What is the reason Nav cannot be directly used in the constructor instead of using @ViewChild(Nav) nav: Nav in Ionic?

As a newcomer to Ionic, I am trying to understand the code snippet provided. My confusion lies in the purpose of "NAV" and why it is used in @viewchild(). While I can access the MenuController by using it in the constructor, I am unable to use NAV in the same manner.

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

import { Platform, MenuController, Nav } from 'ionic-angular';

import { HelloIonicPage } from '../pages/hello-ionic/hello-ionic';
import { ListPage } from '../pages/list/list';

import { StatusBar } from '@ionic-native/status-bar';
import { SplashScreen } from '@ionic-native/splash-screen';


@Component({
  templateUrl: 'app.html'
})
export class MyApp {
  @ViewChild(Nav) nav: Nav;

  // make HelloIonicPage the root (or first) page
  rootPage = HelloIonicPage;
  pages: Array<{title: string, component: any}>;

  constructor(
    public platform: Platform,
    public menu: MenuController,
    public statusBar: StatusBar,
    public splashScreen: SplashScreen
  ) {
    this.initializeApp();

    // set our app's pages
    this.pages = [
      { title: 'Hello Ionic', component: HelloIonicPage },
      { title: 'My First List', component: ListPage }
    ];
  }

  initializeApp() {
    this.platform.ready().then(() => {
      // Okay, so the platform is ready and our plugins are available.
      // Here you can do any higher level native things you might need.
      this.statusBar.styleDefault();
      this.splashScreen.hide();
    });
  }

  openPage(page) {
    // close the menu when clicking a link from the menu
    this.menu.close();
    // navigate to the new page if it is not the current page
    this.nav.setRoot(page.component);
  }
}

Answer №1

When it comes to navigation controllers in Angular, components that act as navigation controllers are typically children of the root component. This means they cannot be directly injected.

One workaround for this limitation is to add a reference variable to the ion-nav element. By using @ViewChild, you can obtain an instance of the Nav component, which serves as a navigation controller (as it extends NavController).

Update:

The use of @ViewChild is especially handy when we need to interact with child components and utilize their methods or access instance variables. While you can access the base class NavController through the constructor, components like Nav and Tab extend NavController. This means you cannot directly access them in the constructor; however, by leveraging @ViewChild, you can successfully access these child components.

Answer №2

Looking for the solution? Check it out here (v3)

Navigating from the Root component

If you need to manage navigation from your main app component, injecting NavController won't work since components that act as navigation controllers are nested within the root component and cannot be injected.

However, you can access the Nav component by adding a reference variable to ion-nav and then utilizing @ViewChild to obtain an instance of Nav, which acts as a navigation controller:

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