Managing the Android back button

How to enable double pressing on the Android physical button to dismiss and kill the app

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var frameModule = require("ui/frame");
var application = require("application")

var activity = application.android.startActivity ||
        application.android.foregroundActivity ||
        frameModule.topmost().android.currentActivity ||
        frameModule.topmost().android.activity;
var lastPress;

activity.onBackPressed = function() {
    var timeDelay = 500
    if (lastPress + timeDelay > java.lang.System.currentTimeMillis()) {
        var startMain = new android.content.Intent(android.content.Intent.ACTION_MAIN);
        startMain.addCategory(android.content.Intent.CATEGORY_HOME);
        startMain.setFlags(android.content.Intent.FLAG_ACTIVITY_NEW_TASK);
        activity.startActivity(startMain);

        // If you want to kill the app totally, use these codes instead of above
        // activity.finish();
        // java.lang.System.exit(0);

    } else {
        frameModule.topmost().goBack();
    }
    lastPress = java.lang.System.currentTimeMillis();
}