iOS - 对地址簿中的每个联系人执行异步请求 - 检查用户是否已注册

I'm a student and working on my first app which needs a contact sync system with the web database i.e to check if the contact in your addressbook is already a registered user or not. So what I'm doing is firstly saving all of the contacts in an Array and then trying to make POST request with each contact in that Array. What I want to ask is :I'm a student and working on my first app which