CODEWITHSUNDEEP
androidangularionic2
hydrococcus
hydrococcus

Reputation: 487

Ionic 2 - NavController canGoBack() allways undefined (Android)

This is my Function:

checkStack(){
    let canGoBack = this.navCtrl.canGoBack()
    console.log('canGoBack: ',canGoBack);

    if(canGoBack === true){
        console.log('Not RootPage.');
    } else {

        let alert = this.alertCtrl.create({
            title: 'Exit',
            message: 'sure?',
            buttons: [
                {
                    text: 'Yes',
                    role: 'cancel',
                    handler: () => {
                        console.log('Cancel clicked');
                    }
                },
                {
                    text: 'No',
                    handler: () => {
                        this.platform.exitApp();
                    }
                }
            ]
        });

        alert.present();
    }
}

i am call in IonViewDidLoad():

this.platform.registerBackButtonAction(this.checkStack);

But if i click the BackButton it fails with Message:

"Uncaught TypeError: Cannot read property 'canGoBack' of undefined"

Whats wrong?

Upvotes: 2

Views: 3435

Answers (3)

SimPHP
SimPHP

Reputation: 65

I had the same issue, and all I needed to solve it was to remove the parenthesis.

Instead of:

this.navCtrl.canGoBack()

Use: 

this.navCtrl.canGoBack

Upvotes: -1

hydrococcus
hydrococcus

Reputation: 487

Here is my Solution:

constructor(public platform: Platform, public app: App){
   initializeApp()
}

this.platform.registerBackButtonAction(() => {

            let activeNav = this.app.getActiveNav();

            if(activeNav.canGoBack()){

                // not Root

            } else {

                // is Root

        }

I am defined this in:

initializeApp(){
   this.platform.ready().then(() => {...})
}

and call in Constructor. Works very well ;-)

Upvotes: 4

nickscheynen
nickscheynen

Reputation: 333

The error you receive indicates that your navCtrl variable is undefined. Make sure you instantiate it in the constructor of your page. Like this:

constructor(
    public navCtrl:NavController
) {
    // constructor logic goes here
}

Note that including public is necessary for the variable to be assigned and available (not be undefined) outside of the constructor method.

Upvotes: 1

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