AngularJS：无法访问$ scope。$ on中的控制器的范围变量

Question

I am trying to access scope variable inside $scope.$on but it returns undefined , but if I use a normal variable it is working fine.

Example: 

app.controller('myCtrl',function($scope){
 this.a = "hello";
 var b = "hi";
 // some code

 $scope.$on('someEvent',function(scope,ele,attrs){
  console.log(this.a); // prints 'undefined'
  console.log(b);  //prints 'hi'
 });
});

Answer 1

Inside your callback for $on , the value of this will be that callback. And since you haven't defined any property with name a in that call back, it will be undefined .

app.controller('myCtrl',function($scope){
 this.a = "hello";
 var b = "hi";
 // some code
 var that = this; // <<<<<<<<< 
 $scope.$on('someEvent',function(scope,ele,attrs){
  console.log(that.a); // prints hello
  console.log(b);  //prints 'hi'
 });
});

Answer 2

you should use $scope.a instead of this.a . also, b is used wrong as well.

your code should look like this:

app.controller('myCtrl',function($scope){
 $scope.a = "hello";
 $scope.b = "hi";
 // some code

 $scope.$on('someEvent',function(scope,ele,attrs){
  console.log($scope.a); 
  console.log($scope.b); 
 });
});

hope that helps.