How to return a custom object in a swift convenience initializer? - objective-c

Question

I'm trying to do something like this:
public extension UIImage {
public convenience init(whatever: Int) {
UIGraphicsBeginImageContextWithOptions(...)
//...
let image = UIGraphicsGetImageFromCurrentContext()
UIGraphicsEndImageContext()
return image // <- impossible
}
}
But this is not possible as "nil" is the only valid return for an initializer... How do i do this?
For example, the Objtive-C method [UIImage imageNamed:] is a class method (that can return whatever it wants in Objective-C) and it was mapped to the swift initializer UIImage(named:).

Answer 1

What you want is a class factory method, not an initializer. Most factory methods in Foundation/Cocoa are automatically bridged to initializers, but if what you want can't be done via init, you can add a new class method:
public extension UIImage {
class func imageWithWhatever(whatever: Int) -> UIImage {
UIGraphicsBeginImageContextWithOptions(...)
//...
let image = UIGraphicsGetImageFromCurrentContext()
UIGraphicsEndImageContext()
return image
}
}

Answer 2

This is because you are returning a new object, not self. The point of init is to create the structure of your object, not a new one, so if you want to do it as a convenience init, you need to do it like this:
public extension UIImage {
public convenience init?(whatever: Int) {
defer {
UIGraphicsEndImageContext()
}
UIGraphicsBeginImageContextWithOptions(...)
//...
guard let currentContext = UIGraphicsGetCurrentContext() else { return nil }
guard let image = currentContext.makeImage() else { return nil }
self.init(cgImage:image)
}
}
perhaps instead of a convenience init, you want to create a class function that is doing what you are asking:
public class func createImage(whatever: Int) -> UIImage? {
defer {
UIGraphicsEndImageContext()
}
UIGraphicsBeginImageContextWithOptions(...)
//...
guard let currentContext = UIGraphicsGetCurrentContext() else { return nil }
guard let cgImage = currentContext.makeImage() else { return nil }
let image = UIImage(cgImage: cgImage)
return image
}
I apologize that this is not 100% to code, but that is basically the gist of it