How to track event changes to a polygon with Google Maps API

I'm currently building a 'search an area' feature for a site we're working on. A bit like the one on Rightmove.

I've got everything up and running except the ability to track event changes to a polygon (the setting of new points and altering existing ones). I need to be able to post the coordinates to the form for submission.

I've tried the Google Code docs for editing events. And every time I try it out, I either get a message about 'set_at' not being possible or my object not being defined.

I suppose the bit I know is wrong is thePolygon variable not being passed through to the new

google.maps.event.addListener(thePolygon, 'set_at', function() {
      // grab paths for infoWindow 
      grabPaths(thePath);
    });

But I don't know why. It's a global variable. Or is not?

So, the question is, how can I track the changes to the polygon to pass the updated coordinates through to my form?

All help is greatly appreciated.

Here is the code I currently have:

var mapOptions = {
  // options
};

var map = new google.maps.Map(document.getElementById('map'), mapOptions);

var drawingManager = new google.maps.drawing.DrawingManager({
  drawingMode: google.maps.drawing.OverlayType.POLYGON,
  drawingControl: false,
  polygonOptions: {
    // drawing options
  }
});

drawingManager.setMap(map);
  
google.maps.event.addListener(drawingManager, 'polygoncomplete', function(polygon) {
  // complete functions
});

google.maps.event.addListener(thePolygon, 'set_at', function() {
  // complete functions
});

google.maps.event.addListener(thePolygon, 'insert_at', function() {
  // complete functions
});