$在JSON Angularjs中观察对象

Question

I want to create a $watch in a directive to a specific part of a json object but it doesn't seem to accept the syntax (no errors appear, but it never goes inside the watch)

link: function (scope, element) {
scope.JsonObject={
    profs:{
        prof1:[{
           name:example1a,
           id:example1b
        }],
        prof2:[{
           name:example2a,
           id:example2b
        }]
     }
 }  

scope.teachers=scope.JsonObject['profs']

//until here all ok

for ( var  teacher in scope.teachers){
    //stuff to do
    console.log("creating watch of " + teacher);

    scope.$watch('teachers[teacher]', function() {  //here seems to be the problem (it doesnt seem to accept JsonObject.teacher)       
    //stuff to do

}, true);
}

Answer 1

The string doesnt get interolated into the object.

function getTeacher(teacher) {
   return $scope.teachers[teacher]
}

scope.$watch(getTeacher(teacher), function() {
//Do stuff
},  true);

Answer 2

Previously, teachers[teacher] was undefined because there were no ' ' (quotes) around the teacher .

for (var teacher in $scope.teachers){
    $scope.$watch("teachers['"+ teacher +"']", function(newValue, oldValue) {
      alert("changed");
   }, true);
}

Answer 3

You can't use use an outside value inside a $watch, specially inside a loop. Picture it as an interval, if you put an interval or a timeout inside a loop, the value of the variable handled outside the interval ('$scope.teachers[teacher]' in this case) will not be accesible from the inside, or at least will take the last iteration value.