feat(material/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 #12339.

Author: crisbeto

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 {}

src/material/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/material/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/material/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/material/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/material/stepper/stepper.md

@@ -180,6 +180,13 @@ will not affect steppers marked as `linear`.
 
 <!-- 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/material/stepper/stepper.spec.ts

@@ -1302,6 +1302,44 @@ describe('MatStepper', () => {
       expect(stepper.selectedIndex).toBe(1);
       expect(stepper.selected).toBeTruthy();
     });
+
+  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. */
@@ -1826,3 +1864,26 @@ class NestedSteppers {
 class StepperWithStaticOutOfBoundsIndex {
   @ViewChild(MatStepper) stepper: MatStepper;
 }
+
+
+@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/material/stepper/stepper.ts

@@ -29,24 +29,28 @@ 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, ThemePalette} 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, switchMap} 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({
   selector: 'mat-step',
@@ -59,20 +63,50 @@ 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) stepLabel: MatStepLabel;
 
   /** Theme color for the particular step. */
   @Input() color: ThemePalette;
 
+  /** 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.steps.changes.pipe(switchMap(() => {
+        return 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);

tools/public_api_guard/material/stepper.d.ts

@@ -16,13 +16,24 @@ export declare class MatHorizontalStepper extends MatStepper {
     static ɵfac: i0.ɵɵFactoryDef<MatHorizontalStepper, never>;
 }
 
-export declare class MatStep extends CdkStep implements ErrorStateMatcher {
+export declare class MatStep extends CdkStep implements ErrorStateMatcher, AfterContentInit, OnDestroy {
+    _lazyContent: MatStepContent;
+    _portal: TemplatePortal;
     color: ThemePalette;
     stepLabel: MatStepLabel;
-    constructor(stepper: MatStepper, _errorStateMatcher: ErrorStateMatcher, stepperOptions?: StepperOptions);
+    constructor(stepper: MatStepper, _errorStateMatcher: ErrorStateMatcher, stepperOptions?: StepperOptions, _viewContainerRef?: ViewContainerRef | undefined);
     isErrorState(control: FormControl | null, form: FormGroupDirective | NgForm | null): boolean;
-    static ɵcmp: i0.ɵɵComponentDefWithMeta<MatStep, "mat-step", ["matStep"], { "color": "color"; }, {}, ["stepLabel"], ["*"]>;
-    static ɵfac: i0.ɵɵFactoryDef<MatStep, [null, { skipSelf: true; }, { optional: true; }]>;
+    ngAfterContentInit(): void;
+    ngOnDestroy(): void;
+    static ɵcmp: i0.ɵɵComponentDefWithMeta<MatStep, "mat-step", ["matStep"], { "color": "color"; }, {}, ["stepLabel", "_lazyContent"], ["*"]>;
+    static ɵfac: i0.ɵɵFactoryDef<MatStep, [null, { skipSelf: true; }, { optional: true; }, null]>;
+}
+
+export declare class MatStepContent {
+    _template: TemplateRef<any>;
+    constructor(_template: TemplateRef<any>);
+    static ɵdir: i0.ɵɵDirectiveDefWithMeta<MatStepContent, "ng-template[matStepContent]", never, {}, {}, never>;
+    static ɵfac: i0.ɵɵFactoryDef<MatStepContent, never>;
 }
 
 export declare class MatStepHeader extends _MatStepHeaderMixinBase implements AfterViewInit, OnDestroy, CanColor {
@@ -105,7 +116,7 @@ export declare class MatStepperIntl {
 
 export declare class MatStepperModule {
     static ɵinj: i0.ɵɵInjectorDef<MatStepperModule>;
-    static ɵmod: i0.ɵɵNgModuleDefWithMeta<MatStepperModule, [typeof i1.MatHorizontalStepper, typeof i1.MatVerticalStepper, typeof i1.MatStep, typeof i2.MatStepLabel, typeof i1.MatStepper, typeof i3.MatStepperNext, typeof i3.MatStepperPrevious, typeof i4.MatStepHeader, typeof i5.MatStepperIcon], [typeof i6.MatCommonModule, typeof i7.CommonModule, typeof i8.PortalModule, typeof i9.MatButtonModule, typeof i10.CdkStepperModule, typeof i11.MatIconModule, typeof i6.MatRippleModule], [typeof i6.MatCommonModule, typeof i1.MatHorizontalStepper, typeof i1.MatVerticalStepper, typeof i1.MatStep, typeof i2.MatStepLabel, typeof i1.MatStepper, typeof i3.MatStepperNext, typeof i3.MatStepperPrevious, typeof i4.MatStepHeader, typeof i5.MatStepperIcon]>;
+    static ɵmod: i0.ɵɵNgModuleDefWithMeta<MatStepperModule, [typeof i1.MatHorizontalStepper, typeof i1.MatVerticalStepper, typeof i1.MatStep, typeof i2.MatStepLabel, typeof i1.MatStepper, typeof i3.MatStepperNext, typeof i3.MatStepperPrevious, typeof i4.MatStepHeader, typeof i5.MatStepperIcon, typeof i6.MatStepContent], [typeof i7.MatCommonModule, typeof i8.CommonModule, typeof i9.PortalModule, typeof i10.MatButtonModule, typeof i11.CdkStepperModule, typeof i12.MatIconModule, typeof i7.MatRippleModule], [typeof i7.MatCommonModule, typeof i1.MatHorizontalStepper, typeof i1.MatVerticalStepper, typeof i1.MatStep, typeof i2.MatStepLabel, typeof i1.MatStepper, typeof i3.MatStepperNext, typeof i3.MatStepperPrevious, typeof i4.MatStepHeader, typeof i5.MatStepperIcon, typeof i6.MatStepContent]>;
 }
 
 export declare class MatStepperNext extends CdkStepperNext {