feat(slider): emit input event when slider thumb moves

* Adds an input event output, which fires each time the thumb changes its position.
  Made similar to the native range element `input` event dispatching.

Closes angular#2296

src/lib/slider/slider.spec.ts

@@ -759,6 +759,55 @@ describe('MdSlider', () => {
     });
   });
 
+  describe('slider with input event', () => {
+    let fixture: ComponentFixture<SliderWithChangeHandler>;
+    let sliderDebugElement: DebugElement;
+    let sliderNativeElement: HTMLElement;
+    let sliderTrackElement: HTMLElement;
+    let testComponent: SliderWithChangeHandler;
+
+    beforeEach(() => {
+      fixture = TestBed.createComponent(SliderWithChangeHandler);
+      fixture.detectChanges();
+
+      testComponent = fixture.debugElement.componentInstance;
+      spyOn(testComponent, 'onInput');
+      spyOn(testComponent, 'onChange');
+
+      sliderDebugElement = fixture.debugElement.query(By.directive(MdSlider));
+      sliderNativeElement = sliderDebugElement.nativeElement;
+      sliderTrackElement = <HTMLElement>sliderNativeElement.querySelector('.md-slider-track');
+    });
+
+    it('should emit an input event while sliding', () => {
+      expect(testComponent.onChange).not.toHaveBeenCalled();
+
+      dispatchMouseenterEvent(sliderNativeElement);
+      dispatchSlideEvent(sliderNativeElement, 0.5, gestureConfig);
+      dispatchSlideEvent(sliderNativeElement, 1, gestureConfig);
+      dispatchSlideEndEvent(sliderNativeElement, 1, gestureConfig);
+
+      fixture.detectChanges();
+
+      // The input event should fire twice, because the slider changed two times.
+      expect(testComponent.onInput).toHaveBeenCalledTimes(2);
+      expect(testComponent.onChange).toHaveBeenCalledTimes(1);
+    });
+
+    it('should emit an input event when clicking', () => {
+      expect(testComponent.onChange).not.toHaveBeenCalled();
+
+      dispatchClickEventSequence(sliderNativeElement, 0.75);
+
+      fixture.detectChanges();
+
+      // The `onInput` event should be emitted once due to a single click.
+      expect(testComponent.onInput).toHaveBeenCalledTimes(1);
+      expect(testComponent.onChange).toHaveBeenCalledTimes(1);
+    });
+
+  });
+
   describe('keyboard support', () => {
     let fixture: ComponentFixture<StandardSlider>;
     let sliderDebugElement: DebugElement;
@@ -1108,11 +1157,12 @@ class SliderWithValueSmallerThanMin { }
 class SliderWithValueGreaterThanMax { }
 
 @Component({
-  template: `<md-slider (change)="onChange($event)"></md-slider>`,
+  template: `<md-slider (change)="onChange($event)" (input)="onInput($event)"></md-slider>`,
   styles: [styles],
 })
 class SliderWithChangeHandler {
-  onChange() { }
+  onChange() { };
+  onInput() { };
 }
 
 @Component({

src/lib/slider/slider.ts

@@ -113,8 +113,9 @@ export class MdSlider implements ControlValueAccessor {
 
   private _controlValueAccessorChangeFn: (value: any) => void = () => {};
 
-  /** The last value for which a change event was emitted. */
-  private _lastEmittedValue: number = null;
+  /** The last values for which a change or input event was emitted. */
+  private _lastChangeValue: number = null;
+  private _lastInputValue: number = null;
 
   /** onTouch function registered via registerOnTouch (ControlValueAccessor). */
   onTouched: () => any = () => {};
@@ -295,6 +296,7 @@ export class MdSlider implements ControlValueAccessor {
   }
 
   @Output() change = new EventEmitter<MdSliderChange>();
+  @Output() input = new EventEmitter<MdSliderChange>();
 
   constructor(@Optional() private _dir: Dir, elementRef: ElementRef) {
     this._renderer = new SliderRenderer(elementRef);
@@ -320,6 +322,9 @@ export class MdSlider implements ControlValueAccessor {
     this._isSliding = false;
     this._renderer.addFocus();
     this._updateValueFromPosition({x: event.clientX, y: event.clientY});
+
+    /* Emits a change and input event if the value changed. */
+    this._emitInputEvent();
     this._emitValueIfChanged();
   }
 
@@ -331,6 +336,9 @@ export class MdSlider implements ControlValueAccessor {
     // Prevent the slide from selecting anything else.
     event.preventDefault();
     this._updateValueFromPosition({x: event.center.x, y: event.center.y});
+
+    // Native range elements always emit `input` events when the value changed while sliding.
+    this._emitInputEvent();
   }
 
   _onSlideStart(event: HammerInput) {
@@ -434,16 +442,23 @@ export class MdSlider implements ControlValueAccessor {
 
   /** Emits a change event if the current value is different from the last emitted value. */
   private _emitValueIfChanged() {
-    if (this.value != this._lastEmittedValue) {
-      let event = new MdSliderChange();
-      event.source = this;
-      event.value = this.value;
-      this._lastEmittedValue = this.value;
+    if (this.value != this._lastChangeValue) {
+      let event = this._createChangeEvent();
+      this._lastChangeValue = this.value;
       this._controlValueAccessorChangeFn(this.value);
       this.change.emit(event);
     }
   }
 
+  /** Emits an input event when the current value is different from the last emitted value. */
+  private _emitInputEvent() {
+    if (this.value != this._lastInputValue) {
+      let event = this._createChangeEvent();
+      this._lastInputValue = this.value;
+      this.input.emit(event);
+    }
+  }
+
   /** Updates the amount of space between ticks as a percentage of the width of the slider. */
   private _updateTickIntervalPercent() {
     if (!this.tickInterval) {
@@ -461,6 +476,16 @@ export class MdSlider implements ControlValueAccessor {
     }
   }
 
+  /** Creates a slider change object from the specified value. */
+  private _createChangeEvent(value = this.value): MdSliderChange {
+    let event = new MdSliderChange();
+
+    event.source = this;
+    event.value = value;
+
+    return event;
+  }
+
   /** Calculates the percentage of the slider that a value is. */
   private _calculatePercentage(value: number) {
     return (value - this.min) / (this.max - this.min);