fix(select): emitting change event twice for reset values

Fixes `mat-select` emitting its change event twice when a reset value is selected, as well as when it's selected twice in a row. This PR covers #10859 which would've introduced another issue.

Fixes #10675.
Fixes #13579.

Author: crisbeto

src/material/select/select.spec.ts

@@ -2904,6 +2904,44 @@ describe('MatSelect', () => {
         }));
   });
 
+  describe('with reset option and a form control', () => {
+    let fixture: ComponentFixture<SelectWithResetOptionAndFormControl>;
+    let options: HTMLElement[];
+
+    beforeEach(fakeAsync(() => {
+      configureMatSelectTestingModule([SelectWithResetOptionAndFormControl]);
+      fixture = TestBed.createComponent(SelectWithResetOptionAndFormControl);
+      fixture.detectChanges();
+      fixture.debugElement.query(By.css('.mat-select-trigger'))!.nativeElement.click();
+      fixture.detectChanges();
+      options = Array.from(overlayContainerElement.querySelectorAll('mat-option'));
+    }));
+
+    it('should set the select value', fakeAsync(() => {
+      fixture.componentInstance.control.setValue('a');
+      fixture.detectChanges();
+      expect(fixture.componentInstance.select.value).toBe('a');
+    }));
+
+    it('should reset the control value', fakeAsync(() => {
+      fixture.componentInstance.control.setValue('a');
+      fixture.detectChanges();
+
+      options[0].click();
+      fixture.detectChanges();
+      flush();
+      expect(fixture.componentInstance.control.value).toBe(undefined);
+    }));
+
+    it('should reflect the value in the form control', fakeAsync(() => {
+      options[1].click();
+      fixture.detectChanges();
+      flush();
+      expect(fixture.componentInstance.select.value).toBe('a');
+      expect(fixture.componentInstance.control.value).toBe('a');
+    }));
+  });
+
   describe('without Angular forms', () => {
     beforeEach(async(() => configureMatSelectTestingModule([
       BasicSelectWithoutForms,
@@ -3181,6 +3219,63 @@ describe('MatSelect', () => {
           .toBeFalsy('Expected no value after tabbing away.');
     }));
 
+    it('should emit once when a reset value is selected', fakeAsync(() => {
+      const fixture = TestBed.createComponent(BasicSelectWithoutForms);
+      const instance = fixture.componentInstance;
+      const spy = jasmine.createSpy('change spy');
+
+      instance.selectedFood = 'sandwich-2';
+      instance.foods[0].value = null;
+      fixture.detectChanges();
+
+      const subscription = instance.select.selectionChange.subscribe(spy);
+
+      fixture.debugElement.query(By.css('.mat-select-trigger')).nativeElement.click();
+      fixture.detectChanges();
+      flush();
+
+      (overlayContainerElement.querySelector('mat-option') as HTMLElement).click();
+      fixture.detectChanges();
+      flush();
+
+      expect(spy).toHaveBeenCalledTimes(1);
+
+      subscription.unsubscribe();
+    }));
+
+    it('should not emit the change event multiple times when a reset option is ' +
+      'selected twice in a row', fakeAsync(() => {
+        const fixture = TestBed.createComponent(BasicSelectWithoutForms);
+        const instance = fixture.componentInstance;
+        const spy = jasmine.createSpy('change spy');
+
+        instance.foods[0].value = null;
+        fixture.detectChanges();
+
+        const subscription = instance.select.selectionChange.subscribe(spy);
+
+        fixture.debugElement.query(By.css('.mat-select-trigger')).nativeElement.click();
+        fixture.detectChanges();
+        flush();
+
+        (overlayContainerElement.querySelector('mat-option') as HTMLElement).click();
+        fixture.detectChanges();
+        flush();
+
+        expect(spy).not.toHaveBeenCalled();
+
+        fixture.debugElement.query(By.css('.mat-select-trigger')).nativeElement.click();
+        fixture.detectChanges();
+        flush();
+
+        (overlayContainerElement.querySelector('mat-option') as HTMLElement).click();
+        fixture.detectChanges();
+        flush();
+
+        expect(spy).not.toHaveBeenCalled();
+
+        subscription.unsubscribe();
+      }));
 
   });
 
@@ -5313,3 +5408,23 @@ class MultiSelectWithLotsOfOptions {
     this.value = [];
   }
 }
+
+
+@Component({
+  selector: 'basic-select-with-reset',
+  template: `
+    <mat-form-field>
+      <mat-select [formControl]="control">
+        <mat-option>Reset</mat-option>
+        <mat-option value="a">A</mat-option>
+        <mat-option value="b">B</mat-option>
+        <mat-option value="c">C</mat-option>
+      </mat-select>
+    </mat-form-field>
+  `
+})
+class SelectWithResetOptionAndFormControl {
+  @ViewChild(MatSelect) select: MatSelect;
+  @ViewChildren(MatOption) options: QueryList<MatOption>;
+  control = new FormControl();
+}

src/material/select/select.ts

@@ -442,7 +442,10 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
   get value(): any { return this._value; }
   set value(newValue: any) {
     if (newValue !== this._value) {
-      this.writeValue(newValue);
+      if (this.options) {
+        this._setSelectionByValue(newValue);
+      }
+
       this._value = newValue;
     }
   }
@@ -677,6 +680,7 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
    */
   writeValue(value: any): void {
     if (this.options) {
+      this._value = value;
       this._setSelectionByValue(value);
     }
   }
@@ -1003,7 +1007,10 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
     if (option.value == null && !this._multiple) {
       option.deselect();
       this._selectionModel.clear();
-      this._propagateChanges(option.value);
+
+      if (this.value != null) {
+        this._propagateChanges(option.value);
+      }
     } else {
       if (wasSelected !== option.selected) {
         option.selected ? this._selectionModel.select(option) :