APP_INITIALIZER in Angular

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APP_INITIALIZER in Angular

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What is an APP_INITIALIZER?

The APP_INITIALIZER is a built in injection token available in angular. The purpose of APP_INITIALIZER is to perform any initialization task when the application loads. Mostly the APP_INITIALIZER will be used to get the application's configuration details before the application is rendered by called the api. The best way to use it is by returning a promise function, so that it will wait until any operation is done and the promise is resolved. Once the promise is resolved, then the application will be initialized.

Let's see it with an example program.

Create a new angular application using angular cli command.

ng new angular-tutorial

Create a service to hold the configuration details. Lets create the service in the services folder by the following angular cli command.

ng g s services/config

Open the config.service.ts file and modify the code as below.

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import { Injectable } from '@angular/core';

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​

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@Injectable({

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  providedIn: 'root'

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})

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​

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interface IConfig {

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  serverUrl: string;

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}

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export class ConfigService {

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​

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​

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  private config: IConfig = { serverUrl:  };

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  constructor() { }

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​

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  init(serverUrl: string) {

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    this.config.serverUrl = serverUrl;

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    console.log('Configuration has been set.');

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  }

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​

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  getConfig() {

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    return this.config;

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  }

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}

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​

We have created a variable config which will be holding the property serverUrl. We have a function init to set the serverUrl property and once the property has been set, then it will log 'Configuration has been set.' to the console.

We also have another function, getConfig to return the configuration in the config variable.

Open the app.module.ts file and modify the code as below.

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​

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import { BrowserModule } from '@angular/platform-browser';

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import { HttpClientModule, HttpClient } from '@angular/common/http';

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import { NgModule } from '@angular/core';

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import { AppRoutingModule } from './app-routing-module';

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import { AppComponent } from './app.component';

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import { APP_INITIALIZER } from '@angular/core';

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import { ConfigService } from './services/config.service';

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​

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const initializeConfig = (configService: ConfigService, http: HttpClient) => {

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  return (): Promise<any> => {

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    return new Promise((resolve, reject) => {

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      http.get('https://reqres.in/api/users/2').toPromise()

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        .then((result: any) => {

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          console.log(result);

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          configService.init(result.ad.url);

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          return resolve('Success');

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        });

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    });

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  }

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};

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​

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@NgModule({

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  declarations: [

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    AppComponent,

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  ],

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  imports: [

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    AppRoutingModule,

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    HttpClientModule,

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  ],

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  providers: [ConfigService,

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    { provide: APP_INITIALIZER, useFactory: initializeConfig, deps: [ConfigService, HttpClient], multi: true }

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  ],

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  bootstrap: [AppComponent]

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})

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export class AppModule { }

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​

In the above code we have imported the APP_INITIALIZER and the ConfigService.

In order to use the APP_INITIALIZER, we have to use the syntax as below.

{ provide: APP_INITIALIZER, useFactory: function, deps: [], multi: true/false }

We have to supply the APP_INITIALIZER as the value to the provide property. For useFactory property we will be supplying function as its value. The function can perform synchronous or asynchronous function. The depsproperty is an array that will hold the dependencies needed for the function and the multi parameter can be true or false. By default it is false. If it is set to false then we use a token more than once, the last to register will override all the previous tokens. i.e you can have only one provider for token. If it is true, then the new providers are added to the previously registered providers making it more than one provider for a token. The angular will execute all of them when the token in invoked.

In our code, we have provided intializeConfig function as the value to useFactory parameter and provided ConfigService and HttpClient as the dependencies in deps array and set the multi parameter to be 'true'.

Now lets take a look at the initializeConfig function

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const initializeConfig = (configService: ConfigService, http: HttpClient) => {

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  return (): Promise<any> => {

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    return new Promise((resolve, reject) => {

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      http.get('https://reqres.in/api/users/2').toPromise()

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        .then((result: any) => {

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          console.log(result);

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          configService.init(result.ad.url);

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          return resolve('Success');

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        });

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    });

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  }

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};

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​

The initializeConfig function takes ConfigService and HttpClient as parameter in configService and http variable name respectively. This function is going to return a function as result and that function is going to return a Promise. Inside the function we make api call using http variable and chain it to return it as promise using toPromise. Once it makes the api call now it returns a promise. To retrieve the value from it we use thenfunction and get the result from it. Then we set a url in it by calling the initfunction available in ConfigService using configService variable and return a resolve with a 'Success' string message.

Now, open the app.component.ts file and inside its constructor add some console to verify that the APP_INITIALIZER has been loaded initially.

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​

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

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​

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@Component({

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  selector: 'app-root',

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  templateUrl: './app.component.html',

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  styleUrls: ['./app.component.css'],

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})

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export class AppComponent implements OnInit {

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​

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  constructor() {

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    console.log('App component loaded');

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  }

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  ngOnInit(): void {

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  }

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}

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​

Now run your application using

ng serve --o

Press F12 and check out the console. It will make the api to log the result to the console. It displays logs as below

Configuration has been set.

App component loaded

This is the way we have to use the APP_INITIALIZER.

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