如何在ng-select Angular2中实现分组？

Question

我正在按照此link在我的 Angular2 项目中实施多个选择下拉列表。但是现在我必须在这个多选下拉列表中显示分组，那么我该如何实现呢？或者是否有任何其他插件或类似的东西我可以遵循。

组件Html：

<form [formGroup]="form" class="selector-form">
    <div class="marTop20">
        <ng-select [options]="options1"
                   [multiple]="multiple1"
                   placeholder="Select multiple"
                   formControlName="selectMultiple"
                   (opened)="onMultipleOpened()"
                   (closed)="onMultipleClosed()"
                   (selected)="onMultipleSelected($event)"
                   (deselected)="onMultipleDeselected($event)">
        </ng-select>
    </div>
</form>

ts文件中的组件代码：

export class FilterClientSelectorComponent implements OnInit {
    form: FormGroup;
    multiple1: boolean = true;
    options1: Array<any> = [];
    selection: Array<string>;
    @ViewChild('preMultiple') preMultiple;
    logMultipleString: string = '';

    constructor() {
        let numOptions = 100;
        let opts = new Array(numOptions);
        for (let i = 0; i < numOptions; i++) {
            opts[i] = {
                value: i.toString(),
                label: i.toString(),
                groupname:'a'
            };
        }
        this.options1 = opts.slice(0);
    }
    ngOnInit() {
        this.form = new FormGroup({});
        this.form.addControl('selectMultiple', new FormControl(''));
    }
    private scrollToBottom(elem) {
        elem.scrollTop = elem.scrollHeight;
    }
}

我需要输出如下面的屏幕截图：

输出页面