CODEWITHSUNDEEP
angularjs
rasmusvhansen
rasmusvhansen

Reputation: 1522

AngularJS: Is it possible to watch a global variable (I.e outside a scope)?

Let me start by saying that what I am trying to do is probably not considered good practice. However, I need to do something like this in order to migrate a large web app to AngularJs in small incremental steps.

I tried doing 

 $scope.$watch(function () { return myVar; }, function (n, old) {
                    alert(n + ' ' + old);
                });

Where myVar is a global variable (defined on window)

And then changing myVar from the console. But it only fires when first setting up the watcher.

It works if I update myVar from within the controller (see http://jsfiddle.net/rasmusvhansen/vsDXz/3/, but not if it is updated from some legacy javascript

Is there any way to achieve this?

Update I like Anders' answer if the legacy code is completely off limits. However, at the moment I am looking at this approach which seems to work and does not include a timer firing every second:

// In legacy code when changing stuff    
$('.angular-component').each(function () {
        $(this).scope().$broadcast('changed');
});

// In angular
$scope.$on('changed', function () {
    $scope.reactToChange();
});

I am awarding points to Anders even though I will go with another solution, since his solution correctly solves the problem stated.

Upvotes: 12

Views: 14790

Answers (1)

Anders Ekdahl
Anders Ekdahl

Reputation: 22933

The issue here is probably that you're modifying myVar from outside of the Angular world. Angular doesn't run digest cycles/dirty checks all the time, only when things happen in an application that should trigger a digest, such as DOM events that Angular knows about. So even if myVar has changed, Angular sees no reason to start a new digest cycle, since nothing has happened (at least that Angular knows about).

So in order to fire your watch, you need to force Angular to run a digest when you change myVar. But that would be a bit cumbersome, I think you would be better of to create a global observable object, something like this:

<!doctype html>
<html ng-app="myApp">
<head>
    <script src="http://code.jquery.com/jquery-1.9.1.min.js"></script>
    <script src="http://code.angularjs.org/1.0.5/angular.min.js"></script>
    <script>
    // Outside of Angular
    window.myVar = {prop: "value1"};
    var myVarWatch = (function() {
        var watches = {};

        return {
            watch: function(callback) {
                var id = Math.random().toString();
                watches[id] = callback;

                // Return a function that removes the listener
                return function() {
                    watches[id] = null;
                    delete watches[id];
                }
            },
            trigger: function() {
                for (var k in watches) {
                    watches[k](window.myVar);
                }
            }
        }
    })();

    setTimeout(function() {
        window.myVar.prop = "new value";
        myVarWatch.trigger();
    }, 1000);

    // Inside of Angular
    angular.module('myApp', []).controller('Ctrl', function($scope) {
        var unbind = myVarWatch.watch(function(newVal) {
            console.log("the value changed!", newVal);
        });

        // Unbind the listener when the scope is destroyed
        $scope.$on('$destroy', unbind);
    });
    </script>
</head>
<body ng-controller="Ctrl">

</body>
</html>

Upvotes: 13

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