refactor(selection): avoid awkward null check for onChange event (#12348)

Doesn't type the `SelectionModel.onChange` event to `Subject | null` anymore, in order to avoid having people null-check it on every consumption.

Fixes #12347.

src/cdk/collections/selection.spec.ts

@@ -43,7 +43,7 @@ describe('SelectionModel', () => {
     it('should be able to select multiple options', () => {
       const onChangeSpy = jasmine.createSpy('onChange spy');
 
-      model.changed!.subscribe(onChangeSpy);
+      model.changed.subscribe(onChangeSpy);
       model.select(1);
       model.select(2);
 
@@ -56,7 +56,7 @@ describe('SelectionModel', () => {
     it('should be able to select multiple options at the same time', () => {
       const onChangeSpy = jasmine.createSpy('onChange spy');
 
-      model.changed!.subscribe(onChangeSpy);
+      model.changed.subscribe(onChangeSpy);
       model.select(1, 2);
 
       expect(model.selected.length).toBe(2);
@@ -97,7 +97,7 @@ describe('SelectionModel', () => {
       let model = new SelectionModel();
       let spy = jasmine.createSpy('SelectionModel change event');
 
-      model.onChange!.subscribe(spy);
+      model.onChange.subscribe(spy);
       model.select(1);
 
       let event = spy.calls.mostRecent().args[0];
@@ -112,7 +112,7 @@ describe('SelectionModel', () => {
 
       model.select(1);
 
-      model.changed!.subscribe(spy);
+      model.changed.subscribe(spy);
 
       model.select(2);
 
@@ -130,7 +130,7 @@ describe('SelectionModel', () => {
       // Note: this assertion is only here to run the getter.
       expect(model.selected).toEqual([]);
 
-      model.onChange!.subscribe(() => spy(model.selected));
+      model.onChange.subscribe(() => spy(model.selected));
       model.select(1);
 
       expect(spy).toHaveBeenCalledWith([1]);
@@ -144,7 +144,7 @@ describe('SelectionModel', () => {
         model = new SelectionModel(true);
         spy = jasmine.createSpy('SelectionModel change event');
 
-        model.changed!.subscribe(spy);
+        model.changed.subscribe(spy);
       });
 
       it('should emit an event when a value is selected', () => {
@@ -167,7 +167,7 @@ describe('SelectionModel', () => {
       it('should not emit an event when preselecting values', () => {
         model = new SelectionModel(false, [1]);
         spy = jasmine.createSpy('SelectionModel initial change event');
-        model.changed!.subscribe(spy);
+        model.changed.subscribe(spy);
 
         expect(spy).not.toHaveBeenCalled();
       });
@@ -181,7 +181,7 @@ describe('SelectionModel', () => {
         model = new SelectionModel(true, [1, 2, 3]);
         spy = jasmine.createSpy('SelectionModel change event');
 
-        model.changed!.subscribe(spy);
+        model.changed.subscribe(spy);
       });
 
       it('should emit an event when a value is deselected', () => {
@@ -217,10 +217,6 @@ describe('SelectionModel', () => {
       model = new SelectionModel(true, undefined, false);
     });
 
-    it('should not have an onChange stream if change events are disabled', () => {
-      expect(model.changed).toBeFalsy();
-    });
-
     it('should still update the select value', () => {
       model.select(1);
       expect(model.selected).toEqual([1]);

src/cdk/collections/selection.ts

@@ -34,14 +34,14 @@ export class SelectionModel<T> {
   }
 
   /** Event emitted when the value has changed. */
-  changed: Subject<SelectionChange<T>> | null = this._emitChanges ? new Subject() : null;
+  changed: Subject<SelectionChange<T>> = new Subject();
 
   /**
    * Event emitted when the value has changed.
    * @deprecated Use `changed` instead.
    * @breaking-change 8.0.0 To be changed to `changed`
    */
-  onChange: Subject<SelectionChange<T>> | null = this.changed;
+  onChange: Subject<SelectionChange<T>> = this.changed;
 
   constructor(
     private _multiple = false,
@@ -136,13 +136,11 @@ export class SelectionModel<T> {
     this._selected = null;
 
     if (this._selectedToEmit.length || this._deselectedToEmit.length) {
-      if (this.changed) {
-        this.changed.next({
-          source: this,
-          added: this._selectedToEmit,
-          removed: this._deselectedToEmit
-        });
-      }
+      this.changed.next({
+        source: this,
+        added: this._selectedToEmit,
+        removed: this._deselectedToEmit
+      });
 
       this._deselectedToEmit = [];
       this._selectedToEmit = [];

src/demo-app/tree/dynamic-tree-demo/dynamic-database.ts

@@ -68,7 +68,7 @@ export class DynamicDataSource {
               private database: DynamicDatabase) {}
 
   connect(collectionViewer: CollectionViewer): Observable<DynamicFlatNode[]> {
-    this.treeControl.expansionModel.onChange!.subscribe(change => {
+    this.treeControl.expansionModel.onChange.subscribe(change => {
       if (change.added || change.removed) {
         this.handleTreeControl(change);
       }

src/lib/list/selection-list.ts

@@ -346,7 +346,7 @@ export class MatSelectionList extends _MatSelectionListMixinBase implements Focu
     }
 
     // Sync external changes to the model back to the options.
-    this._modelChanges = this.selectedOptions.onChange!.subscribe(event => {
+    this._modelChanges = this.selectedOptions.onChange.subscribe(event => {
       if (event.added) {
         for (let item of event.added) {
           item.selected = true;

src/lib/select/select.ts

@@ -527,7 +527,7 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
   ngAfterContentInit() {
     this._initKeyManager();
 
-    this._selectionModel.onChange!.pipe(takeUntil(this._destroy)).subscribe(event => {
+    this._selectionModel.onChange.pipe(takeUntil(this._destroy)).subscribe(event => {
       event.added.forEach(option => option.select());
       event.removed.forEach(option => option.deselect());
     });

src/lib/tree/data-source/flat-data-source.ts

@@ -146,7 +146,7 @@ export class MatTreeFlatDataSource<T, F> extends DataSource<F> {
   connect(collectionViewer: CollectionViewer): Observable<F[]> {
     const changes = [
       collectionViewer.viewChange,
-      this.treeControl.expansionModel.onChange!,
+      this.treeControl.expansionModel.onChange,
       this._flattenedData
     ];
     return merge(...changes).pipe(map(() => {

src/material-examples/tree-dynamic/tree-dynamic-example.ts

@@ -59,7 +59,7 @@ export class DynamicDataSource {
               private database: DynamicDatabase) {}
 
   connect(collectionViewer: CollectionViewer): Observable<DynamicFlatNode[]> {
-    this.treeControl.expansionModel.onChange!.subscribe(change => {
+    this.treeControl.expansionModel.onChange.subscribe(change => {
       if ((change as SelectionChange<DynamicFlatNode>).added ||
         (change as SelectionChange<DynamicFlatNode>).removed) {
         this.handleTreeControl(change as SelectionChange<DynamicFlatNode>);