perf(overlay): only compute and emit position changes if there are subscribers (#11431)

When emitting the `ConnectedOverlayPositionChange`, we have to compute whether the overlay is being clipped by the viewport or one of its parent scrollables. These calculations can be expensive, because they need the `ClientRect` of each element, however they won't necessarily be used by the consumer. These changes move things around so we only calculate the scrollable clipping if the consumer has subscribed to the `positionChanges` observable.

Author: crisbeto

src/cdk/overlay/position/flexible-connected-position-strategy.ts

@@ -108,9 +108,19 @@ export class FlexibleConnectedPositionStrategy implements PositionStrategy {
   /** Selector to be used when finding the elements on which to set the transform origin. */
   private _transformOriginSelector: string;
 
+  /** Amount of subscribers to the `positionChanges` stream. */
+  private _positionChangeSubscriptions = 0;
+
   /** Observable sequence of position changes. */
-  positionChanges: Observable<ConnectedOverlayPositionChange> =
-      this._positionChanges.asObservable();
+  positionChanges: Observable<ConnectedOverlayPositionChange> = Observable.create(observer => {
+    const subscription = this._positionChanges.subscribe(observer);
+    this._positionChangeSubscriptions++;
+
+    return () => {
+      subscription.unsubscribe();
+      this._positionChangeSubscriptions--;
+    };
+  });
 
   /** Ordered list of preferred positions, from most to least desirable. */
   get positions() {
@@ -581,9 +591,14 @@ export class FlexibleConnectedPositionStrategy implements PositionStrategy {
     this._lastPosition = position;
 
     // Notify that the position has been changed along with its change properties.
-    const scrollableViewProperties = this._getScrollVisibility();
-    const changeEvent = new ConnectedOverlayPositionChange(position, scrollableViewProperties);
-    this._positionChanges.next(changeEvent);
+    // We only emit if we've got any subscriptions, because the scroll visibility
+    // calculcations can be somewhat expensive.
+    if (this._positionChangeSubscriptions > 0) {
+      const scrollableViewProperties = this._getScrollVisibility();
+      const changeEvent = new ConnectedOverlayPositionChange(position, scrollableViewProperties);
+      this._positionChanges.next(changeEvent);
+    }
+
     this._isInitialRender = false;
   }