Can one service be injected into another service and vice versa simultaneously?

I encountered a challenging scenario in an actual project where I found the need for two services to access each other's properties and methods. Despite my limited expertise in Angular, I wondered if this is even possible?

I made an attempt, but unfortunately, it didn't succeed. Here's what I tried:

app.component.ts

import { Component } from '@angular/core';
import { FirstService } from './first.service';
import { SecondService } from './second.service';

@Component({
  selector: 'my-app',
  template: '<h1>Hello world!</h1>',
  providers: [FirstService, SecondService]
})
export class AppComponent {

  constructor(public firstService: FirstService, public secondService: SecondService) {
    console.log(firstService.foo);
    console.log(secondService.bar);
  }

}

first.service.ts

import { Injectable } from '@angular/core';
import { SecondService } from './second.service';

@Injectable()
export class FirstService {

  foo: string = 'abc';

  constructor(public secondService: SecondService) {
    this.foo = this.foo + this.secondService.bar;
  }

}

second.service.ts

import { Injectable } from '@angular/core';
import { FirstService } from './first.service';

@Injectable()
export class SecondService {

  bar: string = 'xyz';

  constructor(public firstService: FirstService) {
    this.bar = this.bar + this.firstService.foo;
  }

}

Plunker: http://plnkr.co/edit/PQ7Uw1WHpvzPRf6yyLFd?p=preview

While simply injecting the second service into the first service works fine, incorporating the first service into the second service results in errors being thrown to the console.

What could possibly be the issue here?

An effective solution would display the following log in the console:

abcxyz
xyzabc

Your insights are greatly appreciated!

Answer №1

AngularJS does not allow for circular dependencies to be injected.

According to Miško Hevery, a co-author of AngularJS, it is recommended to identify common elements:

+---------+      +---------+
|    A    |<-----|  B      |
|         |      |  |  +-+ |
|         |      |  +->|C| |
|         |------+---->| | |
|         |      |     +-+ |
+---------+      +---------+

These common elements can be extracted into a third service:

                         +---------+
+---------+              |    B    |
|    A    |<-------------|         |
|         |              |         |
|         |    +---+     |         |
|         |--->| C |<----|         |
|         |    +---+     +---------+
+---------+

For more details, visit Miško Hevery's article on Circular Dependency in Constructors and Dependency Injection.

Answer №2

I don't claim to be an expert in Angular, so is it feasible?

Unfortunately, Angular's DI does not resolve circular dependencies.

Even systems that claim to support it can be inconsistent, such as commonjs https://nodejs.org/docs/latest/api/modules.html#modules_cycles, which may result in an empty object.

Solution

One possible solution is to combine the two services into a single service. You can always move certain elements (like simple functions) out if the combined service becomes too complex.

Answer №3

My opinion aligns with the proposed solution by basarat. Another approach could involve initializing the instances outside of Dependency Injection (DI) and passing them as values:

In order to allow one service to create an instance without requiring the other service as a dependency, some modifications need to be made:

@Injectable()
export class FirstService {

  foo: string = 'abc';
  secondService: SecondService

  constructor() {
    //this.foo = this.foo + this.secondService.bar;
  }

  init(secondService:SecondService) {
    this.foo = this.foo + secondService.bar;
  }
}

Subsequently, the instances should be created imperatively and passed as values:

let firstService = new FirstService();
let secondService = new SecondService(firstService);

@Component({
  selector: 'my-app',
  template: '<h1>Hello world!</h1>',
  providers: [
    provide(FirstService, {useFactory: () => {
      firstService.init(secondService);
      return firstService;
  }}), provide(SecondService, {useValue: secondService})]
})
...

Plunker example

Similar questions

If you have not found the answer to your question or you are interested in this topic, then look at other similar questions below or use the search

ngx-datatable: Exploring the Differences Between PageSize and Limit

Currently in the process of incorporating ng-datatable into my Angular 4 application. I am uncertain about the intended purpose of [limit] and its relationship with pageSize (which is triggered as part of the (page) event). It appears that pageSize depen ...

Turn off Angular form validation for individual input field (URL)

I am in the process of creating an Angular form that includes a URL field which users can input with or without the http:// prefix. The default URL validation in Angular requires the prefix, so I have incorporated a regex pattern that allows for both optio ...

Issue with Angular 4: Mega menu does not automatically close when a menu item is selected from within it

I am currently working on an Angular 4 project that includes a mega menu. My issue is that when I click on a menu within the mega menu, I want it to close. However, in my current setup, the menu always remains open even after clicking on a specific option. ...

