Error in Typescript: Cannot find reference to @viewChild

Question

Error in Typescript: Cannot find reference to @viewChild

I attempted to use the select() method in tabs.ts based on the Ionic Tabs documentation. However, upon running it, I encountered an error stating that "select is undefined". Upon further investigation, I realized that my viewChild was empty or undefined when I logged it using console.log(tabs). I tried to search for reasons why the viewChild is undefined but couldn't grasp the explanation.

For more information, you can refer to the Ionic Tabs documentation: https://ionicframework.com/docs/api/components/tabs/Tabs/

Here is a snippet from tabs.html:

<ion-tabs #tabs>
  <ion-tab [root]="tab1Root" tabTitle="Request" tabIcon="alert"></ion-tab>
  <ion-tab [root]="tab2Root" [rootParams]="detailParam" tabTitle="Pending" 
   tabIcon="repeat"></ion-tab>
  <ion-tab [root]="tab3Root" tabTitle="Completed" tabIcon="done-all"></ion-
   tab>
  <ion-tab [root]="tab4Root" tabTitle="Profile" tabIcon="person"></ion-tab>  
</ion-tabs>

And here is a snippet from tabs.ts:

import { Component, ViewChild } from '@angular/core';
import { NavController, NavParams, AlertController, Tabs } from 'ionic-
angular';
import { PendingJobPage } from '../pending-job/pending-job';
import { CompletedJobPage } from '../completed-job/completed-job';
import { RequestPage } from '../request/request';
import { ProfilePage } from '../profile/profile';

@Component({
  templateUrl: 'tabs.html'
})
export class TabsPage {

  @ViewChild('tabs') tabRef: Tabs;
  pending: any;
  apply: boolean;
  detailsParam: any;

  tab1Root = RequestPage;
  tab2Root = PendingJobPage;
  tab3Root = CompletedJobPage;
  tab4Root = ProfilePage;

  constructor(public navParams: NavParams, public navCtrl: NavController) {
    this.pending = this.navParams.get('param1');
    this.apply = this.navParams.get('apply');
    this.detailsParam = this.navParams.data;
    console.log("a = ", this.tabRef);

    if(this.apply === true){
      this.navCtrl.parent.select(1);
    }
    else{
      this.navCtrl.parent.select(0);
    }
  }
}

angular typescript ionic2 ionic3

Answer 1

Answer №1

As mentioned in the Angular Docs,

The ViewChild property is initialized after the view has been set up.

and

The ViewChild property is updated after the view has been checked.

export class AfterViewComponent implements  AfterViewChecked, AfterViewInit {

  ngAfterViewInit() {
    // The ViewChild property is set after the view has been initialized <- Here!
  }

  ngAfterViewChecked() {
    // The ViewChild property is updated after the view has been checked <- Here!
  }

  // ...

}

The issue with your code is that it tries to interact before the view is fully initialized during the constructor execution. It would be best to move the code that interacts with the tabs into the ngAfterViewInit lifecycle hook:

  ngAfterViewInit() {
    // Now you can access the tabs reference
    console.log("a = ", this.tabRef);
  }

If you prefer to use Ionic custom lifecycle events, consider using the ionViewDidEnter hook:

export class TabsPage {

@ViewChild('myTabs') tabRef: Tabs;

ionViewDidEnter() {
    // Now you can utilize the tabs reference
    console.log("a = ", this.tabRef);
 }

}

Answer 2

As mentioned in the Angular Docs,

The ViewChild property is initialized after the view has been set up.

and

The ViewChild property is updated after the view has been checked.

export class AfterViewComponent implements  AfterViewChecked, AfterViewInit {

  ngAfterViewInit() {
    // The ViewChild property is set after the view has been initialized <- Here!
  }

  ngAfterViewChecked() {
    // The ViewChild property is updated after the view has been checked <- Here!
  }

  // ...

}

The issue with your code is that it tries to interact before the view is fully initialized during the constructor execution. It would be best to move the code that interacts with the tabs into the ngAfterViewInit lifecycle hook:

  ngAfterViewInit() {
    // Now you can access the tabs reference
    console.log("a = ", this.tabRef);
  }

If you prefer to use Ionic custom lifecycle events, consider using the ionViewDidEnter hook:

export class TabsPage {

@ViewChild('myTabs') tabRef: Tabs;

ionViewDidEnter() {
    // Now you can utilize the tabs reference
    console.log("a = ", this.tabRef);
 }

}

Error in Typescript: Cannot find reference to @viewChild

Answer №1

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