virtual-scroll: fix updating when data changes and add trackBy demos

src/cdk-experimental/scrolling/virtual-for-of.ts

@@ -147,8 +147,15 @@ export class CdkVirtualForOf<T> implements CollectionViewer, DoCheck, OnDestroy
       private _differs: IterableDiffers,
       /** The virtual scrolling viewport that these items are being rendered in. */
       @SkipSelf() private _viewport: CdkVirtualScrollViewport) {
-    this.dataStream.subscribe(data => this._data = data);
-    this._viewport.renderedRangeStream.subscribe(range => this._onRenderedRangeChange(range));
+    this.dataStream.subscribe(data => {
+      this._data = data;
+      this._onRenderedDataChange();
+    });
+    this._viewport.renderedRangeStream.subscribe(range => {
+      this._renderedRange = range;
+      this.viewChange.next(this._renderedRange);
+      this._onRenderedDataChange();
+    });
     this._viewport.attach(this);
   }
 
@@ -213,9 +220,10 @@ export class CdkVirtualForOf<T> implements CollectionViewer, DoCheck, OnDestroy
   }
 
   /** React to scroll state changes in the viewport. */
-  private _onRenderedRangeChange(renderedRange: ListRange) {
-    this._renderedRange = renderedRange;
-    this.viewChange.next(this._renderedRange);
+  private _onRenderedDataChange() {
+    if (!this._renderedRange) {
+      return;
+    }
     this._renderedItems = this._data.slice(this._renderedRange.start, this._renderedRange.end);
     if (!this._differ) {
       this._differ = this._differs.find(this._renderedItems).create(this.cdkVirtualForTrackBy);

src/demo-app/virtual-scroll/virtual-scroll-demo.html

@@ -52,8 +52,44 @@ <h2>Fixed size</h2>
 <h2>Observable data</h2>
 
 <cdk-virtual-scroll-viewport class="demo-viewport" [itemSize]="50">
-  <div *cdkVirtualFor="let size of observableData; let i = index" class="demo-item"
+  <div *cdkVirtualFor="let size of observableData | async; let i = index" class="demo-item"
        [style.height.px]="size">
     Item #{{i}} - ({{size}}px)
   </div>
 </cdk-virtual-scroll-viewport>
+
+<h2>No trackBy</h2>
+
+<button (click)="sortBy('name')">Sort by state name</button>
+<button (click)="sortBy('capital')">Sort by state capital</button>
+<cdk-virtual-scroll-viewport class="demo-viewport" [itemSize]="60">
+  <div *cdkVirtualFor="let state of statesObservable | async"
+       class="demo-state-item">
+    <div class="demo-state">{{state.name}}</div>
+    <div class="demo-capital">{{state.capital}}</div>
+  </div>
+</cdk-virtual-scroll-viewport>
+
+<h2>trackBy index</h2>
+
+<button (click)="sortBy('name')">Sort by state name</button>
+<button (click)="sortBy('capital')">Sort by state capital</button>
+<cdk-virtual-scroll-viewport class="demo-viewport" [itemSize]="60">
+  <div *cdkVirtualFor="let state of statesObservable | async; trackBy: indexTrackFn"
+       class="demo-state-item">
+    <div class="demo-state">{{state.name}}</div>
+    <div class="demo-capital">{{state.capital}}</div>
+  </div>
+</cdk-virtual-scroll-viewport>
+
+<h2>trackBy state name</h2>
+
+<button (click)="sortBy('name')">Sort by state name</button>
+<button (click)="sortBy('capital')">Sort by state capital</button>
+<cdk-virtual-scroll-viewport class="demo-viewport" [itemSize]="60">
+  <div *cdkVirtualFor="let state of statesObservable | async; trackBy: nameTrackFn"
+       class="demo-state-item">
+    <div class="demo-state">{{state.name}}</div>
+    <div class="demo-capital">{{state.capital}}</div>
+  </div>
+</cdk-virtual-scroll-viewport>

src/demo-app/virtual-scroll/virtual-scroll-demo.scss

@@ -24,3 +24,19 @@
     writing-mode: vertical-lr;
   }
 }
+
+.demo-state-item {
+  height: 60px;
+  display: flex;
+  flex-direction: column;
+  justify-content: center;
+}
+
+.demo-state {
+  font-size: 20px;
+  font-weight: 500;
+}
+
+.demo-capital {
+  font-size: 14px;
+}

src/demo-app/virtual-scroll/virtual-scroll-demo.ts

@@ -9,6 +9,13 @@
 import {Component, ViewEncapsulation} from '@angular/core';
 import {BehaviorSubject} from 'rxjs/index';
 
