The following list contains events that you can react on:

// Listening for changes of the position to a single element
element1.on('change:position', function(element1, position) {
  alert('element1 moved to ' + position.x + ',' + position.y);
});
// All elements events are also propagated to the graph.
graph.on('change:position', function(element, position) {
  console.log('Element ' + element.id + 'moved to ' + position.x + ',' + position.y);
});
// Using the option parameter and a custom attribute
graph.on('change:custom', function(element, custom, opt) {
  if (opt.consoleOutput) {
    console.log('Custom attribute value changed to "' + custom + '"');
  }
});
element2.prop('custom', 'myValue', { consoleOutput: true });