refactor(cdk/table): add internal opt out for virtual scrolling

Adds an opt out that we can use internally to opt apps out of virtual scrolling. This allows us to keep the public API clean for the majority of users.

Author: crisbeto

src/cdk/table/table.ts

@@ -467,6 +467,14 @@ export class CdkTable<T>
   /** Emits when the footer rows sticky state changes. */
   private readonly _footerRowStickyUpdates = new Subject<StickyUpdate>();
 
+  /**
+   * Whether to explicitly disable virtual scrolling even if there is a virtual scroll viewport
+   * parent. This can't be changed externally, whereas internally it is turned into an input that
+   * we use to opt out existing apps that were implementing virtual scroll before we added support
+   * for it.
+   */
+  private readonly _disableVirtualScrolling = false;
+
   /** Aria role to apply to the table's cells based on the table's own role. */
   _getCellRole(): string | null {
     // Perform this lazily in case the table's role was updated by a directive after construction.
@@ -565,7 +573,7 @@ export class CdkTable<T>
   get fixedLayout(): boolean {
     // Require a fixed layout when virtual scrolling is enabled, otherwise
     // the element the header can jump around as the user is scrolling.
-    return this._virtualScrollViewport ? true : this._fixedLayout;
+    return this._virtualScrollEnabled() ? true : this._fixedLayout;
   }
   set fixedLayout(value: boolean) {
     this._fixedLayout = value;
@@ -640,7 +648,7 @@ export class CdkTable<T>
     this._isNativeHtmlTable = this._elementRef.nativeElement.nodeName === 'TABLE';
     this.viewChange = new BehaviorSubject<ListRange>({
       start: 0,
-      end: this._virtualScrollViewport ? 0 : Number.MAX_VALUE,
+      end: this._virtualScrollEnabled() ? 0 : Number.MAX_VALUE,
     });
 
     // Set up the trackBy function so that it uses the `RenderRow` as its identity by default. If
@@ -649,10 +657,6 @@ export class CdkTable<T>
     this._dataDiffer = this._differs.find([]).create((_i: number, dataRow: RenderRow<T>) => {
       return this.trackBy ? this.trackBy(dataRow.dataIndex, dataRow.data) : dataRow;
     });
-
-    if (this._virtualScrollViewport) {
-      this._setupVirtualScrolling(this._virtualScrollViewport);
-    }
   }
 
   ngOnInit() {
@@ -668,9 +672,14 @@ export class CdkTable<T>
 
   ngAfterContentInit() {
     this._viewRepeater =
-      this.recycleRows || this._virtualScrollViewport
+      this.recycleRows || this._virtualScrollEnabled()
         ? new _RecycleViewRepeaterStrategy()
         : new _DisposeViewRepeaterStrategy();
+
+    if (this._virtualScrollEnabled()) {
+      this._setupVirtualScrolling(this._virtualScrollViewport!);
+    }
+
     this._hasInitialized = true;
   }
 
@@ -1039,24 +1048,22 @@ export class CdkTable<T>
    * so that the differ equates their references.
    */
   private _getAllRenderRows(): RenderRow<T>[] {
-    const dataWithinRange = this._renderedRange
-      ? (this._data || []).slice(this._renderedRange.start, this._renderedRange.end)
-      : [];
+    if (!Array.isArray(this._data) || !this._renderedRange) {
+      return [];
+    }
+
     const renderRows: RenderRow<T>[] = [];
+    const end = Math.min(this._data.length, this._renderedRange.end);
 
     // Store the cache and create a new one. Any re-used RenderRow objects will be moved into the
     // new cache while unused ones can be picked up by garbage collection.
     const prevCachedRenderRows = this._cachedRenderRowsMap;
     this._cachedRenderRowsMap = new Map();
 
-    if (!this._data) {
-      return renderRows;
-    }
-
     // For each data object, get the list of rows that should be rendered, represented by the
     // respective `RenderRow` object which is the pair of `data` and `CdkRowDef`.
-    for (let i = 0; i < dataWithinRange.length; i++) {
-      let data = dataWithinRange[i];
+    for (let i = this._renderedRange.start; i < end; i++) {
+      const data = this._data[i];
       const renderRowsForData = this._getRenderRowsForData(data, i, prevCachedRenderRows.get(data));
 
       if (!this._cachedRenderRowsMap.has(data)) {
@@ -1629,6 +1636,10 @@ export class CdkTable<T>
     const endRect = lastNode?.getBoundingClientRect?.();
     return startRect && endRect ? endRect.bottom - startRect.top : 0;
   }
+
+  private _virtualScrollEnabled(): boolean {
+    return !this._disableVirtualScrolling && this._virtualScrollViewport != null;
+  }
 }
 
 /** Utility function that gets a merged list of the entries in an array and values of a Set. */