无法将从订阅方法接收的值存储在模板变量中。

照片细节组件

import { Component,OnInit,Input } from "@angular/core";
import { PhotoSevice } from "../photo.service";
import { Photo } from "src/app/model/photo.model";

@Component({
  selector: "app-photo-detail",templateUrl: "./photo-detail.component.html",styleUrls: ["./photo-detail.component.css"]
})
export class PhotoDetailComponent implements OnInit {

  url: string;

  constructor(private photoService: PhotoSevice) {
    this.photoService.photoSelected.subscribe(data => {
      this.url = data;
      console.log(this.url);
    });
    console.log(this.url);
  }

  ngOnInit() {

  }
}

外部console.log给出了未定义的内容，视图中未呈现任何内容，但是在subscibe方法内部，我可以看到该值。那么，如何在视图中显示它？

照片组件

import { Component,OnInit } from "@angular/core";
import { activatedRoute,Params,Router } from "@angular/router";
import { FnParam } from "@angular/compiler/src/output/output_ast";
import { AlbumService } from "../service/album.service";
import { Photo } from "../model/photo.model";
import { PhotoSevice } from "./photo.service";

@Component({
  selector: "app-photos",templateUrl: "./photos.component.html",styleUrls: ["./photos.component.css"]
})
export class PhotosComponent implements OnInit {
  selectedAlbumId: string;
  photoList: Photo[] = [];
  photoSelected: Photo;
  isLoading: Boolean;
  constructor(
    private rout: activatedRoute,private albumService: AlbumService,private router: Router,private photoService: PhotoSevice
  ) { }

  ngOnInit() {
    this.isLoading = true;
    this.rout.params.subscribe((params: Params) => {
      this.selectedAlbumId = params["id"];
      this.getPhotos(this.selectedAlbumId);
    });
  }

  getPhotos(id: string) {
    this.albumService.fetchPhotos(this.selectedAlbumId).subscribe(photo => {
      this.photoList = photo;
      this.isLoading = false;
    });
  }

  displayPhoto(url: string,title: string) {

    console.log(url);
    this.photoService.photoSelected.emit(url);
    this.router.navigate(["/photo-detail"]);
  }
}

请向我解释这是如何工作的以及如何解决它，以便我可以在模板视图中存储和显示从订阅和异步调用接收的值。

这是两个组件的视图-

photo.component.html

<div *ngIf="isLoading">
  <h3>Loading...</h3>
</div>

<div class="container" *ngIf="!isLoading">
  <div class="card-columns">
    <div *ngFor="let photo of photoList" class="card">
      <img
        class="card-img-top"
        src="{{ photo.thumbnailUrl }}"
        alt="https://source.unsplash.com/random/300x200"
      />
      <div class="card-body">
        <a
          class="btn btn-primary btn-block"
          (click)="displayPhoto(photo.url,photo.title)"
          >Enlarge Image</a
        >
      </div>
    </div>
  </div>
</div>

photo-detail.component.ts

<div class="container">
  <div class="card-columns">
    <div class="card">
      <img class="card-img-top" src="{{ url }}" />
    </div>
  </div>
</div>

photo.service.ts

import { Injectable } from "@angular/core";
import { EventEmitter } from "@angular/core";

@Injectable({ providedIn: "root" })
export class PhotoSevice {
  photoSelected = new EventEmitter();
 // urlService: string;
}

这是我的github存储库的链接，我将代码保留在注释中，并在那里使用了另一种方法。 如果您检查相册组件，那么我也已经订阅了http请求，并在相册组件的模板变量中分配了值。 还有该值作为未定义的subscibe方法，但我能够在模板中访问它。

https://github.com/Arpan619Banerjee/angular-accelerate

以下是相册组件和服务的详细信息 请将此与事件发射器的情况进行比较，并向我解释有什么区别- albums.component.ts

import { Component,OnInit } from "@angular/core";
import { AlbumService } from "../service/album.service";
import { Album } from "../model/album.model";

@Component({
  selector: "app-albums",templateUrl: "./albums.component.html",styleUrls: ["./albums.component.css"]
})
export class AlbumsComponent implements OnInit {
  constructor(private albumService: AlbumService) {}
  listAlbums: Album[] = [];
  isLoading: Boolean;

  ngOnInit() {
    this.isLoading = true;
    this.getalbums();
  }

  getalbums() {
    this.albumService.fetchAlbums().subscribe(data => {
      this.listAlbums = data;
      console.log("inside subscibe method-->" + this.listAlbums); // we have data here
      this.isLoading = false;
    });
    console.log("outside subscribe method----->" + this.listAlbums); //empty list==== but somehow we have the value in the view,this doesn t work
    //for my photo and photo-detail component.
  }
}

albums.component.html

<div *ngIf="isLoading">
  <h3>Loading...</h3>
</div>
<div class="container" *ngIf="!isLoading">
  <h3>Albums</h3>
  <app-album-details
    [albumDetail]="album"
    *ngFor="let album of listAlbums"
  ></app-album-details>
</div>

album.service.ts

import { Injectable } from "@angular/core";
import { HttpClient,HttpParams } from "@angular/common/http";
import { map,tap } from "rxjs/operators";
import { Album } from "../model/album.model";
import { Observable } from "rxjs";
import { username } from "../model/user.model";

@Injectable({ providedIn: "root" })
export class AlbumService {
  constructor(private http: HttpClient) {}
  albumUrl = "http://jsonplaceholder.typicode.com/albums";
  userUrl = "http://jsonplaceholder.typicode.com/users?id=";
  photoUrl = "http://jsonplaceholder.typicode.com/photos";

  //get the album title along with the user name
  fetchAlbums(): Observable<any> {
    return this.http.get<Album[]>(this.albumUrl).pipe(
      tap(albums => {
        albums.map((album: { userId: String; username: String }) => {
          this.fetchUsers(album.userId).subscribe((user: any) => {
            album.username = user[0].username;
          });
        });
        // console.log(albums);
      })
    );
  }

  //get the user name of the particular album with the help of userId property in albums
  fetchUsers(id: String): Observable<any> {
    //let userId = new HttpParams().set("userId",id);
    return this.http.get(this.userUrl + id);
  }

  //get the photos of a particular album using the albumId
  fetchPhotos(id: string): Observable<any> {
    let selectedId = new HttpParams().set("albumId",id);
    return this.http.get(this.photoUrl,{
      params: selectedId
    });
  }
}

我已经按照注释中的说明在偶数发射器中添加了控制台日志，这就是我所期望的行为。