How to make a synchronous call in angular 5?

问题

So, I was trying to get the solution of this problem. But, somehow I am unable to do so, May be because of the lack of knowledge in angular 5. This is my service:

GetCurrentUserData(): Observable<ResponseData> {
    return this.http.get<ResponseData>(ApplicationURLS.GetCurrentUserInformation)
        .map(response => {
            return response;
        });
    //.catch(error => this.handleError(error));
}

This is my component:

public GetCurrentUserInformation(): any {

        return this.loginService.GetCurrentUserData().subscribe(data => { return data; });
    }

Here I am trying to access the data:

ngAfterViewInit() {
        debugger;                
        this.responseData = this.GetCurrentUserInformation();
        if (this.responseData.code != responseCodes.success) {
            this.googleInit();
        }

    }

When I check the this.responseData it is always returning this instead I want data:

I just want to make a sync call so I can get the data immediately.

I also tried to use the do() in service but its returning do() is not a function.

回答1:

this can be simplify by using async/await 🔥🔥

public GetCurrentUserInformation(): Promise<any>{
    return this.loginService.GetCurrentUserData().toPromise()
}

ngAfterViewInit

async ngAfterViewInit() {    
        this.responseData = await this.GetCurrentUserInformation(); // 🧙‍♂️ 
        if (this.responseData.code != responseCodes.success) {
            this.googleInit();
        }
    }

回答2:

Subscribe to GetCurrentUserData() the http call is async (every browser api call is async, because the javascript engine runs in a single thread (google for browser event loop for more, this is not an angular issue))

this.GetCurrentUserInformation().subscribe((data: ResponseData) => {
        if (this.responseData.code != responseCodes.success) {
            this.googleInit();
        }
});

回答3:

Asynchronous functions cannot be called synchronously, because they are asynchronous.

subscribe generally shouldn't performed in methods that are expected to be chained. Even if it should, an observable and not a subscription should be returned from a method (a subscription can be additionally saved to be unsubscribed on destruction).

GetCurrentUserInformation method is redundant because it is just a wrapper for service call. The code can be refactored to:

ngAfterViewInit() {
    this.loginService.GetCurrentUserData().subscribe(data => {
        this.responseData = data;
        if (this.responseData.code != responseCodes.success) {
            this.googleInit();
        }
    });
}

回答4:

To make sure your async calls are executed before processing response you may use Observable.forkJoin() from ReactiveX.

Observable.forkJoin(
    this.http.get('/links.json').map((response:Response) => response.json()),
    this.http.get('/bookmarks.json').map((response:Response) => response.json())
).subscribe(
    data => {
      this.links = data[0]
      this.bookmarks = data[1]
    },
    error => console.error(error)
);

subscribe() function's onNext handler will execute when all HTTP requests complete successfully.

Required imports:

import {Observable} from 'rxjs/Observable';
import 'rxjs/add/observable/forkJoin';

You may find more in this article.

回答5:

there is no direct way to make a synchronous call, however you can do the following procedure (this code is written in angular 7).

import { Component, OnInit } from '@angular/core';
import {HttpClient} from '@angular/common/http';

export class SampleComponent implements OnInit {
  Request1result:Object;
  Request2result:Object;

  constructor(private http:HttpClient) { }

   ngOnInit() 
   {
    this.http.get("URL1").subscribe((res)=>{
      this.Request1result=res;
      this.after1(); // execution will move to next request only when first is done.
    });
   }
  after1()
  {
    this.http.get("URL2").subscribe((res)=>{
      this.Request2result=res;
      this.after2(); // execution will move to the rest of the code only when both the requests are done.
    });
  }
  after2()
    {
    // rest of the code.
    console.log(this.Request1result);
    console.log(this.Request2result);

    }

}

来源：https://stackoverflow.com/questions/47605737/how-to-make-a-synchronous-call-in-angular-5