共享服务变量未定义

Question

我有一个共享服务，它总是返回undefined。在App.component中，从html页面成功检索到值，但是，欢迎组件会丢失该值。

CSHTML

<pm-app gwLinkID="111"></pm-app>

共享服务

import { Subject } from 'rxjs/Subject';

@Injectable()

export class AppParamasService {
    constructor() {

    }
    private _ntName = new Subject<any>();
    ntNameValue = this._ntName.asObservable();

    private _gwLinkID = new Subject<any>();
    gwLinkIDValue = this._gwLinkID.asObservable();

    gwLinkIDReceived(gwLinkID: number) {
        this._gwLinkID.next(gwLinkID);
    }


}

app.component

import { Component, OnInit, Input, ElementRef } from '@angular/core';
import { AppParamasService } from './shared/app-params.service';
@Component({
    selector: 'pm-app',
    moduleId: module.id,
    template:'<home></home>'


})
export class AppComponent {
    @Input() gwLinkID: number;
    @Input() psnlUID: string;
    @Input() ntName: string;
    //pageTitle: any[];

    errorMessage: string;

    constructor(elm: ElementRef, private _param: AppParamasService) {
        this.gwLinkID = elm.nativeElement.getAttribute('gwLinkID'); //value is coming
        this._param.gwLinkIDReceived (elm.nativeElement.getAttribute('gwLinkID'));



    }


    ngOnInit(): void {



    }

}

welcome.component this.gwLinkID未定义

import { Component, ElementRef,Input} from '@angular/core';
@Component({
   selector: 'home',
    moduleId: module.id,
    templateUrl: 'welcome.component.html'

})
export class WelcomeComponent {
    gwLinkID: number;
    constructor(elm: ElementRef,private _appParams: AppParamasService) {


        this._appParams.gwLinkIDValue.subscribe(gwLinkIDVal => {
            this.gwLinkID = gwLinkIDVal;
        });

    }
}