fix(expansion-panel): toggle not being updated when set programmatically

Fixes the toggle arrow not being flipped when the `expanded` is set programmatically, in addition to the `hideToggle` input not working either. This is a regression from angular#5549 and is a consequence of the fact that the panel header reaches into the panel to determine what to do with the arrow.

Fixes angular#5623.

src/lib/expansion/accordion-item.ts

@@ -35,8 +35,10 @@ export class AccordionItem implements OnDestroy {
   @Output() destroyed = new EventEmitter<void>();
   /** The unique MdAccordionChild id. */
   readonly id = `cdk-accordion-child-${nextId++}`;
+
   /** Whether the MdAccordionChild is expanded. */
-  @Input() get expanded(): boolean { return this._expanded; }
+  @Input()
+  get expanded(): boolean { return this._expanded; }
   set expanded(expanded: boolean) {
     // Only emit events and update the internal value if the value changes.
     if (this._expanded !== expanded) {

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

@@ -12,6 +12,8 @@ import {
   Host,
   ViewEncapsulation,
   ChangeDetectionStrategy,
+  ChangeDetectorRef,
+  OnDestroy,
 } from '@angular/core';
 import {
   trigger,
@@ -22,6 +24,9 @@ import {
 } from '@angular/animations';
 import {SPACE, ENTER} from '../core/keyboard/keycodes';
 import {MdExpansionPanel, EXPANSION_PANEL_ANIMATION_TIMING} from './expansion-panel';
+import {filter} from '../core/rxjs/index';
+import {merge} from 'rxjs/observable/merge';
+import {Subscription} from 'rxjs/Subscription';
 
 
 /**
@@ -62,8 +67,22 @@ import {MdExpansionPanel, EXPANSION_PANEL_ANIMATION_TIMING} from './expansion-pa
     ]),
   ],
 })
-export class MdExpansionPanelHeader {
-  constructor(@Host() public panel: MdExpansionPanel) {}
+export class MdExpansionPanelHeader implements OnDestroy {
+  private _parentChangeSubscription: Subscription | null = null;
+
+  constructor(
+    @Host() public panel: MdExpansionPanel,
+    private _changeDetectorRef: ChangeDetectorRef) {
+
+    // Since the toggle state depends on an @Input on the panel, we
+    // need to  subscribe and trigger change detection manually.
+    this._parentChangeSubscription = merge(
+      panel.opened,
+      panel.closed,
+      filter.call(panel._inputChanges, changes => !!changes.hideToggle)
+    )
+    .subscribe(() => this._changeDetectorRef.markForCheck());
+  }
 
   /** Toggles the expanded state of the panel. */
   _toggle(): void {
@@ -103,6 +122,13 @@ export class MdExpansionPanelHeader {
         return;
     }
   }
+
+  ngOnDestroy() {
+    if (this._parentChangeSubscription) {
+      this._parentChangeSubscription.unsubscribe();
+      this._parentChangeSubscription = null;
+    }
+  }
 }
 
 /**

src/lib/expansion/expansion-panel.ts

@@ -16,6 +16,9 @@ import {
   forwardRef,
   ChangeDetectionStrategy,
   ChangeDetectorRef,
+  SimpleChanges,
+  OnChanges,
+  OnDestroy,
 } from '@angular/core';
 import {
   trigger,
@@ -27,6 +30,7 @@ import {
 import {MdAccordion, MdAccordionDisplayMode} from './accordion';
 import {AccordionItem} from './accordion-item';
 import {UniqueSelectionDispatcher} from '../core';
+import {Subject} from 'rxjs/Subject';
 
 
 /** MdExpansionPanel's states. */
@@ -73,10 +77,13 @@ export const EXPANSION_PANEL_ANIMATION_TIMING = '225ms cubic-bezier(0.4,0.0,0.2,
     ]),
   ],
 })
-export class MdExpansionPanel extends AccordionItem {
+export class MdExpansionPanel extends AccordionItem implements OnChanges, OnDestroy {
   /** Whether the toggle indicator should be hidden. */
   @Input() hideToggle: boolean = false;
 
+  /** Stream that emits for changes in `@Input` properties. */
+  _inputChanges = new Subject<SimpleChanges>();
+
   constructor(@Optional() @Host() accordion: MdAccordion,
               _changeDetectorRef: ChangeDetectorRef,
               _uniqueSelectionDispatcher: UniqueSelectionDispatcher) {
@@ -107,6 +114,14 @@ export class MdExpansionPanel extends AccordionItem {
   _getExpandedState(): MdExpansionPanelState {
     return this.expanded ? 'expanded' : 'collapsed';
   }
+
+  ngOnChanges(changes: SimpleChanges) {
+    this._inputChanges.next(changes);
+  }
+
+  ngOnDestroy() {
+    this._inputChanges.complete();
+  }
 }
 
 @Directive({