通过使用Http,我们调用一个方法来进行网络调用,并返回一个Http可观察对象:

getCustomer() {
    return this.http.get('/someUrl').map(res => res.json());
}

如果我们获取这个可观察对象并向其添加多个订阅者:

let network$ = getCustomer();

let subscriber1 = network$.subscribe(...);
let subscriber2 = network$.subscribe(...);

我们要做的是确保这不会导致多个网络请求。

这似乎是一个不寻常的场景,但实际上很常见:例如,如果调用者订阅了可观察对象以显示错误消息,并使用异步管道将其传递给模板,那么我们已经有两个订阅者了。

在RxJs 5中正确的方法是什么?

也就是说,这似乎工作得很好:

getCustomer() {
    return this.http.get('/someUrl').map(res => res.json()).share();
}

但是这是RxJs 5中惯用的方法吗,或者我们应该用别的方法来代替?

注意:根据Angular 5的新HttpClient,所有示例中的.map(res => res. JSON())部分现在都是无用的,因为现在默认假设JSON结果。


当前回答

编辑:从2021年开始,正确的方法是使用RxJs原生提出的shareReplay操作符。详见下面的回答。


缓存数据,如果可用,则返回此数据,否则发出HTTP请求。

import {Injectable} from '@angular/core';
import {Http, Headers} from '@angular/http';
import {Observable} from 'rxjs/Observable';
import 'rxjs/add/observable/of'; //proper way to import the 'of' operator
import 'rxjs/add/operator/share';
import 'rxjs/add/operator/map';
import {Data} from './data';

@Injectable()
export class DataService {
  private url: string = 'https://cors-test.appspot.com/test';
  
  private data: Data;
  private observable: Observable<any>;

  constructor(private http: Http) {}

  getData() {
    if(this.data) {
      // if `data` is available just return it as `Observable`
      return Observable.of(this.data); 
    } else if(this.observable) {
      // if `this.observable` is set then the request is in progress
      // return the `Observable` for the ongoing request
      return this.observable;
    } else {
      // example header (not necessary)
      let headers = new Headers();
      headers.append('Content-Type', 'application/json');
      // create the request, store the `Observable` for subsequent subscribers
      this.observable = this.http.get(this.url, {
        headers: headers
      })
      .map(response =>  {
        // when the cached data is available we don't need the `Observable` reference anymore
        this.observable = null;

        if(response.status == 400) {
          return "FAILURE";
        } else if(response.status == 200) {
          this.data = new Data(response.json());
          return this.data;
        }
        // make it shared so more than one subscriber can get the result
      })
      .share();
      return this.observable;
    }
  }
}

砰砰作响的例子

这篇文章https://blog.thoughtram.io/angular/2018/03/05/advanced-caching-with-rxjs.html是一个很好的解释如何缓存共享播放。

其他回答

编辑:从2021年开始,正确的方法是使用RxJs原生提出的shareReplay操作符。详见下面的回答。


缓存数据,如果可用,则返回此数据,否则发出HTTP请求。

import {Injectable} from '@angular/core';
import {Http, Headers} from '@angular/http';
import {Observable} from 'rxjs/Observable';
import 'rxjs/add/observable/of'; //proper way to import the 'of' operator
import 'rxjs/add/operator/share';
import 'rxjs/add/operator/map';
import {Data} from './data';

@Injectable()
export class DataService {
  private url: string = 'https://cors-test.appspot.com/test';
  
  private data: Data;
  private observable: Observable<any>;

  constructor(private http: Http) {}

  getData() {
    if(this.data) {
      // if `data` is available just return it as `Observable`
      return Observable.of(this.data); 
    } else if(this.observable) {
      // if `this.observable` is set then the request is in progress
      // return the `Observable` for the ongoing request
      return this.observable;
    } else {
      // example header (not necessary)
      let headers = new Headers();
      headers.append('Content-Type', 'application/json');
      // create the request, store the `Observable` for subsequent subscribers
      this.observable = this.http.get(this.url, {
        headers: headers
      })
      .map(response =>  {
        // when the cached data is available we don't need the `Observable` reference anymore
        this.observable = null;

        if(response.status == 400) {
          return "FAILURE";
        } else if(response.status == 200) {
          this.data = new Data(response.json());
          return this.data;
        }
        // make it shared so more than one subscriber can get the result
      })
      .share();
      return this.observable;
    }
  }
}

砰砰作响的例子

这篇文章https://blog.thoughtram.io/angular/2018/03/05/advanced-caching-with-rxjs.html是一个很好的解释如何缓存共享播放。

您是否试过运行已经拥有的代码?

