No provider for NavController! (MyApp -> NavController)

在使用ionic2编写hybrid应用时,在@App页面使用nav就可能会出现标题所示错误：

import {App, Platform, NavController, Loading} from 'ionic-angular';
import {StatusBar} from 'ionic-native';
import {TabsPage} from './pages/tabs/tabs';


@App({
  template: '',
  config: {} // http://ionicframework.com/docs/v2/api/config/Config/
})
export class MyApp {
  rootPage: any = TabsPage;
  constructor(platform: Platform, public nav:NavController) {
    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.
      StatusBar.styleDefault();
      this.presentLoadingDefault();
    });
  }
  presentLoadingDefault() {
  let loading = Loading.create({
    content: 'Please wait...'
  });

  this.nav.present(loading);

  setTimeout(() => {
    loading.dismiss();
  }, 5000);
  }
}

解决办法：
首先,在文件头中，import {ViewChild} from '@angular/core';
早期版本可能是import {ViewChild} from 'angular2/core.js';
第二,在template或templateUrl内的"ion-nav"标签中,新建局部变量，如#nav
第三,在类中创建成员变量：@ViewChild('nav') nav;
接下来就可以正常使用nav来调用一些功能了，比如：this.nav.present(loading)
全部代码如下：

import {App, Platform, NavController, Loading} from 'ionic-angular';
import {StatusBar} from 'ionic-native';
import {TabsPage} from './pages/tabs/tabs';
import {ViewChild} from '@angular/core';

@App({
  template: '',
  config: {} // http://ionicframework.com/docs/v2/api/config/Config/
})
export class MyApp {
  rootPage: any = TabsPage;
  @ViewChild('nav') nav;
  
  constructor(platform: Platform) {
    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.
      StatusBar.styleDefault();
      this.presentLoadingDefault();
    });
  }
  presentLoadingDefault() {
  let loading = Loading.create({
    content: 'Please wait...'
  });

  this.nav.present(loading);

  setTimeout(() => {
    loading.dismiss();
  }, 5000);
  }
}

参考https://github.com/driftyco/ionic/issues/5543