本示例是ionic3.3,用了懒加载

• 在app.html中，添加#myNav，在app.component.ts文件通过@ViewChild('myNav')获取：
  <ion-nav #myNav [root]="rootPage"></ion-nav>

• 在app.component.ts中：获取myNav，注册返回事件监听

export class MyApp {
    rootPage: any;
    backButtonPressed: boolean = false; 

    @ViewChild('myNav') nav: Nav;

    constructor(public platform: Platform,
        statusBar: StatusBar,
        splashScreen: SplashScreen,
        public ionicApp: IonicApp,
        public toastCtrl: ToastController,
        public util: UtilService,
        public keyboard: Keyboard,
        public sqlHelp: SqlService) {
        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();
            splashScreen.hide();
            this.init();
            this.sqlHelp.createTable();
            this.registerBackButtonAction();
        });
    }
}

• 在tabs.html中，添加#mainTabs，在tabs.ts文件通过@ViewChild('mainTabs') tabs:Tabs获取:

<ion-tabs  #mainTabs>
  <ion-tab [root]="tab1Root" tabUrlPath="work" tabTitle="业务" tabIcon="apps"></ion-tab> 
  <ion-tab [root]="tab2Root" tabUrlPath="report" tabTitle="报表" tabIcon="list-box"></ion-tab>
  <ion-tab [root]="tab3Root" tabUrlPath="alarm" tabTitle="提醒" [tabBadge]="tabBadgeNum"
           tabIcon="alarm"></ion-tab>
  <ion-tab [root]="tab4Root" tabUrlPath="setting" tabTitle="设置" tabIcon="settings"></ion-tab>
</ion-tabs>

• 在tabs.ts中：

import {Component, ViewChild} from '@angular/core';
import {IonicPage,Tabs} from "ionic-angular";
import {UtilService} from '../../providers/util-service';
@IonicPage()
@Component({
    templateUrl: 'tabs.html'
})
export class TabsPage {
    tabBadgeNum:string;
    // this tells the tabs component which Pages
    // should be each tab's root Page
    @ViewChild('mainTabs') tabs: Tabs;
    tab1Root: any = 'WorkPage';
    tab2Root: any = 'ReportPage';
    tab3Root: any = 'AlarmPage';
    tab4Root: any = 'SettingPage';

    constructor( private utilService: UtilService) {
    }
}

• 在app.component.ts写事件：

    registerBackButtonAction() {
        this.platform.registerBackButtonAction(() => {
            if (this.keyboard.isOpen()) {
                //按下返回键时，先关闭键盘
                this.keyboard.close();
                return;
            };
            //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.gaetActive() || this.ionicApp._loadingPortal.getActive() || this.ionicApp._overlayPortal.getActive()
            //不写this.ionicApp._toastPortal.getActive()是因为连按2次退出
            let activePortal = this.ionicApp._modalPortal.getActive() ||this.ionicApp._overlayPortal.getActive();
            let loadingPortal = this.ionicApp._loadingPortal.getActive();
            if (activePortal) {
                //其他的关闭
                activePortal.dismiss().catch(() => {
                });
                activePortal.onDidDismiss(() => {
                });
                return;
            }
            if (loadingPortal) {
                //loading的话，返回键无效
                return;
            }
            let activeVC = this.nav.getActive();
            let page = activeVC.instance;

            if (page instanceof LoginPage || page instanceof WelcomePage) {
                this.platform.exitApp();
                return;
            }
            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: 'bottom',
                cssClass: 'text-align: center'
            }).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
        }
    }