AngularJS資料繫結中資料監控的機制說明

from : http://docs.angularjs.org/guide/scope

When the browser calls into JavaScript the code executes outside the Angular execution context, which means that Angular is unaware of model modifications. To properly process model modifications the execution has to enter the Angular execution context using
the $apply method. Only model
modifications which execute inside the $apply method will be properly accounted for by Angular. For example if a directive listens on DOM events, such
as ng-click it must evaluate the
expression inside the $apply method.

After evaluating the expression, the $apply method performs a $digest.
In the $digest phase the scope examines all of the $watch expressions and compares them with the previous value. This dirty checking is done asynchronously.
This means that assignment such as $scope.username="angular" will not immediately cause a $watch to
be notified, instead the $watch notification is delayed until the $digest phase.
This delay is desirable, since it coalesces multiple model updates into one $watch notification as well as it guarantees that during the $watch notification
no other $watches are running. If a $watch changes
the value of the model, it will force additional $digest cycle.