如何预选md-sidenav中的第一项

<md-sidenav [(ngModel)]="selectedItem">
  <md-list>
    <md-list-item (click)="selectItem('item1')">Item 1</md-list-item>
    <md-list-item (click)="selectItem('item2')">Item 2</md-list-item>
    <md-list-item (click)="selectItem('item3')">Item 3</md-list-item>
  </md-list>
</md-sidenav>

selectedItem: string;

selectItem(item: string) {
  this.selectedItem = item;
}

constructor() {
  this.selectedItem = 'item1'; // 设置为第一项的值
}

// 或者

ngOnInit() {
  this.selectedItem = 'item1'; // 设置为第一项的值
}