在Angular中获取更改的数组元素

Question

我有这个组件：

    export class RequestedProductList implements DoCheck {

        @Input() requests: Request[] = [];
        differ: any;
     constructor(
            private globalVariablesService: GlobalVariablesService,
            private route: ActivatedRoute,
            private requestService: RequestService,
            private loaderService: LoaderService,
            private collectionService: CollectionService,
            private accountService: AccountService,
            private differs: IterableDiffers
        ) {
            this.requests = requestService.requests;
            this.differ = differs.find([]).create(null);

            this.route.params.subscribe(
                params => this.accountId = params["id"]
            );

            this.loadRequests();
            this.loadAccountData();        
        }

ngDoCheck() {
        const change = this.differ.diff(this.requests);
        console.log(change);
    }

请求数组显示在模板中，并且有一些像这样的双向绑定表单字段。

<td><input class="form-control form-control-sm comment" [(ngModel)]="request.comment" type="text" placeholder=".form-control-sm"></td>

当我输入数据时，此表单字段为ngDoCheck（）运行但更改始终为空。

我试图获取数组中已更改的对象，以便我可以在数据库中更新它。为什么差异没有检测到变化？