Cordova/Phonegap - Reads a (json) file from the local cache and fetches it from a remote url if it doesn't exist yet

read-local-file-or-fetch-from-remote.js

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
 
var dataSource = null;

var app = {
    initialize: function() {
        this.bindEvents();
    },

    bindEvents: function() {
        document.addEventListener('deviceready', this.onDeviceReady, false);
    },

    onDeviceReady: function() {
        app.receivedEvent('deviceready');

        // Synchronize the file
        app.synchFile();

        navigator.splashscreen.hide();
    },

    synchFile: function () {

        // first, check if the file exists locally
        var fileName = "datasource";

        // tip: you can test the functionality below simply by uncommenting the line below
        // localStorage.clear();

        // fetch the file from the local cache
        dataSource = localStorage.getItem(fileName);

        // check if something was found
        if (dataSource === null) {

            // nope - file doesn't exist, so fetch it remotely (synchronously)
            var remoteFile = app.fetchRemoteFile("YOUR_REMOTE_URL");

            // persist it to the local cache
            localStorage.setItem(fileName, remoteFile);

            // and finally, read it in memory...
            dataSource = remoteFile;
        }

        if (dataSource === null) {
            console.log("Something went wrong... please add your error handling here...");
            return;
        }

        console.log("Finished, datasource globally available as:");
        console.log(dataSource);
    },

    receivedEvent: function(id) {
        var parentElement = document.getElementById(id);
        var listeningElement = parentElement.querySelector('.listening');
        var receivedElement = parentElement.querySelector('.received');

        listeningElement.setAttribute('style', 'display:none;');
        receivedElement.setAttribute('style', 'display:block;');

        console.log('Received Event: ' + id);
    },

    /*
     * Custom Functions
     */

    fetchRemoteFile: function(path) {

        var returnValue = null;
        console.log("Fetching now!");
        $.ajax({
            url: path,
            async: false,
            dataType: "json",
            type: "GET",
            cache: false,
            success: function (response) {
                console.log("Function: fetchRemoteFile(" + path + "), success.");
                returnValue = response;
            },
            error: function (request, status, error) {
                console.log('Function: fetchRemoteFile(), error: ');
                console.log(request);
            }

        });

        return returnValue;
    }

};