Your case is trying to share data between controllers, rather than watch service's value in controllers, I think directly reference service object to controller's scope is a better way

So your view can be

 <pre ng-controller="cntrl1">Value in cntrl1: {{ myService.value }} <button ng-click="update('value1')">Change to 'value1'</button></pre>

  <pre ng-controller="cntrl2">Value in cntrl2: {{ myService.value }} <button ng-click="update('value2')">Change to 'value2'</button></pre>

and change your controllers to

app.controller('cntrl1', function(myService, $scope) {
  $scope.myService = myService;
  $scope.update = function(str) {
    $scope.myService.setValue(str);
  }
});

app.controller('cntrl2', function(myService, $scope) {
  $scope.myService = myService;
  $scope.update = function(str) {
    $scope.myService.setValue(str);
  }
});

Use service to share data between controllers

Your case is trying to share data between controllers, rather than watch service's value in controllers, I think directly reference service object to controller's scope is a better way

So your view can be

 <pre ng-controller="cntrl1">Value in cntrl1: {{ myService.value }} <button ng-click="update('value1')">Change to 'value1'</button></pre>

  <pre ng-controller="cntrl2">Value in cntrl2: {{ myService.value }} <button ng-click="update('value2')">Change to 'value2'</button></pre>

and change your controllers to

app.controller('cntrl1', function(myService, $scope) {
  $scope.myService = myService;
  $scope.update = function(str) {
    $scope.myService.setValue(str);
  }
});

app.controller('cntrl2', function(myService, $scope) {
  $scope.myService = myService;
  $scope.update = function(str) {
    $scope.myService.setValue(str);
  }
});

#Use $broadcast/$emit Just as @squiroid points out, you can use $broadcast to broadcast events to any controllers who is monitoring targeted events.

Please note here, you'd better not use $rootScope.$broadcast + $scope.$on but rather $rootScope.$emit+ $rootScope.$onas $broadcast event will bubble down through all descendant scopes, which might lead to serious performance problems.

Your case is how to share data between controllers, rather than watch service's value in controllers, I think directly reference service object to controller's scope is a better way

So your view can be

 <pre ng-controller="cntrl1">Value in cntrl1: {{ myService.value }} <button ng-click="update('value1')">Change to 'value1'</button></pre>

  <pre ng-controller="cntrl2">Value in cntrl2: {{ myService.value }} <button ng-click="update('value2')">Change to 'value2'</button></pre>

and change your controllers to

app.controller('cntrl1', function(myService, $scope) {
  $scope.myService = myService;
  $scope.update = function(str) {
    $scope.myService.setValue(str);
  }
});

app.controller('cntrl2', function(myService, $scope) {
  $scope.myService = myService;
  $scope.update = function(str) {
    $scope.myService.setValue(str);
  }
});

Your case is trying to share data between controllers, rather than watch service's value in controllers, I think directly reference service object to controller's scope is a better way

So your view can be

 <pre ng-controller="cntrl1">Value in cntrl1: {{ myService.value }} <button ng-click="update('value1')">Change to 'value1'</button></pre>

  <pre ng-controller="cntrl2">Value in cntrl2: {{ myService.value }} <button ng-click="update('value2')">Change to 'value2'</button></pre>

and change your controllers to

app.controller('cntrl1', function(myService, $scope) {
  $scope.myService = myService;
  $scope.update = function(str) {
    $scope.myService.setValue(str);
  }
});

app.controller('cntrl2', function(myService, $scope) {
  $scope.myService = myService;
  $scope.update = function(str) {
    $scope.myService.setValue(str);
  }
});