fix(overlay): only dispatch position change event if requested (#19785)

The `ConnectedOverlayPositionChange` event can be expensive to calculate since it contains information about the scroll position. We have some logic in the position strategy about not emitting it if there are no observers, but in the `CdkConnectedOverlay` directive we were always subscribing in order to proxy it to the output. These changes add some logic to only subscribe when necessary.

(cherry picked from commit bbf5670)

Author: crisbeto

src/cdk/overlay/overlay-directives.ts

@@ -26,6 +26,7 @@ import {
   ViewContainerRef,
 } from '@angular/core';
 import {Subscription} from 'rxjs';
+import {takeWhile} from 'rxjs/operators';
 import {Overlay} from './overlay';
 import {OverlayConfig} from './overlay-config';
 import {OverlayRef} from './overlay-ref';
@@ -113,6 +114,7 @@ export class CdkConnectedOverlay implements OnDestroy, OnChanges {
   private _backdropSubscription = Subscription.EMPTY;
   private _attachSubscription = Subscription.EMPTY;
   private _detachSubscription = Subscription.EMPTY;
+  private _positionSubscription = Subscription.EMPTY;
   private _offsetX: number;
   private _offsetY: number;
   private _position: FlexibleConnectedPositionStrategy;
@@ -254,6 +256,7 @@ export class CdkConnectedOverlay implements OnDestroy, OnChanges {
     this._attachSubscription.unsubscribe();
     this._detachSubscription.unsubscribe();
     this._backdropSubscription.unsubscribe();
+    this._positionSubscription.unsubscribe();
 
     if (this._overlayRef) {
       this._overlayRef.dispose();
@@ -367,10 +370,7 @@ export class CdkConnectedOverlay implements OnDestroy, OnChanges {
   /** Returns the position strategy of the overlay to be set on the overlay config */
   private _createPositionStrategy(): FlexibleConnectedPositionStrategy {
     const strategy = this._overlay.position().flexibleConnectedTo(this.origin.elementRef);
-
     this._updatePositionStrategy(strategy);
-    strategy.positionChanges.subscribe(p => this.positionChange.emit(p));
-
     return strategy;
   }
 
@@ -394,6 +394,22 @@ export class CdkConnectedOverlay implements OnDestroy, OnChanges {
     } else {
       this._backdropSubscription.unsubscribe();
     }
+
+    this._positionSubscription.unsubscribe();
+
+    // Only subscribe to `positionChanges` if requested, because putting
+    // together all the information for it can be expensive.
+    if (this.positionChange.observers.length > 0) {
+      this._positionSubscription = this._position.positionChanges
+        .pipe(takeWhile(() => this.positionChange.observers.length > 0))
+        .subscribe(position => {
+          this.positionChange.emit(position);
+
+          if (this.positionChange.observers.length === 0) {
+            this._positionSubscription.unsubscribe();
+          }
+        });
+    }
   }
 
   /** Detaches the overlay and unsubscribes to backdrop clicks if backdrop exists */
@@ -403,6 +419,7 @@ export class CdkConnectedOverlay implements OnDestroy, OnChanges {
     }
 
     this._backdropSubscription.unsubscribe();
+    this._positionSubscription.unsubscribe();
   }
 
   static ngAcceptInputType_hasBackdrop: BooleanInput;