Angular2如何在使用ng-for时,仅在第一个元素上设置元素类名

我想把class =“active”设置在第一个li上.我将索引转换为类属性,但这不是我想要的.

import { Component,View,NgFor,Inject,forwardRef,Input,NgIf,FORM_DIRECTIVES } from 'angular2/angular2';
@Component({
    selector: 'tabs'
})
@View({
    template: `
        <ul>
           <li *ng-for="#tab of tabs;#index = index" class="{{index}}" (click)="selectTab(tab)">{{tab.tabTitle}}</li>
        </ul>
        <ng-content></ng-content>
  `,directives: [NgFor]
})

export class Tabs {
    get tabs() {
        return this._tabs;
    }

    set tabs(value) {
        this._tabs = value;
    }
    private _tabs;

    constructor() {
        console.log("ctor.Tabs");
        this._tabs = [];
    }

    selectTab(tab) {
        this._tabs.forEach((tab) => {
            tab.active = false;
        });
        tab.active = true;
    }

    addTab(tab: Tab) {
        if (this._tabs.length === 0) {
            tab.active = true;

        }
        else {
            tab.active = false;
        }
        this._tabs.push(tab);
    }
}

@Component({
    selector: 'tab',properties: ['tabTitle: tab-title']
})
@View({
    template: `
    <div [hidden]="!active" [class]="active">
      <ng-content/>
    </div>
  `
})
export class Tab {
    @Input() index: number;

    constructor(@Inject(forwardRef(() => Tabs)) tabs: Tabs) {
        console.log("ctor.Tab") ;
        tabs.addTab(this);
        console.log(tabs);
    }

    get active() {
        return this._active;
    }
    set active(value) {
        this._active = value;
    }
    private _active;


}