fix(expansion): respect parent accordion hideToggle binding

* Fixes that the expansion panels no longer respect the bindings of a parent `<mat-accordion>`. (this has been accidentally changes in angular#6529)
* Fixes that the expansion panel does not re-render if the `[hideToggle]` binding is set on the accordion.
* Removes the unused `_getHideToggle` method. This method is no longer used because the panel header directly accesses the panel through DI.

src/cdk/accordion/accordion.ts

@@ -6,8 +6,8 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Directive, Input} from '@angular/core';
 import {coerceBooleanProperty} from '@angular/cdk/coercion';
+import {Directive, Input, OnChanges, OnDestroy, SimpleChanges} from '@angular/core';
 import {Subject} from 'rxjs';
 
 /** Used to generate unique ID for each accordion. */
@@ -20,7 +20,10 @@ let nextId = 0;
   selector: 'cdk-accordion, [cdkAccordion]',
   exportAs: 'cdkAccordion',
 })
-export class CdkAccordion {
+export class CdkAccordion implements OnDestroy, OnChanges {
+  /** Emits when the state of the accordion changes */
+  readonly _stateChanges = new Subject<SimpleChanges>();
+
   /** Stream that emits true/false when openAll/closeAll is triggered. */
   readonly _openCloseAllActions: Subject<boolean> = new Subject<boolean>();
 
@@ -43,6 +46,14 @@ export class CdkAccordion {
     this._openCloseAll(false);
   }
 
+  ngOnChanges(changes: SimpleChanges) {
+    this._stateChanges.next(changes);
+  }
+
+  ngOnDestroy() {
+    this._stateChanges.complete();
+  }
+
   private _openCloseAll(expanded: boolean): void {
     if (this.multi) {
       this._openCloseAllActions.next(expanded);

src/demo-app/expansion/expansion-demo.html

@@ -62,17 +62,17 @@ <h1>matAccordion</h1>
   </div>
 </div>
 <br>
-<mat-accordion [displayMode]="displayMode" [multi]="multi"
+<mat-accordion [displayMode]="displayMode" [multi]="multi" [hideToggle]="hideToggle"
                class="demo-expansion-width">
-  <mat-expansion-panel #panel1  [hideToggle]="hideToggle">
+  <mat-expansion-panel #panel1>
     <mat-expansion-panel-header>Section 1</mat-expansion-panel-header>
     <p>This is the content text that makes sense here.</p>
   </mat-expansion-panel>
-  <mat-expansion-panel #panel2 [hideToggle]="hideToggle" [disabled]="disabled">
+  <mat-expansion-panel #panel2 [disabled]="disabled">
     <mat-expansion-panel-header>Section 2</mat-expansion-panel-header>
     <p>This is the content text that makes sense here.</p>
   </mat-expansion-panel>
-  <mat-expansion-panel #panel3 *ngIf="showPanel3" [hideToggle]="hideToggle">
+  <mat-expansion-panel #panel3 *ngIf="showPanel3">
     <mat-expansion-panel-header>Section 3</mat-expansion-panel-header>
     <mat-checkbox #showButtons>Reveal Buttons Below</mat-checkbox>
     <mat-action-row *ngIf="showButtons.checked">

src/lib/expansion/accordion.spec.ts

@@ -13,6 +13,7 @@ describe('MatAccordion', () => {
         MatExpansionModule
       ],
       declarations: [
+        AccordionWithHideToggle,
         NestedPanel,
         SetOfItems,
       ],
@@ -93,6 +94,22 @@ describe('MatAccordion', () => {
 
     expect(innerPanel.accordion).not.toBe(outerPanel.accordion);
   });
+
+  it('should update the expansion panel if hideToggle changed', () => {
+    const fixture = TestBed.createComponent(AccordionWithHideToggle);
+    const panel = fixture.debugElement.query(By.directive(MatExpansionPanel));
+
+    fixture.detectChanges();
+
+    expect(panel.nativeElement.querySelector('.mat-expansion-indicator'))
+      .toBeTruthy('Expected the expansion indicator to be present.');
+
+    fixture.componentInstance.hideToggle = true;
+    fixture.detectChanges();
+
+    expect(panel.nativeElement.querySelector('.mat-expansion-indicator'))
+      .toBeFalsy('Expected the expansion indicator to be removed.');
+  });
 });
 
