Exploring AngularJS for real-time updates from a persistent database

Hello, I'm new to Angular and currently working on an app that requires frequent polling of a URL every second. The retrieved data needs to be stored persistently for access across multiple views and controllers.

To handle this, I've placed the HTTP request in a factory where the data is accessed by controllers through factory functions. However, I'm facing an issue where the factory function is executed before the HTTP request, causing errors in my app.

Below is the code snippet:

App.factory('metrics', function($http){
    var service;
    var users = [{laps:[]}];
    var updateMetrics = function(){
        // Function to update the users array in the factory
    };
    $http.get('data.csv').success(function(data) {
        var temp_array = data.split(" ");
        updateMetrics(0, temp_array);
    });

    service.lastLapInfo = function(){
        var lastlap = [];
        for (var i=0; i<users.length;i++)
        {
            var lap = users[i].laps[users[i].laps.length-1];
            lastlap.push(lap);
        }
        return lastlap;
    };
    return service;
});

App.controller('mainController', function($scope, $http, metrics) {
    $scope.users=metrics.lastLapInfo();
});

The call to "lastLapInfo()" is happening before the HTTP request, which results in errors due to empty data array. Any suggestions on how to resolve this?

Also, if there's a better approach to achieve the desired functionality instead of using a Factory, please advise!

Answer №1

Here is an example of how to use a promise, like Angular's $q service.

This method involves creating a factory that returns a promise:

App.factory('metrics', function($http){
   var service;
   ...
   service.load = function () {
      return $http.get('data.csv');
   });
   return service;
});       

In the controller, you would then call your metrics service and utilize the promise it returns:

App.controller('mainController', function($scope, $http, metrics) {
    metrics.load().then(function(response) {
       // This section confirms that the request has completed
       // You can either call the service again for more information or handle additional logic here.
    });
});

Answer №2

If you are unable to access the data right away, you can handle this by either indicating it in the return value (null, undefined, []), or you could opt to return a promise instead. This promise can be stored and resolved later once the data is retrieved.

For example:

var deferred = null;

$http.get(...).then ({
  if (deferred) {
    deferred.resolve(result);
    deferred = null;
  }
});

service.dataInfo = function(){
  if (no info available) {
    deferred = $q.defer()
    return deferred;
  }
  else {
    return data;
  }
};

This approach should work, depending on your desired structure.

Edit: included link to $q from comment.

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