To observe and react to changes in a value Angular.js has scope.$watch function. When you have to trigger events like callback etc on multiple objects using $watch you can use methods like $watchGroup and $watchCollection. Since Angular 1.3 Angular offers two scope methods $watchGroup and $watchCollection. An example of using $watchGroup is given below.

$scope.foo = 'foo';
$scope.bar = 'bar';

$scope.$watchGroup(['foo','bar'], function(newValues, oldValues, scope) {
  // newValues array contains the current values of the watch expressions
  // with the indexes matching those of the watchExpression array
  // i.e.
  // newValues[0] ->$scope.foo 
  // and 
  // newValues[1] ->$scope.bar 
});

$watchGroup method is a variant of $watch() where it watches array of watchExpressions and if any of those expressions in the collection changes the listener is executed. Through the standard $watch operation the items in the watchExpressions array are observed. Every call to $digest their return values are examined.

Example for watchCollection:

//angular 1.1.4
$scope.$watchCollection(['foo','bar'], function(newValues, oldValues){
    // do what you want here
});

$watchCollection shallow watches the properties of an object and fires whenever any of the property change. When the change is detected the listener callback is fired.