Angular 使用isExpanded（）和ng引导程序手风琴

我正在用Angular 10和ng bootstrap制作手风琴。面板标题上的标题需要根据面板是折叠还是展开显示“+”或“-”favicon。我无法确定isExpanded方法的正确用法

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{{item.key}
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这里解释了示例代码：

检查此StackBlitz：

HTML文件：

<div class="container">
  <ngb-accordion #acc="ngbAccordion" id="accordion" (panelChange)="changeTitle($event, acc)">
    <ngb-panel title="{{ title }}">
      <ng-template ngbPanelContent>
        Content
      </ng-template>
    </ngb-panel>
  </ngb-accordion>
</div>

<div class="container">
  <ngb-accordion #acc="ngbAccordion" id="accordion" (panelChange)="changeTitle($event, acc)">
    <ngb-panel title="{{ title }}">
      <ng-template ngbPanelContent>
        Content
      </ng-template>
    </ngb-panel>
  </ngb-accordion>
</div>

export class AppComponent  {
  name = 'Angular';

  title: string = "TITLE WHEN NOT EXPANDED";

  changeTitle($event: NgbPanelChangeEvent, acc) {
    if (!acc.isExpanded($event.panelId)) {
      this.title = "TITLE WHEN EXPANDED";
    } else {
      this.title = "TITLE WHEN NOT EXPANDED";
    }
  }
}