Swift 2.0 Subclassing a subclass of UIViewController and called convenience initializers - ios

Question

Have a bit of confusion regarding designated and convenience initializers for UIViewController in Swift 2.0/Xcode 7beta3. Our UIViewControllers are all defined in code, there are no Nibs
Currently class A inherits from UIViewController like this
class A : UIViewController {
convenience init() {
...
self.init(nibName:nil, bundle:nil)
...
}
}
Then class B inherits from class A and should override the convenience init and call its as super.init()
class B : A {
convenience init() {
super.init()
...
}
}
The compiler does not allow this with Must call a designated initializer of the superclass '...' error on super.init()

Answer 1

You need to make your initializers designated, not convenience:
class A : UIViewController {
init() {
super.init(nibName:nil, bundle:nil)
}
required init?(coder aDecoder: NSCoder) {
fatalError("")
}
}
class B : A {
override init() {
super.init()
}
required init?(coder aDecoder: NSCoder) {
fatalError("")
}
}
That gives you the inheritance structure you're looking for.

Answer 2

Take a look at these images found in the documentation.
(source: apple.com)
(source: apple.com)
According to the image convenience initializers are not inherited. So if you want to inherit you must make it a designated initializer.
class A : UIViewController {
init() {
super.init(nibName: nil, bundle: nil)
}
required init(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)
}
}
class B : A {
override init() {
super.init()
}
required init(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)
}
}
At this Point you may wonder what the heck is the difference between a designated and convenience initializer? Well, Convenience is used to call a designated initializer in the same class and you are suppose to use this to do some set up.

Answer 3

According to the document
Rule 2 A convenience initializer must call another initializer from the same class.
From https://developer.apple.com/library/mac/documentation/Swift/Conceptual/Swift_Programming_Language/Initialization.html(Initializer Delegation for Class Types)
You should call it's own initializer instead:
class B : A {
convenience init() {
init()
...
}
}
And init() is automatically inherit from it's superclass version

Answer 4

Here are rules how communicating designated initializers and convenience initializers
rule 1:
A designated initializer must call a designated initializer from its immediate superclass.
Rule 2:
A convenience initializer must call another initializer from the same class.
Rule 3:
A convenience initializer must ultimately call a designated initializer.
A simple way to remember this is:
Designated initializers must always delegate up.
Convenience initializers must always delegate across.