fix(button-toggle): initialize falsy value for multiple button group

src/lib/button-toggle/button-toggle.spec.ts

@@ -1,5 +1,5 @@
 import {dispatchMouseEvent} from '@angular/cdk/testing';
-import {Component, DebugElement, QueryList, ViewChild, ViewChildren} from '@angular/core';
+import {Component, DebugElement, OnInit, QueryList, ViewChild, ViewChildren} from '@angular/core';
 import {ComponentFixture, fakeAsync, flush, TestBed, tick} from '@angular/core/testing';
 import {FormControl, FormsModule, NgModel, ReactiveFormsModule} from '@angular/forms';
 import {By} from '@angular/platform-browser';
@@ -19,6 +19,7 @@ describe('MatButtonToggle with forms', () => {
       declarations: [
         ButtonToggleGroupWithNgModel,
         ButtonToggleGroupWithFormControl,
+        ButtonToggleGroupMultipleWithFormControl,
       ],
     });
 
@@ -71,6 +72,46 @@ describe('MatButtonToggle with forms', () => {
     });
   });
 
+  describe('multiple using FormControl', () => {
+    let fixture: ComponentFixture<ButtonToggleGroupMultipleWithFormControl>;
+    let groupDebugElement: DebugElement;
+    let groupInstance: MatButtonToggleGroup;
+    let testComponent: ButtonToggleGroupMultipleWithFormControl;
+
+    beforeEach(fakeAsync(() => {
+      fixture = TestBed.createComponent(ButtonToggleGroupMultipleWithFormControl);
+      fixture.detectChanges();
+
+      testComponent = fixture.debugElement.componentInstance;
+
+      groupDebugElement = fixture.debugElement.query(By.directive(MatButtonToggleGroup));
+      groupInstance = groupDebugElement.injector.get<MatButtonToggleGroup>(MatButtonToggleGroup);
+    }));
+
+    it('should initialize with falsy value', () => {
+      expect(groupInstance.value).toEqual([0, 1, 2]);
+    });
+
+    it('should set the value', () => {
+      testComponent.control.setValue([0, 1]);
+
+      expect(groupInstance.value).toEqual([0, 1]);
+
+      testComponent.control.setValue([3, 5]);
+
+      expect(groupInstance.value).toEqual([3, 5]);
+    });
+
+    it('should register the on change callback', () => {
+      let spy = jasmine.createSpy('onChange callback');
+
+      testComponent.control.registerOnChange(spy);
+      testComponent.control.setValue([0, 1]);
+
+      expect(spy).toHaveBeenCalled();
+    });
+  });
+
   describe('button toggle group with ngModel and change event', () => {
     let fixture: ComponentFixture<ButtonToggleGroupWithNgModel>;
     let groupDebugElement: DebugElement;
@@ -748,6 +789,32 @@ class ButtonTogglesInsideButtonToggleGroupMultiple {
   isVertical: boolean = false;
 }
 
+@Component({
+  template: `
+  <mat-button-toggle-group [formControl]="control" multiple>
+    <mat-button-toggle *ngFor="let option of options" [value]="option.value">
+      {{option.label}}
+    </mat-button-toggle>
+  </mat-button-toggle-group>
+  `
+})
+class ButtonToggleGroupMultipleWithFormControl implements OnInit {
+  control = new FormControl([]);
+  options = [
+    {value: 0, label: 'Sunday'},
+    {value: 1, label: 'Monday'},
+    {value: 2, label: 'Tuesday'},
+    {value: 3, label: 'Wednesday'},
+    {value: 4, label: 'Thursday'},
+    {value: 5, label: 'Friday'},
+    {value: 6, label: 'Saturday'},
+  ];
+
+  ngOnInit(): void {
+    this.control.patchValue([0, 1, 2]);
+  }
+}
+
 @Component({
   template: `
   <mat-button-toggle>Yes</mat-button-toggle>

src/lib/button-toggle/button-toggle.ts

@@ -264,7 +264,11 @@ export class MatButtonToggleGroup extends _MatButtonToggleGroupMixinBase impleme
     }
 
     if (this.multiple && Array.isArray(this._rawValue)) {
-      return !!this._rawValue.find(value => toggle.value != null && value === toggle.value);
+      const correspondingOption = this._rawValue.find(value =>
+        toggle.value != null && value === toggle.value
+      );
+
+      return typeof correspondingOption !== 'undefined';
     }
 
     return toggle.value === this._rawValue;
@@ -303,7 +307,7 @@ export class MatButtonToggleGroup extends _MatButtonToggleGroupMixinBase impleme
       return toggle.value != null && toggle.value === value;
     });
 
-    if (correspondingOption) {
+    if (typeof correspondingOption !== 'undefined') {
       correspondingOption.checked = true;
       this._selectionModel.select(correspondingOption);
     }