NativeScript offline view model example that persists data loaded into the view model

nativescript-offline-view-model.js

var observableArrayModule = require("data/observable-array");
var fsModule = require("file-system");

var offline_view_model = {
    OfflineViewModel: function(fileName, items) {
        var viewModel = new observableArrayModule.ObservableArray(items);
        viewModel._fileName = fileName;
        
        viewModel.loadOffline = function() {
            // clear out current view model
            viewModel.empty();
            
            // refresh view model from memory
            var file = fsModule.knownFolders.documents().getFile(viewModel._fileName);
            return file.readText()
                .then(function(contents) {
                    JSON.parse(contents).forEach(function(item) {
                        viewModel.push(item); 
                    });
                });
        };

        viewModel.loadOnline = function() {
            // include your api call here, assuming data comes into variable apiData as an array of data
            // this is simply sample data right now
            var apiData = [{"id":"1","value":"mike"},{"id":"2","value":"josh"},{"id":"3","value":"jason"}];

            // clear the view model
            viewModel.empty();
            
            // repopulate vm from retrieved data
            apiData.forEach(function(item) {
                // could do more advanced data conversion/mapping here, but i'm assuming data from 
                // the api is exactly what I want to persist and advertise 
                viewModel.push(item); 
            });
            
            // persist retrieved data to file system
            return viewModel.persist();
        };
        
        viewModel.delete = function(index) {
            viewModel.splice(index, 1);
            return viewModel.persist();
        }
        
        viewModel.empty = function() {
            while (viewModel.length) {
                viewModel.pop();
            }
        };
        
        viewModel.persist = function() {
            var file = fsModule.knownFolders.documents().getFile(viewModel._fileName);
            var items = [];
            viewModel.forEach(function(item) {
                items.push(item);
            })
            return file.writeText(JSON.stringify(items));
        };
        
        return viewModel;
    }
};
module.exports = offline_view_model;


// to use, see below
var offlineViewModelModule = require("./offline-view-model");
var offlineViewModel = new offlineViewModelModule.OfflineViewModel("my-custom-file-name.json");

offlineViewModel.loadOnline()
    .then(function() {
        console.log("finished loading view model data from api call...");
        offlineViewModel.forEach(function (item) {
            console.log(JSON.stringify(item));
        })
        var message = "finished logging " + offlineViewModel.length + " items from viewModel...";
        console.log(message);
        alert(message);
    });

offlineViewModel.loadOffline()
    .then(function() {
        console.log("finished loading view model data from file-system...");
        offlineViewModel.forEach(function (item) {
            console.log(JSON.stringify(item));
        })
        var message = "finished logging " + offlineViewModel.length + " items from viewModel...";
        console.log(message);
        alert(message);
    });