feat(aio): animate the leaving/entering documents (#18428)

This commit adds a simple fade-in/out animation.

Fixes #15629

PR Close #18428

aio/src/app/app.component.spec.ts

@@ -56,13 +56,18 @@ describe('AppComponent', () => {
     tocService = de.injector.get(TocService);
   };
 
+
   describe('with proper DocViewer', () => {
 
     beforeEach(() => {
+      DocViewerComponent.animationsEnabled = false;
+
       createTestingModule('a/b');
       initializeTest();
     });
 
+    afterEach(() => DocViewerComponent.animationsEnabled = true);
+
     it('should create', () => {
       expect(component).toBeDefined();
     });
@@ -408,7 +413,6 @@ describe('AppComponent', () => {
     });
 
     describe('currentDocument', () => {
-
       it('should display a guide page (guide/pipes)', () => {
         locationService.go('guide/pipes');
         fixture.detectChanges();

aio/src/app/layout/doc-viewer/doc-viewer.component.spec.ts

@@ -573,6 +573,11 @@ describe('DocViewerComponent', () => {
       expect(docViewerEl.contains(oldNextViewContainer)).toBe(false);
     });
 
+    [true, false].forEach(animationsEnabled => {
+      describe(`(animationsEnabled: ${animationsEnabled})`, () => {
+        beforeEach(() => DocViewerComponent.animationsEnabled = animationsEnabled);
+        afterEach(() => DocViewerComponent.animationsEnabled = true);
+
         it('should return an observable', done => {
           docViewer.swapViews().subscribe(done, done.fail);
         });
@@ -605,5 +610,24 @@ describe('DocViewerComponent', () => {
           expect(docViewer.currViewContainer.innerHTML).toBe('Next view 2');
           expect(docViewer.nextViewContainer.innerHTML).toBe('');
         });
+
+        if (animationsEnabled) {
+          // Without animations, the views are swapped synchronously,
+          // so there is no need (or way) to abort.
+          it('should abort swapping if the returned observable is unsubscribed from', async () => {
+            docViewer.swapViews().subscribe().unsubscribe();
+            await doSwapViews();
+
+            // Since the first call was cancelled, only one swapping should have taken place.
+            expect(docViewerEl.contains(oldCurrViewContainer)).toBe(false);
+            expect(docViewerEl.contains(oldNextViewContainer)).toBe(true);
+            expect(docViewer.currViewContainer).toBe(oldNextViewContainer);
+            expect(docViewer.nextViewContainer).toBe(oldCurrViewContainer);
+            expect(docViewer.currViewContainer.innerHTML).toBe('Next view');
+            expect(docViewer.nextViewContainer.innerHTML).toBe('');
+          });
+        }
+      });
+    });
   });
 });

aio/src/app/layout/doc-viewer/doc-viewer.component.ts

@@ -3,6 +3,7 @@ import { Title } from '@angular/platform-browser';
 
 import { Observable } from 'rxjs/Observable';
 import { of } from 'rxjs/observable/of';
+import { timer } from 'rxjs/observable/timer';
 import 'rxjs/add/operator/catch';
 import 'rxjs/add/operator/do';
 import 'rxjs/add/operator/switchMap';
@@ -25,6 +26,8 @@ const initialDocViewerContent = initialDocViewerElement ? initialDocViewerElemen
   // encapsulation: ViewEncapsulation.Native
 })
 export class DocViewerComponent implements DoCheck, OnDestroy {
+  // Enable/Disable view transition animations.
+  static animationsEnabled = true;
 
   private hostElement: HTMLElement;
 
@@ -138,9 +141,32 @@ export class DocViewerComponent implements DoCheck, OnDestroy {
    *  components.)
    */
   protected swapViews(): Observable<void> {
-    // Placeholders for actual animations.
-    const animateLeave = (elem: HTMLElement) => this.void$;
-    const animateEnter = (elem: HTMLElement) => this.void$;
+    const raf$ = new Observable<void>(subscriber => {
+      const rafId = requestAnimationFrame(() => {
+        subscriber.next();
+        subscriber.complete();
+      });
+      return () => cancelAnimationFrame(rafId);
+    });
+
+    const animateProp =
+        (elem: HTMLElement, prop: string, from: string, to: string, duration = 333) => {
+          elem.style.transition = '';
+          return !DocViewerComponent.animationsEnabled
+              ? this.void$.do(() => elem.style[prop] = to)
+              : this.void$
+                    // In order to ensure that the `from` value will be applied immediately (i.e.
+                    // without transition) and that the `to` value will be affected by the
+                    // `transition` style, we need to ensure an animation frame has passed between
+                    // setting each style.
+                    .switchMap(() => raf$).do(() => elem.style[prop] = from)
+                    .switchMap(() => raf$).do(() => elem.style.transition = `all ${duration}ms ease-in-out`)
+                    .switchMap(() => raf$).do(() => elem.style[prop] = to)
+                    .switchMap(() => timer(duration)).switchMap(() => this.void$);
+        };
+
+    const animateLeave = (elem: HTMLElement) => animateProp(elem, 'opacity', '1', '0.25');
+    const animateEnter = (elem: HTMLElement) => animateProp(elem, 'opacity', '0.25', '1');
 
     let done$ = this.void$;