Angular Material mat-expansion-panel打开事件始终返回undefine

正如您可能从角度材料文档中知道的那样,mat-expansion-panel将@Output命名为“已打开”,这是在打开手风琴项目时发出的.我想抓住那个打开的事件并确定哪个面板被打开了.但是这个简单的代码(下面)总是只返回undefined.我做错了什么？此输出是仅发出未定义还是我错过了什么？材料文档没有说明这种情况.

component.html

<mat-accordion multi>
  <mat-expansion-panel (opened)="openGroup($event)" *ngFor="let element of Data">
    <mat-expansion-panel-header>
      <mat-panel-title>
        <ul class="nav-tabs">
          <li class="col-xs-3">{{element.param1}}</li>
          <li class="col-xs-5">{{element.param2}}</li>
          <li class="col-xs-3">{{element.param3}}</li>
        </ul>
      </mat-panel-title>
    </mat-expansion-panel-header>

    <div>
      <p>{{element.content}}</p>
    </div>
  </mat-expansion-panel>
</mat-accordion>

component.ts

Data = [
  // This returns from backend,this is only for show that the Data is not empty
  { /* some data here*/ },{ /* some data here*/ },{ /* some data here*/ }
];

openGroup(e) {
  console.log(e);
}

<mat-expansion-panel (opened)="openGroup('expansion1')"></mat-expansion-panel>
<mat-expansion-panel (opened)="openGroup('expansion2')"></mat-expansion-panel>

<mat-expansion-panel *ngFor="let element of Data" (opened)="openGroup(element.someProp)">