Swift Extensions Best Practices

Question

I've recently gotten addicted to Extensions. One thing I've found to be a gigantic PITA is figuring where I stashed a useful method for something like an Array, an NSManagedObject, etc.

What I've done is put my extensions in an NSObject class file called CustomExtensions and I list the Class Extensions there in alphabetical order.

It works, but I want to see if it aligns with best practices. If not, what is the best way to track Extensions?

Here's what I've done:

class CustomExtensions: NSObject {
    /*
     Useful extensions, nothing else here
     */
}
// Example extension    
extension Int {
    func formatAsTimeString() -> String {
        let seconds = self % 60
        let minutes = (self / 60) % 60
        let hours = self / 3600
        let stringHours = hours > 9 ? String(hours) : "0" + String(hours)
        let stringMinutes = minutes > 9 ? String(minutes) : "0" + String(minutes)
        let stringSeconds = seconds > 9 ? String(seconds) : "0" + String(seconds)

        if hours > 0 {
            return "\(stringHours):\(stringMinutes):\(stringSeconds)"
        }
        else {
            return "\(stringMinutes):\(stringSeconds)"
        }
    }
}
// More extensions below

Answer 1

I don't think the "best practices" around this issue have been figured out yet. That said, here is MHO:

Ultimately, I treat these extensions the same way I do global functions in C. After all, that's basically what they are. Global functions with a special syntax...

If the extension is only used in one file, then I will put the extension in the same file. I end up with quite a few of these by the time I'm done. For example, your formatAsTimeString is a view model kind of thing. If it is used by a single view controller, I would leave it in the view controller's file.

If the extension is used by multiple classes, then I will break it out into a separate file. I will name the file after the extension. So for example if the formatAsTimeString is used in multiple files, then I will have a file named something like, "Int+formatAsTimeString.swift" where the extension can live.

If there are a number of related functions, then I will put them together in the same file and name the file based on the abstract idea of the functions. I decide that the functions are related by imagining whether they would all have to be moved together if I choose to use them in a different program... For example, maybe I have other time string related functions...

Answer 2

With Swift3, I generally tend to use 'fileprivate' if the extension is only used in one file.

fileprivate extension Date
{
    func toString( dateFormat format  : String ) -> String
    {
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = format
        return dateFormatter.string(from: self)
    }
}