因为你是根据getJSON()的承诺构造Observable,所以网络请求是在任何人订阅之前发出的。最终的承诺由所有订阅者共享。

var promise = jQuery.getJSON(requestUrl); // network call is executed now
var o = Rx.Observable.fromPromise(promise); // just wraps it in an observable
o.subscribe(...); // does not trigger network call
o.subscribe(...); // does not trigger network call
// ...

RXJS 5.3.0

我对.map(myFunction).publishReplay(1).refCount()不满意

对于多个订阅者,.map()在某些情况下执行myFunction两次(我希望它只执行一次)。一个修复似乎是publishReplay(1).refCount().take(1)

你可以做的另一件事,就是不使用refCount(),让Observable立即热:

let obs = this.http.get('my/data.json').publishReplay(1);
obs.connect();
return obs;

这将启动HTTP请求,而不考虑订阅者。我不确定在HTTP GET完成之前取消订阅是否会取消它。

伟大的答案。

或者你可以这样做:

这是rxjs的最新版本。我使用5.5.7版本的RxJS

import {share} from "rxjs/operators";

this.http.get('/someUrl').pipe(share());

只需使用这个缓存层,它就可以完成您需要的一切,甚至还可以管理ajax请求的缓存。

http://www.ravinderpayal.com/blogs/12Jan2017-Ajax-Cache-Mangement-Angular2-Service.html

用起来就这么简单

@Component({
    selector: 'home',
    templateUrl: './html/home.component.html',
    styleUrls: ['./css/home.component.css'],
})
export class HomeComponent {
    constructor(AjaxService:AjaxService){
        AjaxService.postCache("/api/home/articles").subscribe(values=>{console.log(values);this.articles=values;});
    }

    articles={1:[{data:[{title:"first",sort_text:"description"},{title:"second",sort_text:"description"}],type:"Open Source Works"}]};
}

层(作为一个可注入的angular服务)是

import { Injectable }     from '@angular/core';
import { Http, Response} from '@angular/http';
import { Observable }     from 'rxjs/Observable';
import './../rxjs/operator'
@Injectable()
export class AjaxService {
    public data:Object={};
    /*
    private dataObservable:Observable<boolean>;
     */
    private dataObserver:Array<any>=[];
    private loading:Object={};
    private links:Object={};
    counter:number=-1;
    constructor (private http: Http) {
    }
    private loadPostCache(link:string){
     if(!this.loading[link]){
               this.loading[link]=true;
               this.links[link].forEach(a=>this.dataObserver[a].next(false));
               this.http.get(link)
                   .map(this.setValue)
                   .catch(this.handleError).subscribe(
                   values => {
                       this.data[link] = values;
                       delete this.loading[link];
                       this.links[link].forEach(a=>this.dataObserver[a].next(false));
                   },
                   error => {
                       delete this.loading[link];
                   }
               );
           }
    }

    private setValue(res: Response) {
        return res.json() || { };
    }

    private handleError (error: Response | any) {
        // In a real world app, we might use a remote logging infrastructure
        let errMsg: string;
        if (error instanceof Response) {
            const body = error.json() || '';
            const err = body.error || JSON.stringify(body);
            errMsg = `${error.status} - ${error.statusText || ''} ${err}`;
        } else {
            errMsg = error.message ? error.message : error.toString();
        }
        console.error(errMsg);
        return Observable.throw(errMsg);
    }

    postCache(link:string): Observable<Object>{

         return Observable.create(observer=> {
             if(this.data.hasOwnProperty(link)){
                 observer.next(this.data[link]);
             }
             else{
                 let _observable=Observable.create(_observer=>{
                     this.counter=this.counter+1;
                     this.dataObserver[this.counter]=_observer;
                     this.links.hasOwnProperty(link)?this.links[link].push(this.counter):(this.links[link]=[this.counter]);
                     _observer.next(false);
                 });
                 this.loadPostCache(link);
                 _observable.subscribe(status=>{
                     if(status){
                         observer.next(this.data[link]);
                     }
                     }
                 );
             }
            });
        }
}