Loading data from angular service on startup - angularjs

Question

UPDATE
I am currently doing this, and I'm not sure why it works, but I don't think this is the correct approach. I might be abusing digest cycles, whatever those are.
First, I want to have the array navigationList be inside a service so I can pass it anywhere. That service will also update that array.
app.factory('ChapterService', [ 'ExtService', function(ExtService) {
var navigationList = [];
var getNavigation = function() {
ExtService.getUrl('navigation.json').then(function(data) {
angular.copy(data.navigationList, navigationList);
});
}
return{
getNavigation: getNavigation,
navigationList: navigationList,
}
}]);
Then in my controller, I call the service to update the array, then I point the scope variable to it.
ChapterService.getNavigation();
$scope.navigationList = ChapterService.navigationList;
console.log($scope.navigationList);
But this is where it gets weird. console.log returns an empty array [], BUT I have an ng-repeat in my html that uses $scope.navigationList, and it's correctly displaying the items in that array... I think this has something to do with digest cycles, but I'm not sure. Could anyone explain it to me and tell me if I'm approaching this the wrong way?
I have a main factory that runs functions and calculations. I am trying to run
app.factory('ChapterService', [ 'ExtService', function(ExtService) {
var navigation = {
getNavigationData : function () {
ExtService.getUrl('navigation.json').then(function(data) {
return data;
});
}
}
return: {
navigation: navigation
}
I did a console.log on the data before it gets returned and it's the correct data, but for some reason, I can't return it..
The ExtService that has the getUrl method is just the one that's typically used (it returns a promise)
In my controller, I want to do something like
$scope.navigation = ChapterService.navigation.getNavigationData();
in order to load the data from the file when the app initializes,
but that doesn't work and when I run
console.log(ChapterService.navigation.getNavigationData());
I get null, but I don't know why. Should I use app.run() for something like this? I need this data to be loaded before anything else is done and I don't think I'm using the best approach...
EDIT
I'm not sure if I should do something similar to what's being done in this jsfiddle, the pattern is unfamiliar to me, so I'm not sure how to re purpose it for my needs
My code for ExtService is
app.factory('ExtService', function($http, $q, $compile) {
return {
getUrl: function(url) {
var newurl = url + "?nocache=" + (new Date()).getTime();
var deferred = $q.defer();
$http.get(newurl, {cache: false})
.success(function (data) {
deferred.resolve(data);
})
.error(function (error) {
deferred.reject(error);
});
return deferred.promise;
}
}
});
EDIT 2
I'd like to separate the request logic away from the controller, but at the same time, have it done when the app starts. I'd like the service function to just return the data, so I don't have to do further .then or .success on it...

Answer 1

You are using promises incorrectly. Think about what this means:
var navigation = {
getNavigationData : function () {
ExtService.getUrl('navigation.json').then(function(data) {
return data;
});
}
}
getNavigationData is a function that doesn't return anything. When you're in the "then" clause, you're in a different function so return data only returns from the inner function. In fact, .then(function(data) { return data; }) is a no-op.
The important thing to understand about promises is that once you're in the promise paradigm, it's difficult to get out of it - your best bet is to stay inside it.
So first, return a promise from your function:
app.factory('ChapterService', [ 'ExtService', function(ExtService) {
var navigation = {
getNavigationData: function () {
return ExtService.getUrl('navigation.json');
}
}
return {
navigation: navigation
}
}])
Then use that promise in your controller:
app.controller('MyController', function($scope, ExtService) {
ExtService
.navigation
.getNavigationData()
.then(function(data) {
$scope.navigation = data;
});
})
Update
If you really want to avoid the promise paradigm, try the following, although I recommend thoroughly understanding the implications of this approach before doing so. The object you return from the service isn't immediately populated but once the call returns, Angular will complete a digest cycle and any scope bindings will be refreshed.
app.factory('ChapterService', [ 'ExtService', function(ExtService) {
var navigation = {
getNavigationData: function () {
// create an object to return from the function
var returnData = { };
// set the properties of the object when the call has returned
ExtService.getUrl('navigation.json')
.then(function(x) { returnData.nav = x });
// return the object - NB at this point in the function,
// the .nav property has not been set
return returnData;
}
}
return {
navigation: navigation
}
}])
app.controller('MyController', function($scope, ExtService) {
// assign $scope.navigation to the object returned
// from the function - NB at this point the .nav property
// has not been set, your bindings will need to refer to
// $scope.navigation.nav
$scope.navigation = ExtService
.navigation
.getNavigationData();
})

Answer 2

You are using a promise, so return that promise and use the resolve (.then) in the controller:
app.factory('ChapterService', [ 'ExtService', function(ExtService) {
var navigation = {
getNavigationData: function () {
return ExtService.getUrl('navigation.json'); // returns a promise
});
}
return: {
navigation: navigation
}
}
controller:
ChapterService
.navigation
.getNavigationData()
.then(function (data) {
// success
$scope.navigation = data;
}, function (response) {
// fail
});

Answer 3

This is a different approach, I don't know what your data looks like so I am not able to test it for you.
Controller
.controller('homeCtrl', function($scope, $routeParams, ChapterService) {
ChapterService.getNavigationData();
})
Factory
.factory('ChapterService', [ 'ExtService', function(ExtService) {
function makeRequest(response) {
return ExtService.getUrl('navigation.json')
}
function parseResponse(response) {
retries = 0;
if (!response) {
return false;
}
return response.data;
}
var navigation = {
getNavigationData: function () {
return makeRequest()
.then(parseResponse)
.catch(function(err){
console.log(err);
});
}
}
return navigation;
}])