Javascript 返回一个空的 Observable
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Return an empty Observable
提问by Murhaf Sousli
The function more()is supposed to return an Observablefrom a get request
该函数more()应该Observable从获取请求中返回一个
export class Collection{
public more = (): Observable<Response> => {
if (this.hasMore()) {
return this.fetch();
}
else{
// return empty observable
}
}
private fetch = (): Observable<Response> => {
return this.http.get('some-url').map(
(res) => {
return res.json();
}
);
}
}
In this case I can only do a request if hasMore()is true, else I get an error on subscribe()function subscribe is not defined, how can I return an empty observable?
在这种情况下，如果hasMore()为真，我只能做一个请求，否则我在subscribe()函数上得到一个错误subscribe is not defined，我怎样才能返回一个空的 observable？
this.collection.more().subscribe(
(res) =>{
console.log(res);
},
(err) =>{
console.log(err);
}
)
Update
更新
In RXJS 6
在 RXJS 6
import { EMPTY } from 'rxjs'
return EMPTY;
采纳答案by Andrei Petrov
For typescript you can specify generic param of your empty observable like this:
对于打字稿，您可以像这样指定空 observable 的通用参数：
import 'rxjs/add/observable/empty'
Observable.empty<Response>();
回答by Stephen Lautier
With the new syntax of RxJS 5.5+, this becomes as the following:
使用 RxJS 5.5+ 的新语法，这变成如下：
// RxJS 6
import { empty, of } from "rxjs";
// rxjs 5.5+ (<6)
import { empty } from "rxjs/observable/empty";
import { of } from "rxjs/observable/of";
empty();
of({});
Just one thing to keep in mind, empty()completes the observable, so it won't trigger nextin your stream, but only completes. So if you have, for instance, tap, they might not get trigger as you wish (see an example below).
要记住的一件事是empty()完成可观察的，因此它不会next在您的流中触发，而只会完成。因此，例如，如果您有 ，tap它们可能不会如您所愿地触发（请参见下面的示例）。
Whereas of({})creates an Observableand emits next with a value of {}and then it completes the Observable.
而of({})创建 anObservable并发出值为 的 next{}然后它完成Observable.
E.g.:
例如：
empty().pipe(
tap(() => console.warn("i will not reach here, as i am complete"))
).subscribe();
of({}).pipe(
tap(() => console.warn("i will reach here and complete"))
).subscribe();
回答by Simon_Weaver
RxJS6 (without compatibility package installed)
RxJS6（未安装兼容包）
There's now an EMPTYconstant and an emptyfunction.
现在有一个EMPTY常量和一个empty函数。
import { Observable, empty, of } from 'rxjs';
var delay = empty().pipe(delay(1000));
var delay2 = EMPTY.pipe(delay(1000));
Observable.empty()doesn't exist anymore.
Observable.empty()不存在了。
回答by Marcel Tinner
In my case with Angular2 and rxjs, it worked with:
在我使用 Angular2 和 rxjs 的情况下，它适用于：
import {EmptyObservable} from 'rxjs/observable/EmptyObservable';
...
return new EmptyObservable();
...
回答by Toan Nguyen
回答by Andrii Verbytskyi
Several ways to create an Empty Observable:
创建 Empty Observable 的几种方法：
They just differ on how you are going to use it further (what events it will emit after: next, completeor do nothing) e.g.:
它们只是在您将如何进一步使用它（它将在之后发出什么事件：next，complete或do nothing）方面有所不同，例如：
Observable.never()- emits no events and never ends.Observable.empty()- emits onlycomplete.Observable.of({})- emits bothnextandcomplete(Empty object literal passed as an example).
Observable.never()- 不发出任何事件并且永不结束。Observable.empty()- 只发出complete.Observable.of({})- 发出next和complete（作为示例传递的空对象文字）。
Use it on your exact needs)
根据您的确切需求使用它）
回答by Chybie
You can return Observable.of(empty_variable), for example
例如，您可以返回 Observable.of(empty_variable)
Observable.of('');
// or
Observable.of({});
// etc
回答by Tuong Le
Or you can try ignoreElements()as well
或者你可以尝试ignoreElements()以及
回答by Callat
Came here with a similar question, the above didn't work for me in: "rxjs": "^6.0.0", in order to generate an observable that emits no data I needed to do:
带着类似的问题来到这里，上面的内容对我不起作用："rxjs": "^6.0.0"，为了生成一个不发出我需要做的数据的 observable：
import {Observable,empty} from 'rxjs';
class ActivatedRouteStub {
params: Observable<any> = empty();
}
回答by Jorawar Singh
Try this
尝试这个
export class Collection{
public more (): Observable<Response> {
if (this.hasMore()) {
return this.fetch();
}
else{
return this.returnEmpty();
}
}
public returnEmpty(): any {
let subscription = source.subscribe(
function (x) {
console.log('Next: %s', x);
},
function (err) {
console.log('Error: %s', err);
},
function () {
console.log('Completed');
});
}
}
let source = Observable.empty();
