location.js

// http://earth-api-samples.googlecode.com/svn/trunk/demos/geolocation/location.js

/**
 * Watches the user's current location, continually calling the successCallback
 * with a Gears Coords-like object (latitude, longitude, accuracy) or
 * errorCallback with an error object. The second argument to successCallback
 * will be the location provider, i.e. 'gears', 'html5', or 'ip'.
 */
function watchLocation(successCallback, errorCallback, ipFallback) {
  successCallback = successCallback || function(){};
  errorCallback = errorCallback || function(){};

  if (typeof ipFallback == 'undefined')
    ipFallback = true;

  // Set up IP-based fallback
  var tryIPLocation = function(passthruError) {
    var error = passthruError || new Error('Location could not be determined.');

    if (ipFallback) {
      // Try IP-based geolocation.
      if (google.loader.ClientLocation) {
        successCallback({
          latitude: google.loader.ClientLocation.latitude,
          longitude: google.loader.ClientLocation.longitude,
          accuracy: -1
        }, 'ip');
      } else {
        errorCallback(error);
      }
    } else {
      errorCallback(error);
    }
  };

  var locationProvider = 'html5';
  
  // Try HTML5-spec geolocation.
  var geolocation = navigator.geolocation;

  // Try Gears geolocation.
  if (!geolocation && window.google && google.gears) {
    geolocation = google.gears.factory.create('beta.geolocation');
    locationProvider = 'gears';
  }

  if (geolocation) {
    // We have a real geolocation service.
    try {
      function handleSuccess(position) {
        successCallback(position.coords, locationProvider);
      }
      
      geolocation.watchPosition(handleSuccess, errorCallback, {
        enableHighAccuracy: true,
        maximumAge: 5000 // 5 sec.
      });
    } catch (err) {
      tryIPLocation(err);  // Pass-through error.
    }
  } else {
    tryIPLocation();
  }
}