$watch(watchExpression, listener, [objectEquality]);
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 that will be watched. (watchExpression should not change its value when executed multiple times with the same input because it may be executed multiple times by $digest(). That is, watchExpression should be idempotent. The listener is called only when the value from the curren