Ionic 4 alternative for platform.registerBackButtonAction

Update: This was fixed in v4.0.0-beta.8 (dfac9dc)

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Related: how to integrate hardware back button into ionic4 navigation

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This is tracked on GitHub, in the Ionic Forums and Twitter
Until there is an official fix, you can use this workaround:

this.platform.backButton.subscribe(() => {
  // code that is executed when the user pressed the back button
})

// To prevent interference with ionic's own backbutton handling
// you can subscribe with a low priority instead
this.platform.backButton.subscribeWithPriority(0, () => {
  // code that is executed when the user pressed the back button
  // and ionic doesn't already know what to do (close modals etc...)
})

^{Be aware that you need to save the result of subscribe(...) if you ever want to unsubscribe from it again.}

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Old answer: (out of date as of April 2018)

registerBackButtonAction is just a wrapper for the corresponding Cordova call.

So you can just take your old call to registerBackButtonAction:

this.platform.registerBackButtonAction(() => { 
  // code that is executed when the user pressed the back button
});

and replace it with:

this.platform.ready().then(() => {
  document.addEventListener("backbutton", () => { 
    // code that is executed when the user pressed the back button
  });
});