javascript - Reinvoke function on model update in AngularJS -

first off, apologize in advance if terminology or approach off - come structural engineering background , trying learn javascript , angularjs through small pet project.

my question is: how re-invoke function updateme() within service whenever model updates. sample code , plunker below.

initially, code run fine , result calculated. if a or b updated, updated values propagated controllers, value of result not updated.

i've tried playing around $scope.$watch() couldn't work, nor sure right approach.

in actual project, function in question depends on several properties of myservice object, , generates json data plotted n3-charts. want make plot interactive regenerating data when user updates property of myservice object.

angular    .module('app', []);  //some function evaluated when data in myservice changes function updateme(x, y) {   return x * y; }  //define service (to provide data multiple ctrls) function myservice() {    var myservice = this;    myservice.a = 2;    myservice.b = 5;    // trying 'result' update when of input parameters update    myservice.result= updateme(myservice.a, myservice.b);  } angular   .module('app')   .service('myservice', myservice);  // controller uses myservice data in view function firstctrl(myservice) {    var vm = this;    vm.myservice = myservice;  } angular    .module('app')    .controller('firstctrl', firstctrl);  // controller uses myservice data in view       function secondctrl(myservice) {    var vm = this;    vm.myservice = myservice; } angular    .module('app')    .controller('secondctrl', secondctrl); 

here's plunker similar code above.

you can use ng-change function recalculate values

<input type="number" step="1" ng-model="first.myservice.a" ng-change="first.myservice.updateall(first.myservice.a)">     myservice.updateall=function()  {    myservice.result = updateme(myservice.a, myservice.b);  } 

whenever there change on model updateall triggers , can recalculations inside that.

here working example.