linking promises, or one promise triggers another

Question

I'm building a weather app where I first have to get the users location, then make the request to get the weather.

So I have a GeolocationService and a WeatherService. My WeatherService is currently calling the Geolocation Service. How do I make the WeatherService wait until it has the results from the GeolocationService before making the HTTP request?

  app.factory('GeolocationService',function($q,$window,$rootScope){
        return {
            getLatLon: function(){
                var deferred = $q.defer();
                if(!window.navigator){
                    $rootScope.$apply(function(){
                        deferred.reject(new Error("Geolocation not available"));
                    });
                } else {
                    $window.navigator.geolocation.getCurrentPosition(function(position){
                        $rootScope.$apply(function(){
                            deferred.resolve(position);
                        });
                    }, function(error){
                        $rootScope.$apply(function(){
                            deferred.reject(error);
                        });
                    });
                }

                return deferred.promise;
            }
        };
    });

    app.factory("WeatherService", function ($q,$http,$rootScope, GeolocationService) {
        return {
            getWeather: function(){
                var weather;
                var loc = new GeolocationService.getLatLon(); 
                var  lat= loc.lat || 37.4568202221774, 
                lon= loc.lon || -122.201366838789 ;
                var units = '';
                var url = 'http://api.openweathermap.org/data/2.5/forecast/daily?lat='+lat+'&lon='+lon+'&units='+units+'&callback=JSON_CALLBACK';
                $http.jsonp(url)
                    .success(function(data) {
                        weather=data;
                        return weather;
                    })
                    .error(function(err){
                        weather=err;
                        return err;
                    });
            }

        };
    });

Answer 1

You just need to use the promise in your service and then chaine the promise. For sample I have a user with user.company_id and if I want get the name of the company I have to wait that the user is load. It the same that your situation.

this is my service:

    angular.module('UserService', []) 
.factory('UserService', function($q , $http, $rootScope,$timeout) {
var currentUserPromise = null;
var currentCompanyPromise = null;
return {
    getCurrentUser: function() {
        if (currentUserPromise === null) {
            var config = {};
            config.cache = true;
            config.method = "GET"; 
            config.url = "users/get_current_user";
            currentUserPromise = $http(config)
                .then(function(response) {
                    if (typeof response.data === 'object') {
                        return response.data.user;
                    } else {
                        // invalid response
                        return $q.reject(response.data);
                    }  
                }, function(response) {
                    // something went wrong
                    return $q.reject(response.data);
                });
        }
        return currentUserPromise;
    },

    getCurrentCompany: function(company_id) {
        if (currentCompanyPromise === null){
            var config = {};
            var company = {};
            company.id = company_id; 
            config.cache = true;
            config.method = "GET"; 
            config.url = "/companies/show";
            config.params = company;
            currentCompanyPromise = $http(config)
                .then(function(response) {
                    if (typeof response.data === 'object') {
                        return response.data.company;
                    } else {
                        // invalid response
                        return $q.reject(response.data);
                    }

                }, function(response) {
                    // something went wrong
                    return $q.reject(response.data);
                });
            }
            return currentCompanyPromise;

        }
    };     
});

and in my controller I user this like this:

CONTROLLER :

var promiseCurrentUser = UserService.getCurrentUser();
promiseCurrentUser.then(function(user) {
    $scope.currentUser = user;
    UserService.getCurrentCompany(user.company_id).then(function(company){
        $scope.companyName = company.name;
    });
});

And What is cool with the promise is that it is resolve once.

I hope this will help you.

Answer 2

Your GeolocationService's function getLatLon is returning a promise. There is no need to call it with a new operator.

Your getWeather function should look something along the lines of:

app.factory("WeatherService", function ($q,$http,$rootScope, GeolocationService) {
    return {
        getWeather: function(){
            var weather;
            return GeolocationService.getLatLon().then(
                function (loc) {
                    var  lat= loc.lat || 37.4568202221774, 
                    lon= loc.lon || -122.201366838789 ;
                    var units = '';
                    var url = 'http://api.openweathermap.org/data/2.5/forecast/daily?lat='+lat+'&lon='+lon+'&units='+units+'&callback=JSON_CALLBACK';
                    return $http.jsonp(url)
                        .success(function(data) {
                            weather=data;
                            return weather;
                        })
                        .error(function(err){
                            weather=err;
                            return err;
                        });
            });
        }
    }
}

Here, we first call the GeolocationService.getLatLon() to get a promise. Then we chain our processing as a chain to it. The success function will get the position you resolve it with here deferred.resolve(position);.

Also, you do not need to wrap the resolve and reject within an $apply. Hence, your GeoLocationService can be simplified to:

            if(!window.navigator){
                deferred.reject(new Error("Geolocation not available"));
            } else {
                $window.navigator.geolocation.getCurrentPosition(function(position){
                        deferred.resolve(position);
                }, function(error){
                        deferred.reject(error);
                });
            }