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.
$scope.bar = 'bar';
// newValues array contains the current values of the watch expressions
// with the indexes matching those of the watchExpression array
// i.e.
// newValues[0] ->$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:
$scope.$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.
