Swift and using class extension

Question

I don't understand why programmers use the extension keyword in their class implementation. You can read in other topics that code is then more semantically separated and etc. But when I work with my own code, it feels clearer to me to use // MARK - Something. Then when you use methods list (ctrl+6) in Xcode, everything is seen at first look.

In Apple documentation you can read:

“Extensions add new functionality to an existing class, structure, or enumeration type.”

So why not write my own code directly inside my own class? Unlike when I want to extend functionality of some foreign class, like NSURLSession or Dictionary, where you have to use extensions.

Mattt Thompson use extension in his Alamofire library, maybe he can give me little explanation, why he chose this approach.

Answer 1

For me it seems completely reasonable since you can use extensions to expose different parts of logic to different extensions. This can also be used to make class conformance to protocols more readable, for instance

class ViewController: UIViewController {
...
}

extension ViewController: UITableViewDelegate {
...
}

extension ViewController: UITableViewDataSource {
...
}

extension ViewController: UITextFieldDelegate {
...
}

Protocol methods are separated in different extensions for clarity, this seems to be far better to read than lets say:

class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource, UITextFieldDelegate {}

So, I'd say there's no harm in using extensions to make your own code more readable, not just to extend already existing classes from SDK. Using extensions you can avoid having huge chunks of code in your controllers and split functionality into easily readable parts, so there's no disadvantage of using those.