testing123 testing123 - 4 months ago 199
AngularJS Question

AngularJS : Initialize service with asynchronous data

I have an AngularJS service that I want to initialize with some asynchronous data. Something like this:

myModule.service('MyService', function($http) {
var myData = null;

$http.get('data.json').success(function (data) {
myData = data;
});

return {
setData: function (data) {
myData = data;
},
doStuff: function () {
return myData.getSomeData();
}
};
});


Obviously this won't work because if something tries to call
doStuff()
before
myData
gets back I will get a null pointer exception. As far as I can tell from reading some of the other questions asked here and here I have a few options, but none of them seem very clean (perhaps I am missing something):

Setup Service with "run"

When setting up my app do this:

myApp.run(function ($http, MyService) {
$http.get('data.json').success(function (data) {
MyService.setData(data);
});
});


Then my service would look like this:

myModule.service('MyService', function() {
var myData = null;
return {
setData: function (data) {
myData = data;
},
doStuff: function () {
return myData.getSomeData();
}
};
});


This works some of the time but if the asynchronous data happens to take longer than it takes for everything to get initialized I get a null pointer exception when I call
doStuff()


Use promise objects

This would probably work. The only downside it everywhere I call MyService I will have to know that doStuff() returns a promise and all the code will have to us
then
to interact with the promise. I would rather just wait until myData is back before loading the my application.

Manual Bootstrap

angular.element(document).ready(function() {
$.getJSON("data.json", function (data) {
// can't initialize the data here because the service doesn't exist yet
angular.bootstrap(document);
// too late to initialize here because something may have already
// tried to call doStuff() and would have got a null pointer exception
});
});


Global Javascript Var
I could send my JSON directly to a global Javascript variable:

HTML:

<script type="text/javascript" src="data.js"></script>


data.js:

var dataForMyService = {
// myData here
};


Then it would be available when initializing
MyService
:

myModule.service('MyService', function() {
var myData = dataForMyService;
return {
doStuff: function () {
return myData.getSomeData();
}
};
});


This would work too, but then I have a global javascript variable which smells bad.

Are these my only options? Are one of these options better than the others? I know this is a pretty long question, but I wanted to show that I have tried to explore all my options. Any guidance would greatly be appreciated.

Answer

Have you had a look at $routeProvider.when('/path',{ resolve:{...}? It can make the promise approach a bit cleaner:

Expose a promise in your service:

app.service('MyService', function($http) {
    var myData = null;

    var promise = $http.get('data.json').success(function (data) {
      myData = data;
    });

    return {
      promise:promise,
      setData: function (data) {
          myData = data;
      },
      doStuff: function () {
          return myData;//.getSomeData();
      }
    };
});

Add resolve to your route config:

app.config(function($routeProvider){
  $routeProvider
    .when('/',{controller:'MainCtrl',
    template:'<div>From MyService:<pre>{{data | json}}</pre></div>',
    resolve:{
      'MyServiceData':function(MyService){
        // MyServiceData will also be injectable in your controller, if you don't want this you could create a new promise with the $q service
        return MyService.promise;
      }
    }})
  }):

Your controller won't get instantiated before all dependencies are resolved:

app.controller('MainCtrl', function($scope,MyService) {
  console.log('Promise is now resolved: '+MyService.doStuff().data)
  $scope.data = MyService.doStuff();
});

I've made an example at plnkr: http://plnkr.co/edit/GKg21XH0RwCMEQGUdZKH?p=preview

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