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SOLID Part 1

March 02, 2012 - Roundwall Software

The Single Responsibiliy Principle

Robert C. Martin wrote about the principle in his article back in 2002. The principle states:

"There should never be more than one reason for a class to change." — Robert C. Martin

As requirements for your project change, your code will need to change to meet them. The more reasons you have to change a given class, the greater chance you have of introducing bugs. Consider the case of the often misused Application Delegate. According to Apple’s documentation, an Application Delegate’s one responsibility is to respond to application-level events such as your application being sent to the background or your application has finished launching. You might feel tempted to add additional responsibilities to your Application Delegate for convenience. Handling your Core Data stack, downloading content to display in the Application, and other such responsbilities taint the one single job of the Application Delegate. A few problems arise when this happens:

Apple’s own code follows this rule fairly closely. UITableView, for example, is responsible for handling a collection of cells in a vertically scrolling list. It is not responsible for actually drawing each cell, it leaves this job to cell objects that you supply to the table. It doesn’t keep up with the data that is reflected in the list, it leaves that job to the the object who’s job is to be the datasource (which you provide). Because the table view is only responsible for one job, you don’t need to manipulate or generally even subclass UITableView. To change how information is displayed in the list, simply supply a different UITableViewCell. Want to change how your app knows how many rows to show in your table? Simply supply a different datasource (or change your existing one if necessary). Keeping to one responsibility makes UITableView useful in about a bajillion applications that display all kinds of different lists. Generally it doesn’t hurt to mimic Apple’s own code structure, lots of super smart people have, currently do, and will work there.