如何在每次服务调用前运行方法？(Angular 7)

Question

在执行服务方法之前，我正在尝试从本地存储获取“有效负载”。实际上，我正尝试在构造函数方法上这样做，但有时当一个方法调用时，变量"this.payload“不会被设置。

这是我的代码

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import * as Globals from '../../globals';
import { Observable, of } from 'rxjs';
import { LocalStorage } from '@ngx-pwa/local-storage';

@Injectable({
    providedIn: 'root'
})
export class RechargesService {

    public payload;

    constructor(
        private http: HttpClient,
        public localStorage: LocalStorage
    ) {
        this.localStorage.getItem("payload").subscribe((payload) => {
            this.payload = payload;
        })
    }


    list(): Observable<any> {
        return this.http.post(Globals.url_api + "recharges/list", {
            // Sometimes this.payload isn't setted, i don't know why
            "payload": this.payload
        });
    }
}

正确的做法是什么？我知道在控制器中，我不应该在构造函数方法上写任何东西，但是在服务中它是正确的吗？

Answer 1

如果您想在每次list()调用之前获取有效负载，您可以这样做：

首先创建一个方法返回一个从本地存储获取值的observable，然后使用switchMap返回一个内部observable (这是HTTP get请求)

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import * as Globals from '../../globals';
import { Observable, of} from 'rxjs';
import { tap, switchMap } from 'rxjs/operators';
import { LocalStorage } from '@ngx-pwa/local-storage';

@Injectable({
    providedIn: 'root'
})

export class RechargesService {

public payload;

    constructor(
        private http: HttpClient,
        public localStorage: LocalStorage
    ) {

    }

     getPayload = () => {

        return this.localStorage.getItem("payload").pipe(tap(payload => this.payload = payload)); // <--- the tap() may be redundant here. you can simply get payload in list() method

     }


    list(): Observable<any> {

        return this.getPayload().pipe(
            switchMap(payload => {
               return this.http.post(Globals.url_api + "recharges/list", {"payload": this.payload})  // <--- You can use payload instead of this.payload since localStorage already returns payload
            })
        );
    }

}

switchMap：https://www.learnrxjs.io/operators/transformation/switchmap.html

执行/点击：https://www.learnrxjs.io/operators/utility/do.html

Answer 2

创建一个getPayload函数tp获取本地存储数据，并在列表函数中使用switchMap将getPayaload数据传递给api。

getPayload(){
    return this.localStorage.getItem("payload");
}

list(payload): Observable<any> {
    return getPayload().pipe(
        switchMap(
            (payload) => {
                return this.http.post(Globals.url_api + "recharges/list", {
                    payload
                })
            },
        )
    );
}

Answer 3

使用forkJoin操作符

https://www.learnrxjs.io/operators/combination/forkjoin.html

forkJoin(
      this._myService.makeRequest('Request One', 2000),
      this._myService.makeRequest('Request Two', 1000),
      this._myService.makeRequest('Request Three', 3000)
    )
    .subscribe(([res1, res2, res3]) => {
      this.propOne = res1;
      this.propTwo = res2;
      this.propThree = res3;
    });