Extending CollectionType with optional initialiser - ios

Question

protocol Decodable {
init?(data: [String: AnyObject])
}
struct A: Decodable {
var data: [String: AnyObject]!
init?(data: [String: AnyObject]) {
self.data = data
}
}
This works when i want to create an object
let d = ["name":"Rahul"]
let a = A(data: d)
I am trying to achieve the following but it is giving errors at the compile time.
let dArray = [["name":"Rahul"],["name":"Rahul"],["name":"Rahul"]]
let aArray = [A](data: dArray)
The following code is giving me error 'nil is the only return value permitted in an initializer'.
public extension CollectionType where Generator.Element: Decodable {
init?(data: [[String: AnyObject]]) {
var elements: [Generator.Element] = []
for d in data {
let element = Generator.Element(data: d)
if let element = element {
element.append(element)
}
}
return elements
}
}
=================================
Answer : -
public extension Array where Element: Decodable {
init?(data: [String: AnyObject]) {
var elements: [Element] = []
for d in data {
let element = Element(data: d)
if let element = element {
element.append(element)
}
}
self = elements
}
}
This will allow you to initialise using the following code
let dArray = [["name":"Rahul"],["name":"Rahul"],["name":"Rahul"]]
let aArray = [A](data: dArray)

Answer 1

Your error is because CollectionType is a protocol, which cannot be initialised.
Have you tried to create a helper class method to return you the collection, e.g.
func collectionWithData(data: [[String: AnyObject]]) -> [Generator.Element] {

Answer 2

let's define a protocol
protocol P {
init?(b: Bool)
}
as mentioned by Oliver, protocol cannot be initialized, but we still are able to define init in protocol extension
extension P {
init?(b: Bool) {
print("init defined in extension")
if b == false { return nil }
self.init(b: b)
}
}
see, that you cannot return anything from init but nil (if initialization failed)
let's define a class which conforms to our protocol
class C: P {
var b: Bool
required init(b: Bool) {
self.b = b
}
}
and see what happens in next snippet
let c1: C? = C(b: false)
let c2 = C(b: false)
dump(c1)
dump(c2)
/*
init defined in extension
- nil
▿ C #0
- b: false
*/
the same expression C(b: false) give us two different results :-). So, be careful if you define init and init? via protocol extension. If you try to define init and init? with the same parameters without protocol extension, the compiler will complain.