如何在Codeigniter中设置Img文件的Ajax上传

I read many tutorial about uploading files using codeigniter, but the problem is those tutorial are not working in my situation, and i didn't succeed in uploading via ajax, the request failed and the page load in the navbar which is not ajax, i want to find a very detailed tutorial that explain the basics of ajax upload in codeigniter. or some advice in that topic.I read many tutorial about uploading files usin