Typescript error: @viewChild undefined

问题:

Tried using the select() method in tabs.ts from the Ionic Tabs documentation. But it seems that when I tried running it, it says that "select is undefined" and I found out that my viewChild is actually empty/undefined when I tried console.log(tabs). Tried searching for the reason why viewChild is undefined but could not really understand why.

Link to ionic tabs documentation: https://ionicframework.com/docs/api/components/tabs/Tabs/

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>

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);
    }
  }
}

回答1:

Just like you can see in Angular Docs,

ViewChild is set after the view has been initialized

and

ViewChild is updated after the view has been checked

export class AfterViewComponent implements  AfterViewChecked, AfterViewInit {

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

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

  // ...

}

So the issue on your code is that the view has not been initialized when the constructor is executed. You'd need to put all the code that interacts with the tabs in the ngAfterViewInit lifecycle hook:

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

If you just want to use Ionic custom lifecycle events, you'd need to use the ionViewDidEnter hook:

export class TabsPage {

@ViewChild('myTabs') tabRef: Tabs;

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

}