Building the URL
Communication with servers is done via requests. A request encapsulates information about the interaction between the application and the server, and its most important piece of information is the destination URL.
In this section, you will build up the URL for retrieving recent photos from the Flickr web service. The architecture of the application will reflect best practices. For example, each type that you create will encapsulate a single responsibility. This will make your types robust and flexible and your application easier to reason about. To be a good iOS developer, you not only need to get the job done, you also need to get it done thoughtfully and with foresight.
Formatting URLs and requests