Is there a specific instance where it would be more appropriate to utilize the styled API for styling as opposed to the sx prop in Material-

I am currently in the process of migrating an existing codebase to Material UI and am working towards establishing a styling standard for our components moving forward. In a previous project, all components were styled using the sx prop without encounteri ...

Transforming a typical JSON file into a parent-child hierarchical JSON structure similar to the one utilized in d3's flare.json file format

My JSON file has a specific structure: { "a": "b", "c": "d", "e": { "f": "g", "h": "i" } } I want to transform it into the following structure: { "name": "Root", "parent": "null", "children": [ { ...

Connecting to a Postgres database with Typescript using Docker

Incorporating typeorm into my node project has presented some challenges. Initially, I set up the database using a docker container. However, upon stopping and restarting the container, the IP address kept changing. This led me to consider using the contai ...

What is the method for dynamically assigning a name to ngModel?

I have the following code snippet: vmarray[]={'Code','Name','Place','City'} export class VMDetail { lstrData1:string; lstrData2:string; lstrData3:string; lstrData4:string; lstrData5:string; ...

Angular 5: Utilizing distinct router-outlets in separate modules for optimized lazy loading experience

Having two modules with routing module files and router-outlets in their html, I aim to load components based on the current module. The file tree structure is as follows: project |-- module2 |-- -- profil |-- -- - profil.component.html |-- -- - profil. ...

Is there a way to transfer the value of a selected option from an HTML dropdown box within a form to a TypeScript file in Angular without needing to submit the form?

Just starting out with Angular and I have a dropdown box within a form that I'm trying to work with. Specifically, I want to pass the value of 'serial' from my HTML file to my ts file without having to submit the form. So far, I haven't ...

Angular controller handling a resolve operation

I have a scenario where I need to load ngResources in my global/main controller and pause the execution of anything else until they are loaded, similar to how the resolve property works for routes. How can I achieve this? // Implementing one-time action ...

AngularJS anticipates the completion of a lone asynchronous request

Attempting to assign data to the scope after an asynchronous call. There is an array named companies in the factory. factory.getByCategoryId = function (id) { $http.get('http://localhost/campaign?access-token=12345&id=2').then( func ...

Could someone guide me through the Angular code?

webApp.factory('userAPIService', ['$resource', function ($resource) { return $resource( "/api/reportconfig/:Id", {Id: "@Id" }, { // This section allows for changing methods from GET to POST and ...

Implementing dynamic form fields in Angular 2 to efficiently store user input in a database

Currently, I am involved in a project using Angular 2. The task is to include fields with data from the database (specifically rows with the field value 'test8'). If users want to add new fields and values, they need to click the "Add new row" bu ...

Higher-Order Component integrated with HTMLElement

Check out this complex code snippet I created: export type AdvancedHoverElementProps<TElement extends HTMLElement> = React.HTMLProps<TElement> & { hoverDuration: number, onHoverChanged: (isHovering: boolean) => void }; export ...

Validating React Typescript Props: Ensuring that two specific props do not exist simultaneously

Currently, I'm developing a reusable component in React-Typescript and I am looking to validate my props OnClick and component as follows: Both onClick and component prop are optional. These props will only be passed to the component if they need to ...

What is the best way to place a dragged item on top of a dropped item?

I'm attempting to transfer data between two kendo grids using HTML5 drag and drop events. However, when I use ev.target.appendChild(document.getElementById(data));, I receive an error stating that data is not a node type. My goal is for the dropped da ...

What is the method for displaying all items in Angular Material's auto-complete component, even after a selection has been

By default, Angular Material only shows one item from the auto-completion list when selected, hiding the rest. For example, upon clicking the control, it appears like this: https://i.sstatic.net/U8awO.jpg After selecting an item, the remaining list is no ...

Is it possible for me to choose to establish a new scope within a directive?

Encountered an issue while reusing a directive where some tags had a second directive with a new scope created using new or {}, while others did not have one. Attempting to create a new scope when one already existed resulted in an error being thrown by An ...

Toggle visibility of an Angular 4 component based on the current route

Hello there, I'm facing an issue and not sure if it's possible to resolve. Essentially, I am looking to display a component only when the route matches a certain condition, and hide another component when the route matches a different condition. ...

Tips for deleting a localstorage value in AngularJS

I have implemented the [ngStorage][1] module in my AngularJS project. Here is the snippet of code that I am currently using: $localStorage.$reset(); This particular piece of code seems to be removing all variables from the local storage. ...