在创建iOS云集成框架时，我的基于块的方法是否应该返回主线程？

I am in the middle of creating a cloud integration framework for iOS. We allow you to save, query, count and remove with synchronous and asynchronous with selector/callback and block implementations. What is the correct practice? Running the completion blocks on the main thread or a background thread?I am in the middle of creating a cloud integrat