 
@@ -130,3 +147,15 @@ class NestedPanel {
   @ViewChild('outerPanel') outerPanel: MatExpansionPanel;
   @ViewChild('innerPanel') innerPanel: MatExpansionPanel;
 }
+
+@Component({template: `
+  <mat-accordion [hideToggle]="hideToggle">
+    <mat-expansion-panel>
+      <mat-expansion-panel-header>Header</mat-expansion-panel-header>
+      <p>Content</p>
+    </mat-expansion-panel>
+  </mat-accordion>`
+})
+class AccordionWithHideToggle {
+  hideToggle = false;
+}

src/lib/expansion/accordion.ts

@@ -19,6 +19,7 @@ export type MatAccordionDisplayMode = 'default' | 'flat';
 @Directive({
   selector: 'mat-accordion',
   exportAs: 'matAccordion',
+  inputs: ['multi'],
   host: {
     class: 'mat-accordion'
   }

src/lib/expansion/expansion-panel-header.ts

@@ -19,7 +19,7 @@ import {
   OnDestroy,
   ViewEncapsulation,
 } from '@angular/core';
-import {merge, Subscription} from 'rxjs';
+import {merge, Subscription, EMPTY} from 'rxjs';
 import {filter} from 'rxjs/operators';
 import {matExpansionAnimations} from './expansion-animations';
 import {MatExpansionPanel} from './expansion-panel';
@@ -65,16 +65,20 @@ export class MatExpansionPanelHeader implements OnDestroy {
   private _parentChangeSubscription = Subscription.EMPTY;
 
   constructor(
-    @Host() public panel: MatExpansionPanel,
-    private _element: ElementRef,
-    private _focusMonitor: FocusMonitor,
-    private _changeDetectorRef: ChangeDetectorRef) {
+      @Host() public panel: MatExpansionPanel,
+      private _element: ElementRef,
+      private _focusMonitor: FocusMonitor,
+      private _changeDetectorRef: ChangeDetectorRef) {
+
+    const accordionHideToggleChange = panel.accordion ?
+      panel.accordion._stateChanges.pipe(filter(changes => !!changes.hideToggle)) : EMPTY;
 
     // Since the toggle state depends on an @Input on the panel, we
-    // need to  subscribe and trigger change detection manually.
+    // need to subscribe and trigger change detection manually.
     this._parentChangeSubscription = merge(
       panel.opened,
       panel.closed,
+      accordionHideToggleChange,
       panel._inputChanges.pipe(filter(changes => !!(changes.hideToggle || changes.disabled)))
     )
     .subscribe(() => this._changeDetectorRef.markForCheck());

src/lib/expansion/expansion-panel.ts

@@ -74,7 +74,9 @@ export class MatExpansionPanel extends CdkAccordionItem
   implements AfterContentInit, OnChanges, OnDestroy {
   /** Whether the toggle indicator should be hidden. */
   @Input()
-  get hideToggle(): boolean { return this._hideToggle; }
+  get hideToggle(): boolean {
+    return this._hideToggle || (this.accordion && this.accordion.hideToggle);
+  }
   set hideToggle(value: boolean) {
     this._hideToggle = coerceBooleanProperty(value);
   }
@@ -109,17 +111,12 @@ export class MatExpansionPanel extends CdkAccordionItem
     this.accordion = accordion;
   }
 
-  /** Whether the expansion indicator should be hidden. */
-  _getHideToggle(): boolean {
-    if (this.accordion) {
-      return this.accordion.hideToggle;
-    }
-    return this.hideToggle;
-  }
-
   /** Determines whether the expansion panel should have spacing between it and its siblings. */
   _hasSpacing(): boolean {
     if (this.accordion) {
+      // We don't need to subscribe to the `stateChanges` of the parent accordion because each time
+      // the [displayMode] input changes, the change detection will also cover the host bindings
+      // of this expansion panel.
       return (this.expanded ? this.accordion.displayMode : this._getExpandedState()) === 'default';
     }
     return false;