ionic2如何处理android硬件返回按钮

 import {Component, ViewChild} from '@angular/core';
 import {Platform, ToastController, Nav, IonicApp} from 'ionic-angular';
 import {StatusBar, Splashscreen} from 'ionic-native';
 import {TabsPage} from '../pages/tabs/tabs';

 @Component({
  templateUrl: 'app.html'
 })
 export class MyApp {
  rootPage = TabsPage;
  backButtonPressed: boolean = false; //用于判断返回键是否触发
  @ViewChild('myNav') nav: Nav;

  constructor(public ionicApp: IonicApp, public platform: Platform, public toastCtrl: ToastController) {
   platform.ready().then(() => {
    StatusBar.styleDefault();
    Splashscreen.hide();
    this.registerBackButtonAction();//注册返回按键事件
   });
  }

  registerBackButtonAction() {
   this.platform.registerBackButtonAction(() => {
    //如果想点击返回按钮隐藏toast或loading或Overlay就把下面加上
    // this.ionicApp._toastPortal.getActive() || this.ionicApp._loadingPortal.getActive() || this.ionicApp._overlayPortal.getActive()
    let activePortal = this.ionicApp._modalPortal.getActive();
    if (activePortal) {
     activePortal.dismiss().catch(() => {});
     activePortal.onDidDismiss(() => {});
     return;
    }
    let activeVC = this.nav.getActive();
    let tabs = activeVC.instance.tabs;
    let activeNav = tabs.getSelected();
    return activeNav.canGoBack() ? activeNav.pop() : this.showExit();//另外两种方法在这里将this.showExit()改为其他两种的方法的逻辑就好。
   }, 1);
  }

  //双击退出提示框
  showExit() {
   if (this.backButtonPressed) { //当触发标志为true时，即2秒内双击返回按键则退出APP
    this.platform.exitApp();
   } else {
    this.toastCtrl.create({
     message: '再按一次退出应用',
     duration: 2000,
     position: 'top'
    }).present();
    this.backButtonPressed = true;
    setTimeout(() => this.backButtonPressed = false, 2000);//2秒内没有再次点击返回则将触发标志标记为false
   }
  }
}

 <ion-tabs #mainTabs>
  <ion-tab [root]="tab1Root" tabTitle="Home" tabIcon="home"></ion-tab>
  <ion-tab [root]="tab2Root" tabTitle="About" tabIcon="information-circle"></ion-tab>
  <ion-tab [root]="tab3Root" tabTitle="Contact" tabIcon="contacts"></ion-tab>
 </ion-tabs>

 import {Component, ViewChild} from '@angular/core';
 import { HomePage } from '../home/home';
 import { AboutPage } from '../about/about';
 import { ContactPage } from '../contact/contact';
 import {Tabs} from "ionic-angular";

 @Component({
  templateUrl: 'tabs.html'
 })
 export class TabsPage {
  @ViewChild('mainTabs') tabs:Tabs;//加这句以及引用两个模块
  tab1Root: any = HomePage;
  tab2Root: any = AboutPage;
  tab3Root: any = ContactPage;

  constructor() {
  }
 }