feat(stepper): allow for content to be rendered lazily

Adds the `matStepContent` directive that allows consumers to defer rendering the content of a step until it is opened for the first time.

Fixes angular#12339.

src/cdk/stepper/stepper.ts

@@ -193,7 +193,7 @@ export class CdkStep implements OnChanges {
 
   /** @breaking-change 8.0.0 remove the `?` after `stepperOptions` */
   constructor(
-    @Inject(forwardRef(() => CdkStepper)) private _stepper: CdkStepper,
+    @Inject(forwardRef(() => CdkStepper)) protected _stepper: CdkStepper,
     @Optional() @Inject(STEPPER_GLOBAL_OPTIONS) stepperOptions?: StepperOptions) {
     this._stepperOptions = stepperOptions ? stepperOptions : {};
     this._displayDefaultIndicatorType = this._stepperOptions.displayDefaultIndicatorType !== false;

src/lib/stepper/public-api.ts

@@ -14,3 +14,4 @@ export * from './step-header';
 export * from './stepper-intl';
 export * from './stepper-animations';
 export * from './stepper-icon';
+export * from './step-content';

src/lib/stepper/step-content.ts

@@ -0,0 +1,19 @@
+/**
+ * @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, TemplateRef} from '@angular/core';
+
+/**
+ * Content for a `mat-step` that will be rendered lazily.
+ */
+@Directive({
+  selector: 'ng-template[matStepContent]'
+})
+export class MatStepContent {
+  constructor(public _template: TemplateRef<any>) {}
+}

src/lib/stepper/step.html

@@ -1 +1,4 @@
-<ng-template><ng-content></ng-content></ng-template>
+<ng-template>
+  <ng-content></ng-content>
+  <ng-template [cdkPortalOutlet]="_portal"></ng-template>
+</ng-template>

src/lib/stepper/stepper-module.ts

@@ -19,6 +19,7 @@ import {MatHorizontalStepper, MatStep, MatStepper, MatVerticalStepper} from './s
 import {MatStepperNext, MatStepperPrevious} from './stepper-button';
 import {MatStepperIcon} from './stepper-icon';
 import {MAT_STEPPER_INTL_PROVIDER} from './stepper-intl';
+import {MatStepContent} from './step-content';
 
 
 @NgModule({
@@ -42,6 +43,7 @@ import {MAT_STEPPER_INTL_PROVIDER} from './stepper-intl';
     MatStepperPrevious,
     MatStepHeader,
     MatStepperIcon,
+    MatStepContent,
   ],
   declarations: [
     MatHorizontalStepper,
@@ -53,6 +55,7 @@ import {MAT_STEPPER_INTL_PROVIDER} from './stepper-intl';
     MatStepperPrevious,
     MatStepHeader,
     MatStepperIcon,
+    MatStepContent,
   ],
   providers: [MAT_STEPPER_INTL_PROVIDER, ErrorStateMatcher],
 })

src/lib/stepper/stepper.md

@@ -231,6 +231,13 @@ The stepper can now show error states by simply providing the `showError` option
 
 <!-- example(stepper-errors) -->
 
+### Lazy rendering
+By default, the stepper will render all of it's content when it's initialized. If you have some
+content that you want to want to defer until the particular step is opened, you can put it inside
+an `ng-template` with the `matStepContent` attribute.
+
+<!-- example(stepper-lazy-content) -->
+
 ### Keyboard interaction
 - <kbd>LEFT_ARROW</kbd>: Focuses the previous step header
 - <kbd>RIGHT_ARROW</kbd>: Focuses the next step header

src/lib/stepper/stepper.spec.ts

@@ -1064,6 +1064,44 @@ describe('MatStepper', () => {
       expect(stepper._getIndicatorType(1)).toBe(STEP_STATE.EDIT);
     });
   });
+
+  describe('stepper with lazy content', () => {
+    it('should render the content of the selected step on init', () => {
+      const fixture = createComponent(StepperWithLazyContent);
+      const element = fixture.nativeElement;
+      fixture.componentInstance.selectedIndex = 1;
+      fixture.detectChanges();
+
+      expect(element.textContent).not.toContain('Step 1 content');
+      expect(element.textContent).toContain('Step 2 content');
+      expect(element.textContent).not.toContain('Step 3 content');
+    });
+
+    it('should render the content of steps when the user navigates to them', () => {
+      const fixture = createComponent(StepperWithLazyContent);
+      const element = fixture.nativeElement;
+      fixture.componentInstance.selectedIndex = 0;
+      fixture.detectChanges();
+
+      expect(element.textContent).toContain('Step 1 content');
+      expect(element.textContent).not.toContain('Step 2 content');
+      expect(element.textContent).not.toContain('Step 3 content');
+
+      fixture.componentInstance.selectedIndex = 1;
+      fixture.detectChanges();
+
+      expect(element.textContent).toContain('Step 1 content');
+      expect(element.textContent).toContain('Step 2 content');
+      expect(element.textContent).not.toContain('Step 3 content');
+
+      fixture.componentInstance.selectedIndex = 2;
+      fixture.detectChanges();
+
+      expect(element.textContent).toContain('Step 1 content');
+      expect(element.textContent).toContain('Step 2 content');
+      expect(element.textContent).toContain('Step 3 content');
+    });
+  });
 });
 
