$watch(watchExpression, listener, objectEquality);
watchExpression： 被監聽的對象
listener: 包含3個值 newValue , oldValue, scope（可以引用一個作用域）
objectEquality：是否使用angular.equals方法比對
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<script src="http://cdn.static.runoob.com/libs/angular.js/1.4.6/angular.min.js"></script>
</head>
<body ng-app="ap" >
<div ng-controller="cl">
<p>小框子1</p>
<input type="text" ng-model="input_value" />
<p>小框子1的 $watch監聽</p>
<span>{{dv}}</span>
</div>
</body>
<script>
var ap = angular.module("ap", [])
.controller("cl", function($scope){
$scope.input_value = "我是一個初值";
$scope.$watch('input_value', function(newValue, oldValue) {
$scope.dv=newValue;
});
});
</script>
</html>