From 247e7999cf080f57a63baf282c2a73f061e15c5b Mon Sep 17 00:00:00 2001
From: crisbeto <crisbeto@abv.bg>
Date: Thu, 5 Oct 2017 20:54:11 +0200
Subject: [PATCH] perf: remove persistent global scroll listener

* Removes the global scroll listener from the viewport ruler, which was being used to invalidate the scroll position cache. Caching the scroll position is pointless since it'll be invalidated very frequently. Since most Material components depend on the `ViewportRuler` one way or another, it meant that there was a good chance that the scroll listener would stay around forever.
* Fixes an issue that caused the scroll dispatcher not to remove its global scroll listener, if there are any scrollables on the page, even though nothing was subscribed to it.

Fixes #6882.
---
 src/cdk/scrolling/scroll-dispatcher.spec.ts | 20 ++++++++++++++++++++
 src/cdk/scrolling/scroll-dispatcher.ts      |  2 +-
 src/cdk/scrolling/viewport-ruler.ts         | 20 ++++++++------------
 3 files changed, 29 insertions(+), 13 deletions(-)

diff --git a/src/cdk/scrolling/scroll-dispatcher.spec.ts b/src/cdk/scrolling/scroll-dispatcher.spec.ts
index e16d3278afe7..7e8fc9d1a9d0 100644
--- a/src/cdk/scrolling/scroll-dispatcher.spec.ts
+++ b/src/cdk/scrolling/scroll-dispatcher.spec.ts
@@ -143,6 +143,26 @@ describe('Scroll Dispatcher', () => {
           'Expected global listeners to have been removed after the subscription has stopped.');
     });
 
+    it('should remove global listeners on unsubscribe, despite any other live scrollables', () => {
+      const fixture = TestBed.createComponent(NestedScrollingComponent);
+      fixture.detectChanges();
+
+      expect(scroll._globalSubscription).toBeNull('Expected no global listeners on init.');
+      expect(scroll.scrollableReferences.size).toBe(4, 'Expected multiple scrollables');
+
+      const subscription = scroll.scrolled(0).subscribe(() => {});
+
+      expect(scroll._globalSubscription).toBeTruthy(
+          'Expected global listeners after a subscription has been added.');
+
+      subscription.unsubscribe();
+
+      expect(scroll._globalSubscription).toBeNull(
+          'Expected global listeners to have been removed after the subscription has stopped.');
+      expect(scroll.scrollableReferences.size)
+          .toBe(4, 'Expected scrollable count to stay the same');
+    });
+
   });
 });
 
diff --git a/src/cdk/scrolling/scroll-dispatcher.ts b/src/cdk/scrolling/scroll-dispatcher.ts
index 9060f4569bc9..95d5c3667450 100644
--- a/src/cdk/scrolling/scroll-dispatcher.ts
+++ b/src/cdk/scrolling/scroll-dispatcher.ts
@@ -89,7 +89,7 @@ export class ScrollDispatcher {
         subscription.unsubscribe();
         this._scrolledCount--;
 
-        if (this._globalSubscription && !this.scrollableReferences.size && !this._scrolledCount) {
+        if (this._globalSubscription && !this._scrolledCount) {
           this._globalSubscription.unsubscribe();
           this._globalSubscription = null;
         }
diff --git a/src/cdk/scrolling/viewport-ruler.ts b/src/cdk/scrolling/viewport-ruler.ts
index f5d840f4646f..f042899a86e6 100644
--- a/src/cdk/scrolling/viewport-ruler.ts
+++ b/src/cdk/scrolling/viewport-ruler.ts
@@ -8,7 +8,6 @@
 
 import {Injectable, Optional, SkipSelf, NgZone, OnDestroy} from '@angular/core';
 import {Platform} from '@angular/cdk/platform';
-import {ScrollDispatcher} from './scroll-dispatcher';
 import {Observable} from 'rxjs/Observable';
 import {fromEvent} from 'rxjs/observable/fromEvent';
 import {merge} from 'rxjs/observable/merge';
@@ -32,21 +31,19 @@ export class ViewportRuler implements OnDestroy {
   /** Stream of viewport change events. */
   private _change: Observable<Event>;
 
-  /** Subscriptions to streams that invalidate the cached viewport dimensions. */
-  private _invalidateCacheSubscription = Subscription.EMPTY;
+  /** Subscription to streams that invalidate the cached viewport dimensions. */
+  private _invalidateCache: Subscription;
 
-  constructor(platform: Platform, ngZone: NgZone, scrollDispatcher: ScrollDispatcher) {
+  constructor(platform: Platform, ngZone: NgZone) {
     this._change = platform.isBrowser ? ngZone.runOutsideAngular(() => {
       return merge<Event>(fromEvent(window, 'resize'), fromEvent(window, 'orientationchange'));
     }) : observableOf();
 
-    // Subscribe to scroll and resize events and update the document rectangle on changes.
-    this._invalidateCacheSubscription = merge(scrollDispatcher.scrolled(0), this.change())
-      .subscribe(() => this._cacheViewportGeometry());
+    this._invalidateCache = this.change().subscribe(() => this._cacheViewportGeometry());
   }
 
   ngOnDestroy() {
-    this._invalidateCacheSubscription.unsubscribe();
+    this._invalidateCache.unsubscribe();
   }
 
   /** Gets a ClientRect for the viewport's bounds. */
@@ -123,15 +120,14 @@ export class ViewportRuler implements OnDestroy {
 /** @docs-private */
 export function VIEWPORT_RULER_PROVIDER_FACTORY(parentRuler: ViewportRuler,
                                                 platform: Platform,
-                                                ngZone: NgZone,
-                                                scrollDispatcher: ScrollDispatcher) {
-  return parentRuler || new ViewportRuler(platform, ngZone, scrollDispatcher);
+                                                ngZone: NgZone) {
+  return parentRuler || new ViewportRuler(platform, ngZone);
 }
 
 /** @docs-private */
 export const VIEWPORT_RULER_PROVIDER = {
   // If there is already a ViewportRuler available, use that. Otherwise, provide a new one.
   provide: ViewportRuler,
-  deps: [[new Optional(), new SkipSelf(), ViewportRuler], Platform, NgZone, ScrollDispatcher],
+  deps: [[new Optional(), new SkipSelf(), ViewportRuler], Platform, NgZone],
   useFactory: VIEWPORT_RULER_PROVIDER_FACTORY
 };