feat(animations): support :increment and :decrement transition aliases

packages/animations/browser/src/dsl/animation_transition_expr.ts

@@ -24,8 +24,14 @@ export function parseTransitionExpr(
 function parseInnerTransitionStr(
     eventStr: string, expressions: TransitionMatcherFn[], errors: string[]) {
   if (eventStr[0] == ':') {
-    eventStr = parseAnimationAlias(eventStr, errors);
+    const result = parseAnimationAlias(eventStr, errors);
+    if (typeof result == 'function') {
+      expressions.push(result);
+      return;
+    }
+    eventStr = result as string;
   }
+
   const match = eventStr.match(/^(\*|[-\w]+)\s*(<?[=-]>)\s*(\*|[-\w]+)$/);
   if (match == null || match.length < 4) {
     errors.push(`The provided transition expression "${eventStr}" is not supported`);
@@ -43,12 +49,16 @@ function parseInnerTransitionStr(
   }
 }
 
-function parseAnimationAlias(alias: string, errors: string[]): string {
+function parseAnimationAlias(alias: string, errors: string[]): string|TransitionMatcherFn {
   switch (alias) {
     case ':enter':
       return 'void => *';
     case ':leave':
       return '* => void';
+    case ':increment':
+      return (fromState: any, toState: any): boolean => parseFloat(toState) > parseFloat(fromState);
+    case ':decrement':
+      return (fromState: any, toState: any): boolean => parseFloat(toState) < parseFloat(fromState);
     default:
       errors.push(`The transition alias value "${alias}" is not supported`);
       return '* => *';

packages/animations/src/animation_metadata.ts

@@ -707,7 +707,7 @@ export function keyframes(steps: AnimationStyleMetadata[]): AnimationKeyframesSe
  * ])
  * ```
  *
- * ### Transition Aliases (`:enter` and `:leave`)
+ * ### Using :enter and :leave
  *
  * Given that enter (insertion) and leave (removal) animations are so common, the `transition`
  * function accepts both `:enter` and `:leave` values which are aliases for the `void => *` and `*
@@ -717,12 +717,88 @@ export function keyframes(steps: AnimationStyleMetadata[]): AnimationKeyframesSe
  * transition(":enter", [
  *   style({ opacity: 0 }),
  *   animate(500, style({ opacity: 1 }))
- * ])
+ * ]),
  * transition(":leave", [
  *   animate(500, style({ opacity: 0 }))
  * ])
  * ```
  *
+ * ### Using :increment and :decrement
+ * In addition to the :enter and :leave transition aliases, the :increment and :decrement aliases
+ * can be used to kick off a transition when a numeric value has increased or decreased in value.
+ *
+ * ```
+ * import {group, animate, query, transition, style, trigger} from '@angular/animations';
+ * import {Component} from '@angular/core';
+ *
+ * @Component({
+ *   selector: 'banner-carousel-component',
+ *   styles: [`
+ *     .banner-container {
+ *        position:relative;
+ *        height:500px;
+ *        overflow:hidden;
+ *      }
+ *     .banner-container > .banner {
+ *        position:absolute;
+ *        left:0;
+ *        top:0;
+ *        font-size:200px;
+ *        line-height:500px;
+ *        font-weight:bold;
+ *        text-align:center;
+ *        width:100%;
+ *      }
+ *   `],
+ *   template: `
+ *     <button (click)="previous()">Previous</button>
+ *     <button (click)="next()">Next</button>
+ *     <hr>
+ *     <div [@bannerAnimation]="selectedIndex" class="banner-container">
+ *       <div class="banner"> {{ banner }} </div>
+ *     </div>
+ *   `
+ *   animations: [
+ *     trigger('bannerAnimation', [
+ *       transition(":increment", group([
+ *         query(':enter', [
+ *           style({ left: '100%' }),
+ *           animate('0.5s ease-out', style('*'))
+ *         ]),
+ *         query(':leave', [
+ *           animate('0.5s ease-out', style({ left: '-100%' }))
+ *         ])
+ *       ])),
+ *       transition(":decrement", group([
+ *         query(':enter', [
+ *           style({ left: '-100%' }),
+ *           animate('0.5s ease-out', style('*'))
+ *         ]),
+ *         query(':leave', [
+ *           animate('0.5s ease-out', style({ left: '100%' }))
+ *         ])
+ *       ])),
+ *     ])
+ *   ]
+ * })
+ * class BannerCarouselComponent {
+ *   allBanners: string[] = ['1', '2', '3', '4'];
+ *   selectedIndex: number = 0;
+ *
+ *   get banners() {
+ *      return [this.allBanners[this.selectedIndex]];
+ *   }
+ *
+ *   previous() {
+ *     this.selectedIndex = Math.max(this.selectedIndex - 1, 0);
+ *   }
+ *
+ *   next() {
+ *     this.selectedIndex = Math.min(this.selectedIndex + 1, this.allBanners.length - 1);
+ *   }
+ * }
+ * ```
+ *
  * {@example core/animation/ts/dsl/animation_example.ts region='Component'}
  *
  * @experimental Animation support is experimental.

