How to create swift class for category?

Question

I want to create category of my existing swift class, but there is no option in IDE to do so.

Any idea if category exists in swift project? Or how to achieve similar functionality in swift project?

Answer 1

In Swift, you can use Extensions to add new functionality to existing classes, structs and enumeration types.

They differ from Objective-C categories in a few ways, mainly:

• They aren't named
• You don't need to import an Extension explicitly. If you define an extension to add new functionality to an existing type, the new functionality will be available on all existing instances of that type, even if they were created before the extension was defined.
• As stated above, they work not only with classes, but with other types as well.

As it stands today, Extensions can:

• Add computed properties and computed static properties
• Define instance methods and type methods
• Provide new initializers
• Define subscripts
• Define and use new nested types
• Make an existing type conform to a protocol

The basic syntax to declare an extension is as follows:

extension SomeType {     // new functionality to add to SomeType goes here } 

Check Apple's documentation for more information on how to use Extensions in Swift.

Answer 2

In Objective C they were called categories, but in Swift they are called extensions. The purpose of both of them are to give additional functionality to existing classes without having to create subclasses.

I had read about extensions in the documentation, but I didn't really understand how to use one in my project until I watched this tutorial video (YouTube version, github source).

Here is a summary taken from the video of how to do it.

Add a Swift file to your project

• Right click in the Project Navigator and choose "New File..."

• Select "Swift File"

• The convention is to save the file name as the class name you are extending plus (with a "+" sign) what you are doing to it. For example, "UIImage+Cropping".

Write the code for the extension

Open the new Swift file that you just created.

You should import UIKit (instead of Foundation) if you are extending a UIView. Then use the extension keyword before the class name that you want to extend. You can then add your own new methods to the class. (Note, extensions are for adding new methods, not overriding existing methods--hence the name.)

In the video, the example was to add a method that crops a circle from the image and gives it a border.

import UIKit  extension UIImage {          func cropToCircleWithBorderColor(color: UIColor, lineWidth: CGFloat) -> UIImage {                  // code to create the cropped circle with colored border                  return newImage     } } 

See here for the full example.

Use your extension anywhere in your project

Now you can use your new method for that class anywhere in your code, just like it was part of the standard class.

Here is the video's example (on github):

import UIKit class ViewController: UIViewController {                                  @IBOutlet var imageView : UIImageView = nil          override func viewDidLoad() {         super.viewDidLoad()                  let img = imageView.image         imageView.image = img.cropToCircleWithBorderColor(UIColor(red:0.91, green:0.34, blue:0.16, alpha:1.00), lineWidth: 20)     } } 

The method cropToCircleWithBorderColor is not a standard part of UIImage, but as you can see, it is used just like it were.

For further study:

• Three examples of common extensions that I made
• Documentation