Should I download the information in controller or data class?

Question

My application downloads photos from web server. Is downloading photo information from server Controller's business or data's business? I mean should I download data in controller class

Photo.title = [self getPhotoInfoFromServer: url]; 

or in data class?

[Photo getPhotoInfoFromServer: url];

Answer 1

Global point of view

An UIViewController is here to Control a View. It means interactions (give good data to the view, and answer to view interactions).

A model / an entity can be created by independents sources (unless the source itself is semantically linked with your model). A Photo is a Photo, whatever if created from a data retrieved from a server or from a local Camera Roll picture.

You should use a custom DataFetcherController to get a remote data and transform it into a instance.

Example Flow

Your UIViewController subclass needs a list of photos. It asks the PhotoServerFetcherController for getting the data from the Server. The fetcher can look into its cache and perfom a request the a server if needed. When the PhotoServerFetcherController get the data and parse it to instances of your Photo entity and give it back to the UIViewController`.

Data fetcher interactions

The best is to create your own FetcherControllerinterface (@protocol keyword). You can handle 2 async callbacks : success with the data in it and failure with the error in it (connection ? bad input ?).

Here you have many ways to implements that, the main 2 patterns for 1 to 1 interactions are delegate or block callbacks.

Answer 2

You should download the photos in to data class then the viewControllers those needs that data can easily get from data class. Actually you should follow MVC where

M= Modal (Your Data Class)

C= Controller class

V= view what your controller displays

Answer 3

I prefer using a dedicated server class, which instantiates models, and is used from the controller: [photoServer getPhotoFromURL:URL], or [photoServer getPhotoFromURL:URL completion:^(Photo *photo, NSError *error){...}]

Why a dedicated server class?

• photos may come from several sources, and you don't want to write many +[Photo getPhotoFrom...] methods and hinder the reusability and maintainability of your Photo class.
• Whenever the server implementation changes, you just have to modify a single class, the class which is responsible for the server. The Photo model remains unaware there is even a server, and is not concerned by the server implementation. This is the principle of single responsibility in action.

Why should this code be called from the controller?

• server access is inherently asynchronous, and may fail. Only controllers know how to deal with delay and errors when we talk about user feedback.