incremental UI updates from multiple promises

Question

I have an AngularJS service that's used to retrieve individual contacts (/contacts/:id) based on an index (/contacts):

app.service("CollectionService", function($http, $q) {
    this.fetch = function(collectionURL) {
        return $http.get(collectionURL).then(function(response) {
            var urls = response.data;
            var entities = urls.map(function(url) {
                return $http.get(url);
            });
            return $q.all(entities);
        }).then(function(responses) {
            return responses.map(function(response) {
                return response.data;
            });
        });
    };
});

// used within the controller:
CollectionService.fetch("/contacts").then(function(contacts) {
    $scope.contacts = contacts;
});

The results are displayed in a simple list (<li ng-repeat="contact in contacts">{{ contact }}</li>).

However, due to the use of $q.all, that list is not updated until the last (slowest) response has been received. How would one switch from this bulk update to incremental updates as individual contacts are received?

Answer 1

You could just pass the contacts list into fetch() and have it popupate the list.

app.service("CollectionService", function($http, $q) {
    this.fetch = function(collectionURL, resultList) {
        $http.get(collectionURL).then(function(response) {
            var urls = response.data;
            urls.forEach(function(url) {
                $http.get(url).then(function(response) {
                    resultList.push(response.data);
                });
            });
        };
    };
});

// used within the controller:
$scope.contacts = [];
CollectionService.fetch("/contacts", $scope.contacts);

Answer 2

You could use your own promise to return and then hook into a notification of the promise to give you an update on the overall loading progress, and still use $q.all to determine when this has finished. It is basically what you have now with slightly different way of handling and using a custom promise.

Fiddle: http://jsfiddle.net/U4XPU/1/

HTML

<div class="wrapper" ng-app="stackExample">
    <div class="loading" ng-show="loading">Loading</div>
    <div class="contacts" ng-controller="ContactController">
        <div class="contact" ng-repeat="contact in contacts">    {{contact.name}} - {{contact.dob}}</div>
    </div>
</div> 

Controller

.controller("ContactController", ["$scope", "CollectionService", function (scope, service) {
    scope.contacts = [];
    scope.loading = true;

    service.fetch("/contacts")
        .then(
    // All complete handler
    function () {
        console.log("Loaded all contacts");
        scope.loading = false;
    },
    // Error handler
    function () {
        scope.error = "Ruh roh";
        scope.loading = false;
    },
    // Incremental handler with .notify
    function (newContacts) {
        console.log("New contacts found");
        scope.contacts = scope.contacts.concat(newContacts);
    });
}])

Service

.service("CollectionService", ["$q", "$http", function (q, http) {

    this.fetch = function (collectionUrl) {

        var deferred = q.defer();

        http.get(collectionUrl)
        .then(function (response) {

            // Still map all your responses to an array of requests
            var allRequests = response.data.map(function (url) {
                return http.get(url)
                    .then(function (innerResponse) {
                        deferred.notify(innerResponse.data);
                    });
            });

            // I haven't here, but you could still pass all of your data to resolve().
            q.all(allRequests).then(function () {
                deferred.resolve();
            });

        });

        return deferred.promise;

    }

}]);

You can also handle the errors as you see fit and .reject() the promise:

http://docs.angularjs.org/api/ng/service/$q