在最新版下如何实现5-2课程中的功能？根据id获取控件

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

import  { HomePage }     from '../home/home';

import  { AboutPage }    from '../about/about';

import  { ContactPage }  from '../contact/contact';

import  { IonicApp }     from 'ionic-angular';

@Component({

templateUrl: 'tabs.html'

})

export class TabsPage {

static get parameters(){

return [[IonicApp]];

}

// this tells the tabs component which Pages

// should be each tab's root Page

tab1Root: any = HomePage;

tab2Root: any = AboutPage;

tab3Root: any = ContactPage;

app     : any = IonicApp;

// app     : any = IonicApp;

constructor(app:IonicApp) {

debugger;

this.app = app;

}

ionViewDidEnter(){

let mainTabs = this.app.getComponent("mainTabs");

mainTabs.select(1);

}

}

代码如上所示，目前是报错说this.app没有getComponent方法，求正确写法和大致语法（文档链接即可）