+
+type State = {
+  name: string,
+  capital: string
+};
+
+
 @Component({
   moduleId: module.id,
   selector: 'virtual-scroll-demo',
@@ -23,6 +30,61 @@ export class VirtualScrollDemo {
       .map((_, i) => (1 + Math.floor((10000 - i) / 1000)) * 20);
   randomData = Array(10000).fill(0).map(() => Math.round(Math.random() * 100));
   observableData = new BehaviorSubject<number[]>([]);
+  states = [
+    {name: 'Alabama', capital: 'Montgomery'},
+    {name: 'Alaska', capital: 'Juneau'},
+    {name: 'Arizona', capital: 'Phoenix'},
+    {name: 'Arkansas', capital: 'Little Rock'},
+    {name: 'California', capital: 'Sacramento'},
+    {name: 'Colorado', capital: 'Denver'},
+    {name: 'Connecticut', capital: 'Hartford'},
+    {name: 'Delaware', capital: 'Dover'},
+    {name: 'Florida', capital: 'Tallahassee'},
+    {name: 'Georgia', capital: 'Atlanta'},
+    {name: 'Hawaii', capital: 'Honolulu'},
+    {name: 'Idaho', capital: 'Boise'},
+    {name: 'Illinois', capital: 'Springfield'},
+    {name: 'Indiana', capital: 'Indianapolis'},
+    {name: 'Iowa', capital: 'Des Moines'},
+    {name: 'Kansas', capital: 'Topeka'},
+    {name: 'Kentucky', capital: 'Frankfort'},
+    {name: 'Louisiana', capital: 'Baton Rouge'},
+    {name: 'Maine', capital: 'Augusta'},
+    {name: 'Maryland', capital: 'Annapolis'},
+    {name: 'Massachusetts', capital: 'Boston'},
+    {name: 'Michigan', capital: 'Lansing'},
+    {name: 'Minnesota', capital: 'St. Paul'},
+    {name: 'Mississippi', capital: 'Jackson'},
+    {name: 'Missouri', capital: 'Jefferson City'},
+    {name: 'Montana', capital: 'Helena'},
+    {name: 'Nebraska', capital: 'Lincoln'},
+    {name: 'Nevada', capital: 'Carson City'},
+    {name: 'New Hampshire', capital: 'Concord'},
+    {name: 'New Jersey', capital: 'Trenton'},
+    {name: 'New Mexico', capital: 'Santa Fe'},
+    {name: 'New York', capital: 'Albany'},
+    {name: 'North Carolina', capital: 'Raleigh'},
+    {name: 'North Dakota', capital: 'Bismarck'},
+    {name: 'Ohio', capital: 'Columbus'},
+    {name: 'Oklahoma', capital: 'Oklahoma City'},
+    {name: 'Oregon', capital: 'Salem'},
+    {name: 'Pennsylvania', capital: 'Harrisburg'},
+    {name: 'Rhode Island', capital: 'Providence'},
+    {name: 'South Carolina', capital: 'Columbia'},
+    {name: 'South Dakota', capital: 'Pierre'},
+    {name: 'Tennessee', capital: 'Nashville'},
+    {name: 'Texas', capital: 'Austin'},
+    {name: 'Utah', capital: 'Salt Lake City'},
+    {name: 'Vermont', capital: 'Montpelier'},
+    {name: 'Virginia', capital: 'Richmond'},
+    {name: 'Washington', capital: 'Olympia'},
+    {name: 'West Virginia', capital: 'Charleston'},
+    {name: 'Wisconsin', capital: 'Madison'},
+    {name: 'Wyoming', capital: 'Cheyenne'},
+  ];
+  statesObservable = new BehaviorSubject(this.states);
+  indexTrackFn = (index: number) => index;
+  nameTrackFn = (_: number, item: State) => item.name;
 
   constructor() {
     this.emitData();
@@ -35,4 +97,11 @@ export class VirtualScrollDemo {
       setTimeout(() => this.emitData(), 1000);
     }
   }
+
+  sortBy(prop: 'name' | 'capital') {
+    this.statesObservable.next(this.states.map(s => ({...s})).sort((a, b) => {
+      const aProp = a[prop], bProp = b[prop];
+      return aProp < bProp ? -1 : (aProp > bProp ? 1 : 0);
+    }));
+  }
 }