将服务导入另一个服务时出现循环依赖错误 [英] cyclic dependency error when importing service into another service

问题描述

我正在尝试使用 Angular 2 提供的 DefaultRequestOptions 类为我的 http 请求设置默认标头.可以在此处找到文档:https://angular.io/docs/ts/latest/guide/server-communication.html#!#override-默认请求选项

I'm trying to set default headers for my http requests using the DefaultRequestOptions class provided by Angular 2. Documentation can be found here: https://angular.io/docs/ts/latest/guide/server-communication.html#!#override-default-request-options

我想添加一个默认的不记名令牌，该令牌设置在我的一项服务中，但这样做会在我的浏览器控制台中出现以下错误:

I want to add a default bearer token, which gets set in one of my services, but doing so gives me the following error in my browser console:

未处理的承诺拒绝:提供程序解析错误:无法实例化循环依赖！http: 在 NgModule AppModule 中的 ./AppModule ；区:;任务:Promise.then；价值:

Unhandled Promise rejection: Provider parse errors: Cannot instantiate cyclic dependency! Http: in NgModule AppModule in ./AppModule ; Zone: ; Task: Promise.then ; Value:

这是我的 default-request-options.service.ts 文件:

import { Injectable }                         from '@angular/core';
import { BaseRequestOptions, RequestOptions } from '@angular/http';
import { UserService }                        from './user.service';

@Injectable()
export class DefaultRequestOptions extends BaseRequestOptions {

  constructor(private userService: UserService) {
    super();

    // Set the default 'Content-Type' header
    this.headers.set('Content-Type', 'application/json');
    this.headers.set('Accept', 'application/json');
    this.headers.set('Authorization', 'Bearer ' + this.userService.idToken);
  }
}

export const requestOptionsProvider = { provide: RequestOptions, useClass: DefaultRequestOptions };

这是我的app.module.ts文件中的相关代码:

Here is the relevant code in my app.module.ts file:

import { requestOptionsProvider }         from './default-request-options.service';
import { UserService }                    from './user.service';

@NgModule({ 
    imports: [
        ...
    ],  
    declarations: [
        ...
    ],    
    providers: [        
        ...        
        requestOptionsProvider,        
        UserService
    ],
    bootstrap: [ AppComponent ]
})

我做错了什么?

推荐答案

Imagine 3 services ..

Imagine 3 services ..

服务 1、服务 2、服务 3

Service1, Service2, Service3

• Service1 导入 Service2
• Service2 导入 Service3
• Service3 导入 Service1

Service1 尝试导入 2，2 尝试导入 3，3 尝试导入 1，这将永远持续下去.这是一个循环依赖.

Service1 tries to import 2, 2 tries to import 3, 3 tries to import 1 and this continues forever. This is a cyclic dependency.

打破循环来修复它.

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