fix(select): handle async changes to the option label

Currently `mat-select` doesn't react to changes in the label of its options, which is problematic, because the option label might be populated at a later point by a pipe or it might have changed while the select is closed. These changes add a `Subject` to the `MatOption` that will emit whenever the label changes and allows the select to react accordingly.

Fixes angular#7923.

src/lib/core/option/option.ts

@@ -8,6 +8,7 @@
 
 import {coerceBooleanProperty} from '@angular/cdk/coercion';
 import {ENTER, SPACE} from '@angular/cdk/keycodes';
+import {Subject} from 'rxjs/Subject';
 import {
   ChangeDetectionStrategy,
   ChangeDetectorRef,
@@ -21,6 +22,7 @@ import {
   ViewEncapsulation,
   InjectionToken,
   Inject,
+  AfterViewChecked,
 } from '@angular/core';
 import {MatOptgroup} from './optgroup';
 
@@ -81,11 +83,12 @@ export const MAT_OPTION_PARENT_COMPONENT =
   preserveWhitespaces: false,
   changeDetection: ChangeDetectionStrategy.OnPush,
 })
-export class MatOption {
+export class MatOption implements AfterViewChecked {
   private _selected = false;
   private _active = false;
   private _disabled = false;
   private _id = `mat-option-${_uniqueIdCounter++}`;
+  private _oldViewValue = '';
 
   /** Whether the wrapping component is in multiple selection mode. */
   get multiple() { return this._parent && this._parent.multiple; }
@@ -110,6 +113,9 @@ export class MatOption {
   /** Event emitted when the option is selected or deselected. */
   @Output() onSelectionChange = new EventEmitter<MatOptionSelectionChange>();
 
+  /** Emits when the state of the option changes and any parents have to be notified. */
+  _stateChanges = new Subject<void>();
+
   constructor(
     private _element: ElementRef,
     private _changeDetectorRef: ChangeDetectorRef,
@@ -219,6 +225,22 @@ export class MatOption {
     return this._element.nativeElement;
   }
 
+  ngAfterViewChecked() {
+    // Since parent components could be using the option's label to display the selected values
+    // (e.g. `mat-select`) and they don't have a way of knowing if the option's label has changed
+    // we have to check for changes in the DOM ourselves and dispatch an event. These checks are
+    // relatively cheap, however we still limit them only to selected options in order to avoid
+    // hitting the DOM too often.
+    if (this._selected) {
+      const viewValue = this.viewValue;
+
+      if (viewValue !== this._oldViewValue) {
+        this._oldViewValue = viewValue;
+        this._stateChanges.next();
+      }
+    }
+  }
+
   /** Emits the selection change event. */
   private _emitSelectionChangeEvent(isUserInput = false): void {
     this.onSelectionChange.emit(new MatOptionSelectionChange(this, isUserInput));

src/lib/select/select.spec.ts

@@ -997,6 +997,18 @@ describe('MatSelect', () => {
             .toBe(fixture.componentInstance.options.last);
       }));
 
+      it('should update the trigger when the selected option label is changed', fakeAsync(() => {
+        fixture.componentInstance.control.setValue('pizza-1');
+        fixture.detectChanges();
+
+        expect(trigger.textContent!.trim()).toBe('Pizza');
+
+        fixture.componentInstance.foods[1].viewValue = 'Calzone';
+        fixture.detectChanges();
+
+        expect(trigger.textContent!.trim()).toBe('Calzone');
+      }));
+
       it('should not select disabled options', fakeAsync(() => {
         trigger.click();
         fixture.detectChanges();

src/lib/select/select.ts

@@ -829,16 +829,24 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
 
   /** Drops current option subscriptions and IDs and resets from scratch. */
   private _resetOptions(): void {
-    this.optionSelectionChanges.pipe(
-      takeUntil(merge(this._destroy, this.options.changes)),
-      filter(event => event.isUserInput)
-    ).subscribe(event => {
-      this._onSelect(event.source);
-
-      if (!this.multiple) {
-        this.close();
-      }
-    });
+    const changedOrDestroyed = merge(this.options.changes, this._destroy);
+
+    this.optionSelectionChanges
+      .pipe(takeUntil(changedOrDestroyed), filter(event => event.isUserInput))
+      .subscribe(event => {
+        this._onSelect(event.source);
+
+        if (!this.multiple) {
+          this.close();
+        }
+      });
+
+    merge(...this.options.map(option => option._stateChanges))
+      .pipe(takeUntil(changedOrDestroyed))
+      .subscribe(() => {
+        this._changeDetectorRef.markForCheck();
+        this.stateChanges.next();
+      });
 
     this._setOptionIds();
   }