virtual-scroll: add onContentRendered hook to VirtualScrollStrategy

src/cdk-experimental/scrolling/auto-size-virtual-scroll.ts

@@ -39,11 +39,10 @@ export class ItemSizeAverager {
    * @param size The measured size of the given range in pixels.
    */
   addSample(range: ListRange, size: number) {
-    const weight = range.end - range.start;
-    const newTotalWeight = this._totalWeight + weight;
+    const newTotalWeight = this._totalWeight + range.end - range.start;
     if (newTotalWeight) {
       const newAverageItemSize =
-          (size * weight + this._averageItemSize * this._totalWeight) / newTotalWeight;
+          (size + this._averageItemSize * this._totalWeight) / newTotalWeight;
       if (newAverageItemSize) {
         this._averageItemSize = newAverageItemSize;
         this._totalWeight = newTotalWeight;
@@ -87,7 +86,6 @@ export class AutoSizeVirtualScrollStrategy implements VirtualScrollStrategy {
    */
   attach(viewport: CdkVirtualScrollViewport) {
     this._viewport = viewport;
-    this._updateTotalContentSize();
     this._renderContentForOffset(this._viewport.measureScrollOffset());
   }
 
@@ -96,21 +94,27 @@ export class AutoSizeVirtualScrollStrategy implements VirtualScrollStrategy {
     this._viewport = null;
   }
 
-  /** Called when the viewport is scrolled. */
+  /** Implemented as part of VirtualScrollStrategy. */
   onContentScrolled() {
     if (this._viewport) {
       this._renderContentForOffset(this._viewport.measureScrollOffset());
     }
   }
 
-  /** Called when the length of the data changes. */
+  /** Implemented as part of VirtualScrollStrategy. */
   onDataLengthChanged() {
     if (this._viewport) {
-      this._updateTotalContentSize();
       this._renderContentForOffset(this._viewport.measureScrollOffset());
     }
   }
 
+  /** Implemented as part of VirtualScrollStrategy. */
+  onContentRendered() {
+    if (this._viewport) {
+      this._checkRenderedContentSize();
+    }
+  }
+
   /**
    * Update the buffer parameters.
    * @param minBufferPx The minimum amount of buffer rendered beyond the viewport (in pixels).
@@ -122,6 +126,17 @@ export class AutoSizeVirtualScrollStrategy implements VirtualScrollStrategy {
     this._addBufferPx = addBufferPx;
   }
 
+  /**
+   * Checks the size of the currently rendered content and uses it to update the estimated item size
+   * and estimated total content size.
+   */
+  private _checkRenderedContentSize() {
+    const viewport = this._viewport!;
+    const renderedContentSize = viewport.measureRenderedContentSize();
+    this._averager.addSample(viewport.getRenderedRange(), renderedContentSize);
+    this._updateTotalContentSize(renderedContentSize);
+  }
+
   /**
    * Render the content that we estimate should be shown for the given scroll offset.
    * Note: must not be called if `this._viewport` is null
@@ -179,9 +194,13 @@ export class AutoSizeVirtualScrollStrategy implements VirtualScrollStrategy {
   }
 
   /** Update the viewport's total content size. */
-  private _updateTotalContentSize() {
+  private _updateTotalContentSize(renderedContentSize: number) {
     const viewport = this._viewport!;
-    viewport.setTotalContentSize(viewport.getDataLength() * this._averager.getAverageItemSize());
+    const renderedRange = viewport.getRenderedRange();
+    const totalSize = renderedContentSize +
+        (viewport.getDataLength() - (renderedRange.end - renderedRange.start)) *
+        this._averager.getAverageItemSize();
+    viewport.setTotalContentSize(totalSize);
   }
 }
 

src/cdk-experimental/scrolling/fixed-size-virtual-scroll.ts

@@ -70,6 +70,9 @@ export class FixedSizeVirtualScrollStrategy implements VirtualScrollStrategy {
     this._updateRenderedRange();
   }
 
+  /** Called when the range of items rendered in the DOM has changed. */
+  onContentRendered() { /* no-op */ }
+
   /** Update the viewport's total content size. */
   private _updateTotalContentSize() {
     if (!this._viewport) {

src/cdk-experimental/scrolling/virtual-scroll-strategy.ts

@@ -31,4 +31,7 @@ export interface VirtualScrollStrategy {
 
   /** Called when the length of the data changes. */
   onDataLengthChanged();
+
+  /** Called when the range of items rendered in the DOM has changed. */
+  onContentRendered();
 }

src/cdk-experimental/scrolling/virtual-scroll-viewport.ts

@@ -23,6 +23,7 @@ import {
 import {Observable} from 'rxjs/Observable';
 import {fromEvent} from 'rxjs/observable/fromEvent';
 import {sampleTime} from 'rxjs/operators/sampleTime';
+import {take} from 'rxjs/operators/take';
 import {takeUntil} from 'rxjs/operators/takeUntil';
 import {animationFrame} from 'rxjs/scheduler/animationFrame';
 import {Subject} from 'rxjs/Subject';
@@ -99,6 +100,11 @@ export class CdkVirtualScrollViewport implements OnInit, OnDestroy {
     return this._viewportSize;
   }
 
+  /** Get the current rendered range of items. */
+  getRenderedRange(): ListRange {
+    return this._renderedRange;
+  }
+
   // TODO(mmalebra): Consider calling `detectChanges()` directly rather than the methods below.
 
   /**
@@ -122,6 +128,9 @@ export class CdkVirtualScrollViewport implements OnInit, OnDestroy {
       this._ngZone.run(() => {
         this._renderedRangeSubject.next(this._renderedRange = range);
         this._changeDetectorRef.markForCheck();
+        this._ngZone.runOutsideAngular(() => this._ngZone.onStable.pipe(take(1)).subscribe(() => {
+          this._scrollStrategy.onContentRendered();
+        }));
       });
     }
   }
@@ -169,6 +178,12 @@ export class CdkVirtualScrollViewport implements OnInit, OnDestroy {
         this.elementRef.nativeElement.scrollLeft : this.elementRef.nativeElement.scrollTop;
   }
 
+  /** Measure the combined size of all of the rendered items. */
+  measureRenderedContentSize() {
+    const contentEl = this._contentWrapper.nativeElement;
+    return this.orientation === 'horizontal' ? contentEl.offsetWidth : contentEl.offsetHeight;
+  }
+
   ngOnInit() {
     Promise.resolve().then(() => {
       this._viewportSize = this.orientation === 'horizontal' ?