data being released from variables in angular ionic

Question

data being released from variables in angular ionic

I am truly perplexed as to why the variables aren't holding their values.
I've declared a variable user and initialized it with data from the userData() function. However, it appears as undefined when I log it within the constructor as well as in the ProfilePage.

https://i.sstatic.net/L1w17.png

share-service.ts

export class ShareService {
  uid: string;
  user;

  constructor(public db: AngularFireDatabase) {
    let currentUser = firebase.auth().currentUser;
    this.uid = currentUser.uid;
    this.userData();
    console.log('[constructor]user: ', this.user);
  }

  userData() {
    this.db.object(`users/${this.uid}`).snapshotChanges().subscribe(data => {
      this.user = data.payload.val();
      console.log('[userData]user: ', this.user);
    });
  }

  getUser() {
    return this.user;
  }

}

profile.ts

export class ProfilePage {
  user;

  constructor(public navCtrl: NavController, public firebaseService: FirebaseService, shareService: ShareService) {
    console.log('[profile.ts]', shareService.getUser());
  }

}

"angularfire2": "5.0.0-rc.4",
"firebase": "4.8.2",
"ionic-angular": "3.9.2",
"rxjs": "^5.5.6",
"@ionic/app-scripts": "3.1.8",
"typescript": "2.4.2"

I have attempted to find solutions by researching similar issues such as this one. It seems that the problem lies in Angular's asynchronous nature, leading me to explore concepts like promises as suggested in posts like this. There was also mention of using .then().

Despite my efforts to implement these suggestions in my specific case, I encountered various syntax errors.

Your guidance would be greatly appreciated.

javascript angular typescript asynchronous ionic-framework

Answer 1

Answer №1

Angular offers the Async Pipe specifically designed for handling asynchronous methods.

User information is retrieved from the Firebase server, and this process takes some time. Meanwhile, JavaScript continues its execution.

This explains why you may receive this.user as undefined, as the data has not arrived from the server at that point of execution.

In order to manage such scenarios, there are different approaches available:

Callback -- Convert callbacks to promises (callback to promises)
Promises -- Learn about promises
Async-Await -- Explore how async-await works
Observables -- Especially in Angular applications, consider using observables

Here's an example implementation:

share-service.ts

export class ShareService {
 uid: string;

 constructor(public db: AngularFireDatabase) {
 let currentUser = firebase.auth().currentUser;
 this.uid = currentUser.uid;
 }

 getUser() {
   return this.db.object(`users/${this.uid}`)
     .snapshotChanges()
     .map(data => data.payload.val());
 }
}

profile.ts

export class ProfilePage {
 user;

 constructor(public navCtrl: NavController, public firebaseService: 
  FirebaseService, shareService: ShareService) {
  this.user = shareService.getUser();
 }
}

Template Usage

<div *ngIf="user | async as _user; else loadingUsers">
  {{_user.username}}
</div>
<ng-template #loadingUsers>
  Loading...
</ng-template>

Answer 2

Angular offers the Async Pipe specifically designed for handling asynchronous methods.

User information is retrieved from the Firebase server, and this process takes some time. Meanwhile, JavaScript continues its execution.

This explains why you may receive this.user as undefined, as the data has not arrived from the server at that point of execution.

In order to manage such scenarios, there are different approaches available:

Callback -- Convert callbacks to promises (callback to promises)
Promises -- Learn about promises
Async-Await -- Explore how async-await works
Observables -- Especially in Angular applications, consider using observables

Here's an example implementation:

share-service.ts

export class ShareService {
 uid: string;

 constructor(public db: AngularFireDatabase) {
 let currentUser = firebase.auth().currentUser;
 this.uid = currentUser.uid;
 }

 getUser() {
   return this.db.object(`users/${this.uid}`)
     .snapshotChanges()
     .map(data => data.payload.val());
 }
}

profile.ts

export class ProfilePage {
 user;

 constructor(public navCtrl: NavController, public firebaseService: 
  FirebaseService, shareService: ShareService) {
  this.user = shareService.getUser();
 }
}

Template Usage

<div *ngIf="user | async as _user; else loadingUsers">
  {{_user.username}}
</div>
<ng-template #loadingUsers>
  Loading...
</ng-template>

Answer 3

Answer №2

Below is the breakdown of how to interpret the code snippet:

 userData() {
    return this.db.object(`users/${this.uid}`).snapshotChanges()
    .pipe(map(data => {

      this.user = data.payload.val();
      return this.user;
      console.log('[userData]user: ', this.user);
    }));//import { map } from 'rxjs/operators/map';

}

To access it anywhere, utilize subscribe() on getUser().

  getUser(){
    if(!this.user){
        return this.userData();//return the db call if not set
    }else{
    return of(this.user);//import {of} from 'rxjs/Observable/of;'
  }

In the constructor method,

  constructor(public db: AngularFireDatabase) {
    let currentUser = firebase.auth().currentUser;
    this.uid = currentUser.uid;
    this.getUser().subscribe(u=>{
        console.log('[constructor]user: ', u);
    });//subscribe to the observable.
  }

Answer 4

Below is the breakdown of how to interpret the code snippet:

 userData() {
    return this.db.object(`users/${this.uid}`).snapshotChanges()
    .pipe(map(data => {

      this.user = data.payload.val();
      return this.user;
      console.log('[userData]user: ', this.user);
    }));//import { map } from 'rxjs/operators/map';

}

To access it anywhere, utilize subscribe() on getUser().

  getUser(){
    if(!this.user){
        return this.userData();//return the db call if not set
    }else{
    return of(this.user);//import {of} from 'rxjs/Observable/of;'
  }

In the constructor method,

  constructor(public db: AngularFireDatabase) {
    let currentUser = firebase.auth().currentUser;
    this.uid = currentUser.uid;
    this.getUser().subscribe(u=>{
        console.log('[constructor]user: ', u);
    });//subscribe to the observable.
  }

data being released from variables in angular ionic

Answer №1

Answer №2

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