fix(form-field): not updating outline when prefix/suffix is added or removed

src/lib/form-field/form-field.ts

@@ -34,7 +34,7 @@ import {
   MAT_LABEL_GLOBAL_OPTIONS,
   mixinColor,
 } from '@angular/material/core';
-import {EMPTY, fromEvent, merge} from 'rxjs';
+import {fromEvent, merge} from 'rxjs';
 import {startWith, take} from 'rxjs/operators';
 import {MatError} from './error';
 import {matFormFieldAnimations} from './form-field-animations';
@@ -153,14 +153,7 @@ export class MatFormField extends _MatFormFieldMixinBase
     this._appearance = value || (this._defaults && this._defaults.appearance) || 'legacy';
 
     if (this._appearance === 'outline' && oldValue !== value) {
-      // @breaking-change 7.0.0 Remove this check and else block once _ngZone is required.
-      if (this._ngZone) {
-        this._ngZone!.onStable.pipe(take(1)).subscribe(() => {
-          this._ngZone!.runOutsideAngular(() => this.updateOutlineGap());
-        });
-      } else {
-        Promise.resolve().then(() => this.updateOutlineGap());
-      }
+      this._updateOutlineGapOnStable();
     }
   }
   _appearance: MatFormFieldAppearance;
@@ -273,22 +266,30 @@ export class MatFormField extends _MatFormFieldMixinBase
 
   ngAfterContentInit() {
     this._validateControlChild();
-    if (this._control.controlType) {
-      this._elementRef.nativeElement.classList
-          .add(`mat-form-field-type-${this._control.controlType}`);
+
+    const control = this._control;
+
+    if (control.controlType) {
+      this._elementRef.nativeElement.classList.add(`mat-form-field-type-${control.controlType}`);
     }
 
     // Subscribe to changes in the child control state in order to update the form field UI.
-    this._control.stateChanges.pipe(startWith<void>(null!)).subscribe(() => {
+    control.stateChanges.pipe(startWith<void>(null!)).subscribe(() => {
       this._validatePlaceholders();
       this._syncDescribedByIds();
       this._changeDetectorRef.markForCheck();
     });
 
-    // Run change detection if the value, prefix, or suffix changes.
-    const valueChanges = this._control.ngControl && this._control.ngControl.valueChanges || EMPTY;
-    merge(valueChanges, this._prefixChildren.changes, this._suffixChildren.changes)
-        .subscribe(() => this._changeDetectorRef.markForCheck());
+    // Run change detection if the value changes.
+    if (control.ngControl && control.ngControl.valueChanges) {
+      control.ngControl.valueChanges.subscribe(() => this._changeDetectorRef.markForCheck());
+    }
+
+    // Run change detection and update the outline if the suffix or prefix changes.
+    merge(this._prefixChildren.changes, this._suffixChildren.changes).subscribe(() => {
+      this._updateOutlineGapOnStable();
+      this._changeDetectorRef.markForCheck();
+    });
 
     // Re-validate when the number of hints changes.
     this._hintChildren.changes.pipe(startWith(null)).subscribe(() => {
@@ -503,4 +504,14 @@ export class MatFormField extends _MatFormFieldMixinBase
   private _getStartEnd(rect: ClientRect): number {
     return this._dir && this._dir.value === 'rtl' ? rect.right : rect.left;
   }
+
+  /** Updates the outline gap the new time the zone stabilizes. */
+  private _updateOutlineGapOnStable() {
+    // @breaking-change 7.0.0 Remove this check and else block once _ngZone is required.
+    if (this._ngZone) {
+      this._ngZone.onStable.pipe(take(1)).subscribe(() => this.updateOutlineGap());
+    } else {
+      Promise.resolve().then(() => this.updateOutlineGap());
+    }
+  }
 }

src/lib/input/input.spec.ts

@@ -1281,6 +1281,28 @@ describe('MatInput with appearance', () => {
     expect(parseInt(outlineGap.style.width)).toBeGreaterThan(0);
   }));
 
+  it('should update the outline gap when the prefix/suffix is added or removed', fakeAsync(() => {
+    fixture.destroy();
+    TestBed.resetTestingModule();
+
+    const outlineFixture = createComponent(MatInputWithAppearanceAndLabel);
+
+    outlineFixture.componentInstance.appearance = 'outline';
+    outlineFixture.detectChanges();
+    flush();
+    outlineFixture.detectChanges();
+
+    spyOn(outlineFixture.componentInstance.formField, 'updateOutlineGap');
+
+    outlineFixture.componentInstance.showPrefix = true;
+    outlineFixture.detectChanges();
+    flush();
+    outlineFixture.detectChanges();
+
+    expect(outlineFixture.componentInstance.formField.updateOutlineGap).toHaveBeenCalled();
+  }));
+
+
 });
 
 describe('MatFormField default options', () => {
@@ -1735,13 +1757,16 @@ class MatInputWithAppearance {
 @Component({
   template: `
     <mat-form-field [appearance]="appearance">
+      <span matPrefix *ngIf="showPrefix">Somewhat long prefix</span>
       <mat-label>{{labelContent}}</mat-label>
       <input matInput>
     </mat-form-field>
   `
 })
 class MatInputWithAppearanceAndLabel {
+  @ViewChild(MatFormField) formField: MatFormField;
   appearance: MatFormFieldAppearance;
+  showPrefix: boolean;
   labelContent = 'Label';
 }