feat(overlay): emit position change event (#1832)

src/lib/core/overlay/position/connected-position-strategy.spec.ts

@@ -2,6 +2,7 @@ import {ElementRef} from '@angular/core';
 import {ConnectedPositionStrategy} from './connected-position-strategy';
 import {ViewportRuler} from './viewport-ruler';
 import {OverlayPositionBuilder} from './overlay-position-builder';
+import {ConnectedOverlayPositionChange} from './connected-position';
 
 
 // Default width and height of the overlay and origin panels throughout these tests.
@@ -259,6 +260,33 @@ describe('ConnectedPositionStrategy', () => {
 
   });
 
+  it('should emit onPositionChange event when position changes', () => {
+    // force the overlay to open in a fallback position
+    fakeViewportRuler.fakeRect = {
+      top: 0, left: 0, width: 500, height: 500, right: 500, bottom: 500
+    };
+    positionBuilder = new OverlayPositionBuilder(fakeViewportRuler);
+    originElement.style.top = '200px';
+    originElement.style.left = '475px';
+
+    strategy = positionBuilder.connectedTo(
+        fakeElementRef,
+        {originX: 'end', originY: 'center'},
+        {overlayX: 'start', overlayY: 'center'})
+        .withFallbackPosition(
+            {originX: 'start', originY: 'bottom'},
+            {overlayX: 'end', overlayY: 'top'});
+
+    const positionChangeHandler = jasmine.createSpy('positionChangeHandler');
+    strategy.onPositionChange.first().subscribe(positionChangeHandler);
+
+    strategy.apply(overlayElement);
+    expect(positionChangeHandler).toHaveBeenCalled();
+    expect(positionChangeHandler.calls.mostRecent().args[0])
+        .toEqual(jasmine.any(ConnectedOverlayPositionChange),
+            `Expected strategy to emit an instance of ConnectedOverlayPositionChange.`);
+  });
+
 
   /**
    * Run all tests for connecting the overlay to the origin such that first preferred

src/lib/core/overlay/position/connected-position-strategy.ts

@@ -5,9 +5,11 @@ import {applyCssTransform} from '../../style/apply-transform';
 import {
     ConnectionPositionPair,
     OriginConnectionPosition,
-    OverlayConnectionPosition
+    OverlayConnectionPosition,
+    ConnectedOverlayPositionChange
 } from './connected-position';
-
+import {Subject} from 'rxjs/Subject';
+import {Observable} from 'rxjs/Observable';
 
 /**
  * A strategy for positioning overlays. Using this strategy, an overlay is given an
@@ -36,6 +38,13 @@ export class ConnectedPositionStrategy implements PositionStrategy {
   /** The origin element against which the overlay will be positioned. */
   private _origin: HTMLElement;
 
+  private _onPositionChange:
+      Subject<ConnectedOverlayPositionChange> = new Subject<ConnectedOverlayPositionChange>();
+
+  /** Emits an event when the connection point changes. */
+  get onPositionChange(): Observable<ConnectedOverlayPositionChange> {
+    return this._onPositionChange.asObservable();
+  }
 
   constructor(
       private _connectedTo: ElementRef,
@@ -64,6 +73,7 @@ export class ConnectedPositionStrategy implements PositionStrategy {
     // We use the viewport rect to determine whether a position would go off-screen.
     const viewportRect = this._viewportRuler.getViewportRect();
     let firstOverlayPoint: Point = null;
+    let isFirstPosition = true;
 
     // We want to place the overlay in the first of the preferred positions such that the
     // overlay fits on-screen.
@@ -77,8 +87,12 @@ export class ConnectedPositionStrategy implements PositionStrategy {
       // If the overlay in the calculated position fits on-screen, put it there and we're done.
       if (this._willOverlayFitWithinViewport(overlayPoint, overlayRect, viewportRect)) {
         this._setElementPosition(element, overlayPoint);
+        if (!isFirstPosition) {
+          this._onPositionChange.next(new ConnectedOverlayPositionChange(pos));
+        }
         return Promise.resolve(null);
       }
+      isFirstPosition = false;
     }
 
     // TODO(jelbourn): fallback behavior for when none of the preferred positions fit on-screen.

src/lib/core/overlay/position/connected-position.ts

@@ -31,3 +31,8 @@ export class ConnectionPositionPair {
     this.overlayY = overlay.overlayY;
   }
 }
+
+/** The change event emitted by the strategy when a fallback position is used. */
+export class ConnectedOverlayPositionChange {
+  constructor(public connectionPair: ConnectionPositionPair) {}
+}