想要在ngbTypeahead中显示消息“找不到记录”

Question

我正在使用ngbTypeahead。在我的示例中，如果搜索文本不匹配，我想显示消息。能否请您提出建议，我该如何显示消息？ 这是我的代码。

add.tool.component.ts

search = (text$: Observable<string>) =>
    text$.pipe(
        debounceTime(200),
        distinctUntilChanged(),
        merge(this.focus$),
        merge(this.click$.pipe(filter(() => !this.instance.isPopupOpen()))),
        map(term => (term === '' ? ''
            : this.requestedTools.filter(v => v.name.toLowerCase().indexOf(term.toLowerCase()) > -1)).slice(0, 10))
    );

formatter = (x: { name: string }) => x.name;

add.tool.component.html

<div class="input-group input-width">
                <input type="text" class="form-control" #instance="ngbTypeahead" placeholder="Search" aria-label="Search" 
                [(ngModel)]="requestedTool"
                 [ngbTypeahead]="search" />
                <span class="search-icon-position">
                    <button type="button" class="btn btn-primary bg-transparent">
                        <span class="fa fa-search search-icon-right"></span>
                    </button>
                </span>
            </div>