Android : How to show confirm alert when Android Back button is pressed?

I want to implement an alert on Webview page, when back button is pressed on Android. What code shall I use?

I Wrote below code in view1.js, but on pressing back button on Android alert is not being shown, it simply goes to the home screen:

$.webview1.url=Alloy.Globals.baseurl;
if(Ti.Platform.osname=='android'){
    $.webview1.enableZoomControls=false;
}

$.webview1.addEventListener('androidback',function(e){
    // show alert
    alert("Do you want to exit?");
    dialog.show();
});

But its not working. Any solutions?