feat(collections): add isMultipleSelection function to SelectionModel (#11560)

* feat(collections): Add public getter for _multiple on SelectionModel

If your component receives a SelectionModel instance through Input, it was previously not possible to have different behavior based on whether multiple selected items is allowed (e.g.: rendering radio buttons instead of checkboxes). You had to use a second Input property to the component. This pull request allows developers to check the SelectionModel instance property.

* Change multiple property to isMultipleSelection function

* Fix linting errors

* Change description of isMultipleSelection function

* Newline to trigger circle ci

* Revert: Newline to trigger circle ci

Author: glenstaes

src/cdk/collections/selection.spec.ts

@@ -269,4 +269,12 @@ describe('SelectionModel', () => {
   it('should be empty if an empty array is passed for the preselected values', () => {
     expect(new SelectionModel(false, []).selected).toEqual([]);
   });
+
+  it('should be able to determine whether multiple values can be selected', () => {
+    let multipleSelectionModel = new SelectionModel(true);
+    expect(multipleSelectionModel.isMultipleSelection()).toBe(true);
+
+    let singleSelectionModel = new SelectionModel();
+    expect(singleSelectionModel.isMultipleSelection()).toBe(false);
+  });
 });

src/cdk/collections/selection.ts

@@ -116,6 +116,13 @@ export class SelectionModel<T> {
     }
   }
 
+  /**
+   * Gets whether multiple values can be selected.
+   */
+  isMultipleSelection() {
+    return this._multiple;
+  }
+
   /** Emits a change event and clears the records of selected and deselected values. */
   private _emitChangeEvent() {
     // Clear the selected values so they can be re-cached.