take(1)与first() [英] take(1) vs first()

问题描述

我发现了一些使用take(1)的AuthGuard的实现.在我的项目中，我使用了first().

I found a few implementation of AuthGuards that use take(1). In my project, I used first().

两者的工作方式相同吗?

Do both work the same way?

import 'rxjs/add/operator/map';
import 'rxjs/add/operator/first';
import { Observable } from 'rxjs/Observable';

import { Injectable } from '@angular/core';
import { CanActivate, Router, ActivatedRouteSnapshot, RouterStateSnapshot } from '@angular/router';
import { AngularFire } from 'angularfire2';

@Injectable()
export class AuthGuard implements CanActivate {

    constructor(private angularFire: AngularFire, private router: Router) { }

    canActivate(route: ActivatedRouteSnapshot, state: RouterStateSnapshot): Observable<boolean> | boolean {
        return this.angularFire.auth.map(
            (auth) =>  {
                if (auth) {
                    this.router.navigate(['/dashboard']);
                    return false;
                } else {
                    return true;
                }
            }
        ).first(); // Just change this to .take(1)
    }
}

推荐答案

运算符first()和take(1)不同.

first()运算符采用可选的predicate函数，并在源完成时没有值匹配时发出error通知.

The first() operator takes an optional predicate function and emits an error notification when no value matched when the source completed.

例如，这将发出错误:

import { EMPTY, range } from 'rxjs';
import { first, take } from 'rxjs/operators';

EMPTY.pipe(
  first(),
).subscribe(console.log, err => console.log('Error', err));

...以及此:

range(1, 5).pipe(
  first(val => val > 6),
).subscribe(console.log, err => console.log('Error', err));

这将与发出的第一个值匹配:

While this will match the first value emitted:

range(1, 5).pipe(
  first(),
).subscribe(console.log, err => console.log('Error', err));

另一方面，take(1)仅采用第一个值并完成.无需进一步的逻辑.

On the other hand take(1) just takes the first value and completes. No further logic is involved.

range(1, 5).pipe(
  take(1),
).subscribe(console.log, err => console.log('Error', err));

然后使用可观察到的空源它将不会发出任何错误:

Then with empty source Observable it won't emit any error:

EMPTY.pipe(
  take(1),
).subscribe(console.log, err => console.log('Error', err));

2019年1月:已针对RxJS 6更新

Jan 2019: Updated for RxJS 6

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