Adding failing optional initializer in Swift in extension - swift

Question

I'm trying to add a convenience failing initializer to UIFont as an extension. So UIFont already has a failing initializer:
open class UIFont : NSObject, NSCopying {
...
public init?(name fontName: String, size fontSize: CGFloat)
...
}
So I want to create a convenience initializer that would call this one, something similar to the code below:
public convenience init?(name fontName: String, size fontSize: CGFloat, weight fontWeight: String) {
self.init(name: fontName + "-" + fontWeight, size: fontSize)
}
So far so good, this works perfectly. Now I want to add some custom logic, for example:
public convenience init?(name fontName: String, size fontSize: CGFloat, weight fontWeight: String) {
if let font = self.init(name: fontName + "-" + fontWeight, size: fontSize) {
self = font
}
else if let font = self.init(name: fontName, size: fontSize) {
self = font
}
else {
//
// Add some failback code
//
return nil
}
}
This is where problems start. What I want to do is a basic convenience API, but I cannot manage to get the font conditionally initialized. In the upper example there is a syntax error in if let font = self.init line, and it says: Initializer for conditional binding must have Optional type, not '()'. But this initializer actually is optional, as defined by UIFont class (see above), so I don't see the problem. I tried returning font instead of assigning it to self, but does not make a difference.
So my question is: How to correctly implement conditional logic in a failing initializer?
It could be that I am doing something wrong here, or this could just be Swift missing feature/bug?

Answer 1

Swift initialisation is one of the most confusing part of the language, especially if you're coming from Objective-C. The short answer is that:
Unlike Objective-C initializers, Swift initializers do not return a value.
(from https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Initialization.html).
Therefore when you write:
public convenience init?(
name fontName: String, size fontSize: CGFloat, weight fontWeight: String
) {
let font = self.init(name: fontName + "-" + fontWeight, size: fontSize)
the type of font is Void. Yeah, I know, it does raise an eyebrow, since it's different model than in ObjC. In Swift you delegate the work of initialisation to another initialiser, but you're not managing the instance directly. So you can call it a missing feature.
The easy workaround is to make a factory method instead of initialiser:
public static func from(
name fontName: String, size fontSize: CGFloat, weight fontWeight: String
) -> UIFont? {
if let font = self.init(name: fontName + "-" + fontWeight, size: fontSize) {
return font
}
else if let font = self.init(name: fontName, size: fontSize) {
return font
}
else {
//
// Add some failback code
//
return nil
}
}