通过 pipe() 订阅时，Angular 没有 http 请求

Question

I am quite new to angular and tried to check if a username already exists by sending a http request to my server, but my server isn't getting any requests.

in my Service:

checkUsername(name: string): Observable<boolean> {
return this.http.get(
  `${this.api}/checkusername/${name}`
).pipe(
  catchError(this.errorHandler)
);
}

in my Validator:

export class UsernameExistsVallidatorService implements AsyncValidator{
constructor(private vs: VallidatorsService) { }

  validate(control: FormControl): Observable<ValidationErrors | null> {
    return this.vs.checkUsername(control.value).pipe(
      map(exists => (exists === false) ? null : {
          isbnExists: { valid: false }
      }),
      catchError(() => of(null))
    );
  }
}

In my server:

app.get("/api/checkusername/:name", (req: express.Request, res: express.Response) => {
  console.log(req.params.name);
  res.json({exists: false});
});

As you can see the server prints every name and for now it answers with {exists: false}.

If I put

this.vs.checkUsername(control.value).subscribe(val => console.log(val));

inside my validate function, the server receives everything correctly.

Answer 1

For the getting request from serve you always needs subscribe

http.post (and get, put, delete, etc) returns a cold Observable , ie an Observable for which:

its underlying producer is created and activated during subscription

This means the function represented by the Observable is activated only with the subscribe() method.

or

 post() : Promise<any> {
     return this.http.post(
       '/api, 
        {}
     ).toPromise();

or

post() {
    var observable = this.http.post('/api', {})
        .map(response => response.json()) // in case you care about returned json       
        .publishReplay(); // would be .publish().replay() 
    observable.connect();
    return observable;
}