fix(tree,scrolling): don't require user DataSource to extend DataSource (#14966)

Currently the way we verify that the passed in data source is by using `instanceof` which assumes that the consumer has extended our `DataSource`. In `cdk/table` we do the verification by checking whether the object matches the interface of a `DataSource` instead.

These changes add a utility method to check whether something is a data source and refactor the `tree` and `scrolling` entry points to be consistent with `table`.

src/cdk/collections/data-source.ts

@@ -29,3 +29,11 @@ export abstract class DataSource<T> {
    */
   abstract disconnect(collectionViewer: CollectionViewer): void;
 }
+
+/** Checks whether an object is a data source. */
+export function isDataSource(value: any): value is DataSource<any> {
+  // Check if the value is a DataSource by observing if it has a connect function. Cannot
+  // be checked as an `instanceof DataSource` since people could create their own sources
+  // that match the interface, but don't extend DataSource.
+  return value && typeof value.connect === 'function';
+}

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

@@ -6,7 +6,13 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {ArrayDataSource, CollectionViewer, DataSource, ListRange} from '@angular/cdk/collections';
+import {
+  ArrayDataSource,
+  CollectionViewer,
+  DataSource,
+  ListRange,
+  isDataSource,
+} from '@angular/cdk/collections';
 import {
   Directive,
   DoCheck,
@@ -82,7 +88,7 @@ export class CdkVirtualForOf<T> implements CollectionViewer, DoCheck, OnDestroy
   }
   set cdkVirtualForOf(value: DataSource<T> | Observable<T[]> | NgIterable<T>) {
     this._cdkVirtualForOf = value;
-    const ds = value instanceof DataSource ? value :
+    const ds = isDataSource(value) ? value :
         // Slice the value if its an NgIterable to ensure we're working with an array.
         new ArrayDataSource<T>(
             value instanceof Observable ? value : Array.prototype.slice.call(value || []));

src/cdk/table/table.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {CollectionViewer, DataSource} from '@angular/cdk/collections';
+import {CollectionViewer, DataSource, isDataSource} from '@angular/cdk/collections';
 import {
   AfterContentChecked,
   Attribute,
@@ -449,7 +449,7 @@ export class CdkTable<T> implements AfterContentChecked, CollectionViewer, OnDes
     this._onDestroy.next();
     this._onDestroy.complete();
 
-    if (this.dataSource instanceof DataSource) {
+    if (isDataSource(this.dataSource)) {
       this.dataSource.disconnect(this);
     }
   }
@@ -769,7 +769,7 @@ export class CdkTable<T> implements AfterContentChecked, CollectionViewer, OnDes
   private _switchDataSource(dataSource: CdkTableDataSourceInput<T>) {
     this._data = [];
 
-    if (this.dataSource instanceof DataSource) {
+    if (isDataSource(this.dataSource)) {
       this.dataSource.disconnect(this);
     }
 
@@ -796,12 +796,8 @@ export class CdkTable<T> implements AfterContentChecked, CollectionViewer, OnDes
 
     let dataStream: Observable<T[] | ReadonlyArray<T>> | undefined;
 
-    // Check if the datasource is a DataSource object by observing if it has a connect function.
-    // Cannot check this.dataSource['connect'] due to potential property renaming, nor can it
-    // checked as an instanceof DataSource<T> since the table should allow for data sources
-    // that did not explicitly extend DataSource<T>.
-    if ((this.dataSource as DataSource<T>).connect instanceof Function) {
-      dataStream = (this.dataSource as DataSource<T>).connect(this);
+    if (isDataSource(this.dataSource)) {
+      dataStream = this.dataSource.connect(this);
     } else if (this.dataSource instanceof Observable) {
       dataStream = this.dataSource;
     } else if (Array.isArray(this.dataSource)) {

src/cdk/tree/tree.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 import {FocusableOption} from '@angular/cdk/a11y';
-import {CollectionViewer, DataSource} from '@angular/cdk/collections';
+import {CollectionViewer, DataSource, isDataSource} from '@angular/cdk/collections';
 import {
   AfterContentChecked,
   ChangeDetectionStrategy,
@@ -185,10 +185,8 @@ export class CdkTree<T>
   private _observeRenderChanges() {
     let dataStream: Observable<T[] | ReadonlyArray<T>> | undefined;
 
-    // Cannot use `instanceof DataSource` since the data source could be a literal with
-    // `connect` function and may not extends DataSource.
-    if (typeof (this._dataSource as DataSource<T>).connect === 'function') {
-      dataStream = (this._dataSource as DataSource<T>).connect(this);
+    if (isDataSource(this._dataSource)) {
+      dataStream = this._dataSource.connect(this);
     } else if (this._dataSource instanceof Observable) {
       dataStream = this._dataSource;
     } else if (Array.isArray(this._dataSource)) {

tools/public_api_guard/cdk/collections.d.ts

@@ -15,6 +15,8 @@ export declare abstract class DataSource<T> {
 
 export declare function getMultipleValuesInSingleSelectionError(): Error;
 
+export declare function isDataSource(value: any): value is DataSource<any>;
+
 export declare type ListRange = {
     start: number;
     end: number;