Back button in ionic app didn't work

Ionic Framework ionic-v3
1

In my ionic App, I used the double press back button(Hardware) to exit the app,
in my Homepage.

> constructor (public navCtrl: NavController,

               public navParams: NavParams,
               private platform: Platform,
............................
 ) {
    this.platform.registerBackButtonAction(() => {
      if (this.counter == 0) {
        this.counter++;
        this.pressAgainToast();
        setTimeout(() => {
          this.counter = 0
        }, 3000)
      } else {
        // console.log("exitapp");
        this.platform.exitApp();
      }
    }, 0);
}

when I open the app and press the back the button, it works properly.
if I goto another page and back to home, and I tried back button, nothing happens.

in another page, i used like this

constructor(public navCtrl: NavController,
              public navParams: NavParams,
              private platform: Platform,
              private view:ViewController) {
     this.platform.registerBackButtonAction(() => {
     this.navCtrl.pop()
     });

  }

How can I use it correctly? pls, help…