Page onNavigatingFrom should allow for canceling

[jkappel71]

Say you have a data entry form that a user has filled out, but they have not filled out all required fields. Pressing the back button (on android) causes the app to go back, no way to cancel it at all. If the form is not properly filled out, you can't save the data so it's just lost.

We need to be able to respond to the onNavigatingFrom event in some manner that will cause the navigation event to be cancelled.

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[stonecourier]

Was just about to raise this as an issue also. I would like to see this implemented.

[sitefinitysteve]

+1

[spierala]

I think on Android there are (in the meantime) some good ways to cancel back navigation:

• Use of activityBackPressed: https://docs.nativescript.org/core-concepts/application-lifecycle
• Use of NavigationButton and overwrite the tap function: https://docs.nativescript.org/ui/action-bar

On iOS as far as I know the only solution is to hide the BackButton (NavigationButton) with visibility="collapsed" and mimic the Back Button by adding a custom ActionItem which can have an icon and tap action of your choice.
The tap action can then cancel or allow the navigation based on some condition.
You can navigate programatically by using this.routerExtensions.backToPreviousPage() (also see: https://docs.nativescript.org/core-concepts/angular-navigation).
The biggest challenge is then to style the custom back button the same way as the default back button. E.g. I am still on the search for the back button icon on iOS :)