如何延迟方法，直到NSData在NSUrlSession中加载完成？

I am working on a project where I call one method from another. In the 2nd method I fetch data from a server using a NSURLSession. When the 2nd method returns the NSData to the first method, the data is converted into JSON and then returned to the viewcontroller that made the inital call on the first method. The problem I am having is that the first method is returning a null object because the NSData doesn't load fast enough. I'm not sure what to do about it. I am working on a project where I call one meth