Registers a listener callback to be executed whenever the watchExpression changes.

• The watchExpression is called on every call to $digest() and should return the value which will be watched. (Since $digest() reruns when it detects changes the watchExpression can execute multiple times per $digest() and should be idempotent.)
• The listener is called only when the value from the current watchExpression and the previous call to watchExpression are not equal (with the exception of the initial run, see below). The inequality is determined according to angular.equals function. To save the value of the object for later comparison, the angular.copy function is used. It also means that watching complex options will have adverse memory and performance implications.
• The watch listener may change the model, which may trigger other listener s to fire. This is achieved by rerunning the watchers until no changes are detected. The rerun iteration limit is 10 to prevent an infinite loop deadlock.

If you want to be notified whenever $digest is called, you can register a watchExpression function with no listener . (Since watchExpression can execute multiple times per $digest cycle when a change is detected, be prepared for multiple calls to your listener.)

After a watcher is registered with the scope, the listener fn is called asynchronously (via $evalAsync) to initialize the watcher. In rare cases, this is undesirable because the listener is called when the result of watchExpression didn't change. To detect this scenario within the listener fn, you can compare the newVal and oldVal . If these two values are identical ( === ) then the listener was called due to initialization.

Example

           // let's assume that scope was dependency injected as the $rootScope
           var scope = $rootScope;
           scope.name = 'misko';
           scope.counter = 0;

           expect(scope.counter).toEqual(0);
           scope.$watch('name', function(newValue, oldValue) { scope.counter = scope.counter + 1; });
           expect(scope.counter).toEqual(0);

           scope.$digest();
           // no variable change
           expect(scope.counter).toEqual(0);

           scope.name = 'adam';
           scope.$digest();
           expect(scope.counter).toEqual(1);

Scope#$watch(watchExpression[, listener][, objectEquality]);