fix(animations): always trigger animations after the change detection check (#12713)

This patch ensures that animations are run outside of change detection
thus allowing for start and done callbacks to modify application data
without causing a cycle loop.

Closes #12713

modules/@angular/core/src/animation/animation_queue.ts

@@ -17,6 +17,15 @@ export function queueAnimation(player: AnimationPlayer) {
 
 /** @internal */
 export function triggerQueuedAnimations() {
+  // this code is wrapped into a single promise such that the
+  // onStart and onDone player callbacks are triggered outside
+  // of the digest cycle of animations
+  if (_queuedAnimations.length) {
+    Promise.resolve(null).then(_triggerAnimations);
+  }
+}
+
+function _triggerAnimations() {
   for (var i = 0; i < _queuedAnimations.length; i++) {
     var player = _queuedAnimations[i];
     player.play();

modules/@angular/core/test/animation/animation_integration_spec.ts

@@ -1151,7 +1151,7 @@ function declareTests({useJit}: {useJit: boolean}) {
 
     describe('animation output events', () => {
       it('should fire the associated animation output expression when the animation starts even if no animation is fired',
-         () => {
+         fakeAsync(() => {
            TestBed.overrideComponent(DummyIfCmp, {
              set: {
                template: `
@@ -1175,16 +1175,18 @@ function declareTests({useJit}: {useJit: boolean}) {
 
            cmp.exp = 'one';
            fixture.detectChanges();
+           flushMicrotasks();
 
            expect(calls).toEqual(1);
            expect(isAnimationRunning).toEqual(false);
 
            cmp.exp = 'two';
            fixture.detectChanges();
+           flushMicrotasks();
 
            expect(calls).toEqual(2);
            expect(isAnimationRunning).toEqual(true);
-         });
+         }));
 
       it('should fire the associated animation output expression when the animation ends even if no animation is fired',
          fakeAsync(() => {
@@ -1296,6 +1298,36 @@ function declareTests({useJit}: {useJit: boolean}) {
            expect(eventData2.totalTime).toEqual(0);
          }));
 
+      it('should successfully update the component view when an animation start callback is fired',
+         fakeAsync(() => {
+           TestBed.overrideComponent(DummyIfCmp, {
+             set: {
+               template: `
+              <div [@trigger]="exp" (@trigger.start)="exp2='look at me'">{{ exp2 }}</div>
+            `,
+               animations: [
+                 trigger(
+                     'trigger',
+                     [transition(
+                         '* => *',
+                         [animate('1s 750ms', style({})), animate('2000ms 0ms', style({}))])]),
+               ]
+             }
+           });
+
+           let fixture = TestBed.createComponent(DummyIfCmp);
+           var cmp = fixture.componentInstance;
+           cmp.exp2 = 'ignore me';
+           cmp.exp = 'one';
+
+           fixture.detectChanges();
+           flushMicrotasks();
+           fixture.detectChanges();
+
+           var container = fixture.nativeElement;
+           expect(getDOM().getInnerHTML(container)).toContain('look at me');
+         }));
+
       it('should throw an error if an animation output is referenced is not defined within the component',
          () => {
            TestBed.overrideComponent(DummyIfCmp, {