Angular 6 - 可观察订阅不起作用

Question

我有以下案例。服务通常是从服务器获取数据，并且该数据需要在其他组件中更新。

该组件仅获取一次订阅值，但服务每 2 秒获取一次数据。我测试了一下，服务做得很好。

在我的情况下，如果订阅是在 ngOnInit 或构造函数中，则不会

成分：

import {Component, OnInit} from '@angular/core';
import {TaskService} from "../../services/task.service";
import {Task} from "../../models/task";

@Component({
  selector: 'app-tasks',
  templateUrl: './tasks.component.html',
  styleUrls: ['./tasks.component.css']
})
export class TasksComponent implements OnInit {
  tasks: Task[];

  constructor(private taskService: TaskService) {
    this.taskService.getTasks().subscribe(tasks => {  // this is triggered only once, why ?
      this.tasks = tasks;
      console.log(tasks);
    });
  }

  ngOnInit() {

  }

  updateTask(task: Task) {
    try {
      task.completed = !task.completed;
      this.taskService.updateTask(task).subscribe();
    }
    catch (e) {
      task.completed = !task.completed;
    }
  }

}

服务：

import {Injectable} from '@angular/core';
import {HttpClient, HttpHeaders} from "@angular/common/http";
import {Observable, of, timer} from "rxjs";
import {Task} from "../models/task";


const httpOptions = {
  headers: new HttpHeaders({'Content-Type': 'application/json'})
};


@Injectable({
  providedIn: 'root'
})


export class TaskService {

  tasks: Task[];
  tasksURL = 'http://localhost:8080/api/tasks/';

  constructor(private http: HttpClient) {

    timer(1000, 2000).subscribe(() => {
      this.http.get<Task[]>(this.tasksURL).subscribe(value => this.tasks = value)
    }); // fetches and update the array every 2 seconds
  }

  getTasks(): Observable<Task[]> {
    return of(this.tasks); //returns observable which is than used by the component
  }

  updateTask(task: Task): Observable<Task> {
    const url = `${this.tasksURL}`;

    return this.http.post<Task>(url, task, httpOptions)

  }
}

Answer 1

您可以通过使用主题/行为来做到您想做的事情：只需对您的服务进行一些更改

import { BehaviorSubject } from 'rxjs/BehaviorSubject';

export class TaskService {

  tasks: new BehaviorSubject<Task[]>([]);
  tasksURL = 'http://localhost:8080/api/tasks/';

  constructor(private http: HttpClient) {

     timer(1000, 2000).subscribe(() => {
       this.http.get<Task[]>(this.tasksURL).subscribe( (value) => { 
         this.tasks.next(value);
       })
     }); // fetches and update the array every 2 seconds
  }

  getTasks(): Observable<Task[]> {
        return tasks.asObservable(); //returns observable which is then used by the component
  }