 /** Asserts that keyboard interaction works correctly. */
@@ -1498,3 +1536,26 @@ class StepperWithAriaInputs {
   ariaLabel: string;
   ariaLabelledby: string;
 }
+
+@Component({
+  template: `
+    <mat-vertical-stepper [selectedIndex]="selectedIndex">
+      <mat-step>
+        <ng-template matStepLabel>Step 1</ng-template>
+        <ng-template matStepContent>Step 1 content</ng-template>
+      </mat-step>
+      <mat-step>
+        <ng-template matStepLabel>Step 2</ng-template>
+        <ng-template matStepContent>Step 2 content</ng-template>
+      </mat-step>
+      <mat-step>
+        <ng-template matStepLabel>Step 3</ng-template>
+        <ng-template matStepContent>Step 3 content</ng-template>
+      </mat-step>
+    </mat-vertical-stepper>
+  `
+})
+class StepperWithLazyContent {
+  selectedIndex = 0;
+}
+

src/lib/stepper/stepper.ts

@@ -28,24 +28,28 @@ import {
   forwardRef,
   Inject,
   Input,
+  OnDestroy,
   Optional,
   Output,
   QueryList,
   SkipSelf,
   TemplateRef,
   ViewChildren,
+  ViewContainerRef,
   ViewEncapsulation,
 } from '@angular/core';
 import {FormControl, FormGroupDirective, NgForm} from '@angular/forms';
 import {DOCUMENT} from '@angular/common';
 import {ErrorStateMatcher} from '@angular/material/core';
-import {Subject} from 'rxjs';
-import {takeUntil, distinctUntilChanged} from 'rxjs/operators';
+import {TemplatePortal} from '@angular/cdk/portal';
+import {Subject, Subscription} from 'rxjs';
+import {takeUntil, distinctUntilChanged, map, startWith} from 'rxjs/operators';
 
 import {MatStepHeader} from './step-header';
 import {MatStepLabel} from './step-label';
 import {matStepperAnimations} from './stepper-animations';
 import {MatStepperIcon, MatStepperIconContext} from './stepper-icon';
+import {MatStepContent} from './step-content';
 
 @Component({
   moduleId: module.id,
@@ -56,17 +60,45 @@ import {MatStepperIcon, MatStepperIconContext} from './stepper-icon';
   exportAs: 'matStep',
   changeDetection: ChangeDetectionStrategy.OnPush,
 })
-export class MatStep extends CdkStep implements ErrorStateMatcher {
+export class MatStep extends CdkStep implements ErrorStateMatcher, AfterContentInit, OnDestroy {
+  private _isSelected = Subscription.EMPTY;
+
   /** Content for step label given by `<ng-template matStepLabel>`. */
   @ContentChild(MatStepLabel, {static: false}) stepLabel: MatStepLabel;
 
+  /** Content that will be rendered lazily. */
+  @ContentChild(MatStepContent, {static: false}) _lazyContent: MatStepContent;
+
+  /** Currently-attached portal containing the lazy content. */
+  _portal: TemplatePortal;
+
   /** @breaking-change 8.0.0 remove the `?` after `stepperOptions` */
+  /** @breaking-change 9.0.0 _viewContainerRef parameter to become required. */
   constructor(@Inject(forwardRef(() => MatStepper)) stepper: MatStepper,
               @SkipSelf() private _errorStateMatcher: ErrorStateMatcher,
-              @Optional() @Inject(STEPPER_GLOBAL_OPTIONS) stepperOptions?: StepperOptions) {
+              @Optional() @Inject(STEPPER_GLOBAL_OPTIONS) stepperOptions?: StepperOptions,
+              private _viewContainerRef?: ViewContainerRef) {
     super(stepper, stepperOptions);
   }
 
+  ngAfterContentInit() {
+    /** @breaking-change 9.0.0 Null check for _viewContainerRef to be removed. */
+    if (this._viewContainerRef) {
+      this._isSelected = this._stepper.selectionChange.pipe(
+        map(event => event.selectedStep === this),
+        startWith(this._stepper.selected === this)
+      ).subscribe(isSelected => {
+        if (isSelected && this._lazyContent && !this._portal) {
+          this._portal = new TemplatePortal(this._lazyContent._template, this._viewContainerRef!);
+        }
+      });
+    }
+  }
+
+  ngOnDestroy() {
+    this._isSelected.unsubscribe();
+  }
+
   /** Custom error state matcher that additionally checks for validity of interacted form. */
   isErrorState(control: FormControl | null, form: FormGroupDirective | NgForm | null): boolean {
     const originalErrorState = this._errorStateMatcher.isErrorState(control, form);

src/material-examples/stepper-lazy-content/stepper-lazy-content-example.css

@@ -0,0 +1 @@
+/** No CSS for this example */

src/material-examples/stepper-lazy-content/stepper-lazy-content-example.html

@@ -0,0 +1,22 @@
+<mat-vertical-stepper>
+  <mat-step>
+    <ng-template matStepLabel>Step 1</ng-template>
+    <ng-template matStepContent>
+      <p>This content was rendered lazily</p>
+      <button mat-button matStepperNext>Next</button>
+    </ng-template>
+  </mat-step>
+  <mat-step>
+    <ng-template matStepLabel>Step 2</ng-template>
+    <ng-template matStepContent>
+      <p>This content was also rendered lazily</p>
+      <button mat-button matStepperPrevious>Back</button>
+      <button mat-button matStepperNext>Next</button>
+    </ng-template>
+  </mat-step>
+  <mat-step>
+    <ng-template matStepLabel>Step 3</ng-template>
+    <p>This content was rendered eagerly</p>
+    <button mat-button matStepperPrevious>Back</button>
+  </mat-step>
+</mat-vertical-stepper>

src/material-examples/stepper-lazy-content/stepper-lazy-content-example.ts

@@ -0,0 +1,11 @@
+import {Component} from '@angular/core';
+
+/**
+ * @title Stepper lazy content rendering
+ */
+@Component({
+  selector: 'stepper-lazy-content-example',
+  templateUrl: 'stepper-lazy-content-example.html',
+  styleUrls: ['stepper-lazy-content-example.css'],
+})
+export class StepperLazyContentExample {}