Is there a way to make this AngularJS service wait until it has a value before returning it?

Question

Is there a way to make this AngularJS service wait until it has a value before returning it?

Multiple controllers call a service that loads data into an object named categories:

.service('DataService', ['$http', '$q', function($http, $q) {

    var categories = {};

    // Public API.
    return({
        setCategory: setCategory,
        getCategory: getCategory,
        getJSON: getJSON,
        categories: categories
    });


    function setCategory(name, category) {
        console.log("setting category");
        console.log(name, category)
        categories[name] = category;
    }

    function getCategory(name) {
        console.log("getCategories:");
        console.log(categories[name]);
        return categories[name];
    }

    function getJSON() {
    //JSON stuff to initialize categories values.
    }

The issue arises when calling getCategory(name) before categories is populated:

 $scope.category = DataService.getCategory(name);
 //$scope.category is undefined

How can I modify the Service so that getCategory waits until categories is defined? Alternatively, how can I adjust the Controller so that getCategory only executes once categories has a value? Attempts with $scope.$watch in the controller have not been successful as it does not update the value.

javascript angularjs

Answer 1

Answer №1

Utilize the promises that are already included in your service. Here is a suggested approach among many possibilities:

var pendingQueue = [];
var loaded = false;
var self = this;
function getCategory(name) {
    var deferred = $q.defer();

    if (loaded) {
        // Resolve immediately
        console.log('Categories already loaded, resolving immediately...');
        deferred.resolve(self.categories[name]);
        return deferred.promise;
    }

    // Queue the request
    pendingQueue.push({
        promise: deferred.promise,
        name: name
    });

    if (pendingQueue.length === 1) {
        console.log('First request for a category, sending request...');

        // We are the FIRST request. Make the necessary call to load the data.
        // This wouldn't be thread-safe in a 'real' language, but with only one thread it's fine.
        $http.get('/my-data').then(function(data) {
            self.categories = data;
            console.log('Categories loaded', self.categories);
            loaded = true;

            pendingQueue.map(function(entry) {
                entry.promise.resolve(entry.name);
            });

            pendingQueue.length = 0;
        });
    }

    return deferred.promise;
}

In your controller:

DataService.getCategory(name).then(function(category) {
    // Handle the category here
});

This process will:

For the first request, initiate the asynchronous request and resolve the promise once the data is fetched.
For subsequent requests before the data is available, queue them to avoid duplicate requests.
For requests made after the data is loaded, promptly resolve with the requested information.

No error handling has been incorporated - remember to use deferred.reject() for error responses, and .catch() / .finally() to manage them within the controller(s).

There are multiple approaches; this is just one possibility.

Answer 2

Utilize the promises that are already included in your service. Here is a suggested approach among many possibilities:

var pendingQueue = [];
var loaded = false;
var self = this;
function getCategory(name) {
    var deferred = $q.defer();

    if (loaded) {
        // Resolve immediately
        console.log('Categories already loaded, resolving immediately...');
        deferred.resolve(self.categories[name]);
        return deferred.promise;
    }

    // Queue the request
    pendingQueue.push({
        promise: deferred.promise,
        name: name
    });

    if (pendingQueue.length === 1) {
        console.log('First request for a category, sending request...');

        // We are the FIRST request. Make the necessary call to load the data.
        // This wouldn't be thread-safe in a 'real' language, but with only one thread it's fine.
        $http.get('/my-data').then(function(data) {
            self.categories = data;
            console.log('Categories loaded', self.categories);
            loaded = true;

            pendingQueue.map(function(entry) {
                entry.promise.resolve(entry.name);
            });

            pendingQueue.length = 0;
        });
    }

    return deferred.promise;
}

In your controller:

DataService.getCategory(name).then(function(category) {
    // Handle the category here
});

This process will:

For the first request, initiate the asynchronous request and resolve the promise once the data is fetched.
For subsequent requests before the data is available, queue them to avoid duplicate requests.
For requests made after the data is loaded, promptly resolve with the requested information.

No error handling has been incorporated - remember to use deferred.reject() for error responses, and .catch() / .finally() to manage them within the controller(s).

There are multiple approaches; this is just one possibility.

Answer 3

Answer №2

Within DataService Component

myApp.service('DataService', function($resource, $q) {
var resource = $resource('/api/category/:id', {}, {
    query: {
        method: 'GET',
        isArray: false,
        cache: false
    },
    save: {
        method: 'POST',
        isArray: false,
        cache: false
    }
});
return {
    getCategory: function(id) {
        var deferred = $q.defer();
        resource.query({id: id},
            function(response) {
                deferred.resolve(response);
            },
            function(response) {
                deferred.reject(response);
            }
        );
        return deferred.promise;
    },
    setCategory: function(categoryObj) {
        var deferred = $q.defer();
        resource.save(categoryObj,
            function(response) {
                deferred.resolve(response);
            },
            function(response) {
                deferred.reject(response);
            }
        );
        return deferred.promise;
    },
    getJSON: function() {
        // operations to be performed
    }
};
});

Inside DataController Section:

myApp.controller('DataCtrl', function($scope, DataService) {

    $scope.handleSuccessResponse = function(response) {
        $scope.data = response;
    };

    $scope.handleErrorResponse = function(response) {
        $scope.error = response;
    };

    DataService.getCategory(123).then($scope.handleSuccessResponse, $scope.handleErrorResponse);
});

Answer 4

Within DataService Component

myApp.service('DataService', function($resource, $q) {
var resource = $resource('/api/category/:id', {}, {
    query: {
        method: 'GET',
        isArray: false,
        cache: false
    },
    save: {
        method: 'POST',
        isArray: false,
        cache: false
    }
});
return {
    getCategory: function(id) {
        var deferred = $q.defer();
        resource.query({id: id},
            function(response) {
                deferred.resolve(response);
            },
            function(response) {
                deferred.reject(response);
            }
        );
        return deferred.promise;
    },
    setCategory: function(categoryObj) {
        var deferred = $q.defer();
        resource.save(categoryObj,
            function(response) {
                deferred.resolve(response);
            },
            function(response) {
                deferred.reject(response);
            }
        );
        return deferred.promise;
    },
    getJSON: function() {
        // operations to be performed
    }
};
});

Inside DataController Section:

myApp.controller('DataCtrl', function($scope, DataService) {

    $scope.handleSuccessResponse = function(response) {
        $scope.data = response;
    };

    $scope.handleErrorResponse = function(response) {
        $scope.error = response;
    };

    DataService.getCategory(123).then($scope.handleSuccessResponse, $scope.handleErrorResponse);
});

Is there a way to make this AngularJS service wait until it has a value before returning it?

Answer №1

Answer №2

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