packages/core/test/animation/animation_integration_spec.ts

@@ -1541,6 +1541,98 @@ export function main() {
         const players = getLog();
         expect(players.length).toEqual(2);
       });
+
+      describe('transition aliases', () => {
+        describe(':increment', () => {
+          it('should detect when a value has incremented', () => {
+            @Component({
+              selector: 'if-cmp',
+              template: `
+          <div [@myAnimation]="exp"></div>
+        `,
+              animations: [
+                trigger(
+                    'myAnimation',
+                    [
+                      transition(
+                          ':increment',
+                          [
+                            animate(1234, style({background: 'red'})),
+                          ]),
+                    ]),
+              ]
+            })
+            class Cmp {
+              exp: number = 0;
+            }
+
+            TestBed.configureTestingModule({declarations: [Cmp]});
+            const fixture = TestBed.createComponent(Cmp);
+            const cmp = fixture.componentInstance;
+            fixture.detectChanges();
+            let players = getLog();
+            expect(players.length).toEqual(0);
+
+            cmp.exp++;
+            fixture.detectChanges();
+            players = getLog();
+            expect(players.length).toEqual(1);
+            expect(players[0].duration).toEqual(1234);
+            resetLog();
+
+            cmp.exp = 5;
+            fixture.detectChanges();
+            players = getLog();
+            expect(players.length).toEqual(1);
+            expect(players[0].duration).toEqual(1234);
+          });
+        });
+
+        describe(':decrement', () => {
+          it('should detect when a value has decremented', () => {
+            @Component({
+              selector: 'if-cmp',
+              template: `
+          <div [@myAnimation]="exp"></div>
+        `,
+              animations: [
+                trigger(
+                    'myAnimation',
+                    [
+                      transition(
+                          ':decrement',
+                          [
+                            animate(1234, style({background: 'red'})),
+                          ]),
+                    ]),
+              ]
+            })
+            class Cmp {
+              exp: number = 5;
+            }
+
+            TestBed.configureTestingModule({declarations: [Cmp]});
+            const fixture = TestBed.createComponent(Cmp);
+            const cmp = fixture.componentInstance;
+            fixture.detectChanges();
+            let players = getLog();
+            expect(players.length).toEqual(0);
+
+            cmp.exp--;
+            fixture.detectChanges();
+            players = getLog();
+            expect(players.length).toEqual(1);
+            expect(players[0].duration).toEqual(1234);
+            resetLog();
+
+            cmp.exp = 0;
+            fixture.detectChanges();
+            players = getLog();
+            expect(players.length).toEqual(1);
+            expect(players[0].duration).toEqual(1234);
+          });
+        });
+      });
     });
 
     describe('animation listeners', () => {