Asynchronous handling of services who provide data and retrieve it via http when necessary

Question

I have an angularJS app that utilizes two services to retrieve data from a DB.

session.js

angular.module('RiskAssessment').service('session', ['dbInterface', function(dbInterface) {
this.getBatches = function () {
    if (!this.batches) {
        console.log("Retrieved Batches");
        var that = this;
        return this.pullBatches().then(function (data) {
            that.batches = data; //Is this EVEN possible?
        });
    } else {
        console.log("Didn't retrieve Batches");
    }
    return this.batches;
};
this.pullBatches = function () {
    return dbInterface.pullBatches(this.getUserId());
};}]);

dbInterface.js

        pullBatches: function(userId){
            return $http.post('db_queries/get_batches.php', userId)
                .then(function (response) {
                    console.log("get_batches.php POST Result: ", response.data);
                    return response.data;
                })
                .catch(function (response) {
                    console.log("Error post");
                });
        }

I want to able to get this.batches via getBatches() if it has already been retrieved and set. Otherwise, I'd like to use pullBatches() to retrieve and set this.batches. The answer is probably some mix of promises, but I am struggling with this.

Thank you for reading!

EDIT ::

How do I set this.batches within a .then() of my call to .pullBatches()?

this.getBatches = function(){
    if(!this.batches) {
        console.log("Retrieved Batches");
        var deferred = $q.defer();
        deferred = this.pullBatches().then(function(data){
            //this.batches = data;  <---------------------------- HERE
        });
        return deferred.promise;
    }else{
        console.log("Didn't retrieve Batches");
    }
    return this.batches;
};

EDIT 2 :: With great help from @Jahirul_Islam_Bhuiyan I fixed my issue.

this.getBatches = function(){
    var deferred = $q.defer();
    if(!this.batches){
        console.log("Retrieved Batches");
        dbInterface.pullBatches(this.getUserId()).then(function(payload){
            deferred.resolve(payload.data);
            service.setBatches(payload.data);
        });
    }else{
        console.log("Didn't retrieve Batches");
        deferred.resolve(this.batches);
    }
    return deferred.promise;
};

this.setBatches = function(batches){
    this.batches = batches;
};

In Controller...

        session.getBatches().then(function(data){
            //console.log("getBatches.then() : " + JSON.stringify(data));
            $scope.batches = data;
        });

I now have a much greater understanding of promises!

use $q to ensure promise. call resolve method in both return from dbInterface with data and return from cache. docs.angularjs.org/api/ng/service/$q — Jahirul Islam Bhuiyan
– Jahirul Islam Bhuiyan, Commented May 11, 2016 at 16:27
Is it true that $http post returns a promise? In that case, wouldn't I remove the .then() from dbInterface; and just have them in the cache and return from cache? — Phiat
– Phiat, Commented May 11, 2016 at 16:36
yes, $http post returns a promise, you can do as var deferred = $q.defer(); and later, deferred =$http post(); — Jahirul Islam Bhuiyan
– Jahirul Islam Bhuiyan, Commented May 11, 2016 at 16:43
Thank you so much for your assistance. Can you look at my EDIT for slightly revised code. I don't have the promises figured out yet, but I still don't understand how I can set this.batches within a .then()? — Phiat
– Phiat, Commented May 11, 2016 at 16:51

Jahirul Islam Bhuiyan · Accepted Answer · 2016-05-11 17:25:34Z

1

Try following

this.getBatches = function(){
   var deferred = $q.defer();
    if(!this.batches) {
        console.log("Retrieved Batches");

        this.pullBatches().then(function(data){
           deferred.resolve(data);
        });
    }else{
        console.log("Didn't retrieve Batches");
        deferred.resolve(this.batches);
    }
   var promises = [
            deferred.promise
        ];
    var promise = $q.all(promises);
        return promise;
};

hope this help

edited May 11, 2016 at 17:25

answered May 11, 2016 at 17:02

Jahirul Islam Bhuiyan

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2 Comments

Phiat Over a year ago

Thank you so much! You're helping me understand this! But I am getting an odd error with this code, saying that deferred.resolve is not a function. I have $q correctly injected...

Jahirul Islam Bhuiyan Over a year ago

i hv edit the code. No need to assign deferred from $http.

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