How to make the method getItem from the offlineStorage of the Kendo Datasource work with LocalForage - kendo-ui

Question

I need to setup my datasource to use localForage to manage my offline data storage.
The problem I'm having is that localForage is asynchronous by nature, only allowing us to use a callback or a promise to retrieve data.
This is how my code is set-up:
(function () {
'use strict';
var serviceId = 'employeeDataSource';
angular.module('app').factory(serviceId, function () {
var crudServiceBaseUrl = 'Data/Employees';
var dataSource = new kendo.data.DataSource({
type: "odata",
offlineStorage: {
getItem: function () {
localforage.getItem('employees-key').then(function (value) {
console.log(value);
return JSON.parse(value);
});
},
setItem: function (item) {
if (item.length > 0) {
localforage.setItem("employees-key", JSON.stringify(item));
}
}
},
transport: {
read: {
url: crudServiceBaseUrl + "/",
dataType: "json"
},
update: {
url: function (data) {
return crudServiceBaseUrl + "(guid'" + data.EmployeeId + "')";
}
},
create: {
url: crudServiceBaseUrl
},
destroy: {
url: function (data) {
return crudServiceBaseUrl + "(guid'" + data.EmployeeId + "')";
}
}
},
batch: false,
pageSize: 5,
serverPaging: true,
schema: {
data: function (data) {
return data.value;
},
total: function (data) {
return data['odata.count'];
},
model: {
id: "EmployeeId",
fields: {
EmployeeId: { editable: false, nullable: true },
Name: { validation: { required: true } },
Email: { validation: { required: true } },
IsManager: { type: "boolean" },
MaxHolidaysPerYear: { editable: false, type: "number", validation: { min: 0, required: true } },
HolidaysLeftThisYear: { type: "number", validation: { min: 0, required: true } }
}
}
},
error: function (e) {
if (e.xhr.responseText !== undefined) {
console.log(e.xhr.responseText);
}
}
});
dataSource.online(navigator.onLine);
$(window).on("offline", function () {
dataSource.online(false);
});
$(window).on("online", function () {
dataSource.online(true);
});
return dataSource;
});
})();
When off-line, the getItem gets called, then the setItem gets call as well with an empty array, hence the:
if (item.length > 0) {
localforage.setItem("employees-key", JSON.stringify(item));
}
When the promise finally returns the off-line data (with the correct values I expected), the Grid displays no results.
This behaviour is presumably because of the promise ?
I tried the same thing with sessionStorage and its worked perfectly... i.e.:
getItem: function () {
return JSON.parse(sessionStorage.getItem("employees-key"));
},
setItem: function (item) {
sessionStorage.setItem("employees-key", JSON.stringify(item));
}
What can I do to get around this?

Answer 1

Just got a heads-up from Telerik
There is an issue logged in their GitHub repo about the same problem.
You can keep track on the progress here:
https://github.com/telerik/kendo-ui-core/issues/326