup">
-  <mat-vertical-stepper formArrayName="formArray" [linear]="!isNonLinear">
+  <mat-vertical-stepper #linearVerticalStepper="matVerticalStepper" formArrayName="formArray" [linear]="!isNonLinear">
     <mat-step formGroupName="0" [stepControl]="formArray?.get([0])">
       <ng-template matStepLabel>Fill out your name</ng-template>
       <mat-form-field>
@@ -38,13 +38,14 @@ <h3>Linear Vertical Stepper Demo using a single form</h3>
       Everything seems correct.
       <div>
         <button mat-button>Done</button>
+        <button type="button" mat-button (click)="linearVerticalStepper.reset()">Reset</button>
       </div>
     </mat-step>
   </mat-vertical-stepper>
 </form>
 
 <h3>Linear Horizontal Stepper Demo using a different form for each step</h3>
-<mat-horizontal-stepper [linear]="!isNonLinear">
+<mat-horizontal-stepper #linearHorizontalStepper="matHorizontalStepper" [linear]="!isNonLinear">
   <mat-step [stepControl]="nameFormGroup">
     <form [formGroup]="nameFormGroup">
       <ng-template matStepLabel>Fill out your name</ng-template>
@@ -82,6 +83,7 @@ <h3>Linear Horizontal Stepper Demo using a different form for each step</h3>
       Everything seems correct.
       <div>
         <button mat-button>Done</button>
+        <button type="button" mat-button (click)="linearHorizontalStepper.reset()">Reset</button>
       </div>
     </form>
   </mat-step>

src/lib/stepper/stepper.spec.ts

@@ -250,6 +250,10 @@ describe('MatHorizontalStepper', () => {
 
           expect(stepper.selectedIndex).toBe(1);
         });
+
+    it('should be able to reset the stepper to its initial state', () => {
+      assertLinearStepperResetable(fixture);
+    });
   });
 });
 
@@ -413,6 +417,10 @@ describe('MatVerticalStepper', () => {
     it('should be able to move to next step even when invalid if current step is optional', () => {
       assertOptionalStepValidity(testComponent, fixture);
     });
+
+    it('should be able to reset the stepper to its initial state', () => {
+      assertLinearStepperResetable(fixture);
+    });
   });
 });
 
@@ -850,6 +858,49 @@ function asyncValidator(minLength: number, validationTrigger: Observable<any>):
   };
 }
 
+
+/** Asserts that a stepper can be reset. */
+function assertLinearStepperResetable(
+    fixture: ComponentFixture<LinearMatHorizontalStepperApp|LinearMatVerticalStepperApp>) {
+
+  const testComponent = fixture.componentInstance;
+  const stepperComponent = fixture.debugElement.query(By.directive(MatStepper)).componentInstance;
+  const steps = stepperComponent._steps.toArray();
+
+  testComponent.oneGroup.get('oneCtrl')!.setValue('value');
+  fixture.detectChanges();
+
+  stepperComponent.next();
+  fixture.detectChanges();
+
+  stepperComponent.next();
+  fixture.detectChanges();
+
+  expect(stepperComponent.selectedIndex).toBe(1);
+  expect(steps[0].interacted).toBe(true);
+  expect(steps[0].completed).toBe(true);
+  expect(testComponent.oneGroup.get('oneCtrl')!.valid).toBe(true);
+  expect(testComponent.oneGroup.get('oneCtrl')!.value).toBe('value');
+
+  expect(steps[1].interacted).toBe(true);
+  expect(steps[1].completed).toBe(false);
+  expect(testComponent.twoGroup.get('twoCtrl')!.valid).toBe(false);
+
+  stepperComponent.reset();
+  fixture.detectChanges();
+
+  expect(stepperComponent.selectedIndex).toBe(0);
+  expect(steps[0].interacted).toBe(false);
+  expect(steps[0].completed).toBe(false);
+  expect(testComponent.oneGroup.get('oneCtrl')!.valid).toBe(false);
+  expect(testComponent.oneGroup.get('oneCtrl')!.value).toBeFalsy();
+
+  expect(steps[1].interacted).toBe(false);
+  expect(steps[1].completed).toBe(false);
+  expect(testComponent.twoGroup.get('twoCtrl')!.valid).toBe(false);
+}
+
+
 @Component({
   template: `
     <mat-horizontal-stepper>

src/material-examples/stepper-overview/stepper-overview-example.html

@@ -1,6 +1,6 @@
 <button mat-raised-button (click)="isLinear = true" id="toggle-linear">Enable linear mode</button>
 
-<mat-horizontal-stepper [linear]="isLinear">
+<mat-horizontal-stepper [linear]="isLinear" #stepper="matHorizontalStepper">
   <mat-step [stepControl]="firstFormGroup">
     <form [formGroup]="firstFormGroup">
       <ng-template matStepLabel>Fill out your name</ng-template>
@@ -29,6 +29,7 @@
     You are now done.
     <div>
       <button mat-button matStepperPrevious>Back</button>
+      <button mat-button (click)="stepper.reset()">Reset</button>
     </div>
   </mat-step>
 </mat-horizontal-stepper>