Add "for in" support to iterate over Swift custom classes - swift

Question

As we know, we can use the for..in loop to iterate across Arrays or Dictionaries. However, I would like to iterate over my own CustomClass like this:
for i in CustomClass {
someFunction(i)
}
What operations/protocols does CustomClass have to support for this to be possible?

Answer 1

Say you have a class "Cars" that you want to be able to iterate over using a for..in loop:
let cars = Cars()
for car in cars {
println(car.name)
}
The simplest way is to use AnyGenerator with the classes like this:
class Car {
var name : String
init(name : String) {
self.name = name
}
}
class Cars : SequenceType {
var carList : [Car] = []
func generate() -> AnyGenerator<Car> {
// keep the index of the next car in the iteration
var nextIndex = carList.count-1
// Construct a AnyGenerator<Car> instance, passing a closure that returns the next car in the iteration
return anyGenerator {
if (nextIndex < 0) {
return nil
}
return self.carList[nextIndex--]
}
}
}
To try a complete working sample add the two classes above and then try to use them like this, adding a couple of test items:
let cars = Cars()
cars.carList.append(Car(name: "Honda"))
cars.carList.append(Car(name: "Toyota"))
for car in cars {
println(car.name)
}
That's it, simple.
More info: http://lillylabs.no/2014/09/30/make-iterable-swift-collection-type-sequencetype

Answer 2

All of the above answers can be a little tricky. If you have an array in your class, which you want to iterate over (like in #Lee Whitney's answer), there's a much simpler way to implement it. You have the following class, CustomClass:
class CustomClass: SequenceType {
let array: [String]
init(array: [String]) {
self.array = array
}
func generate() -> IndexingGenerator<[String]> {
return array.generate()
}
}
Simple as that. Tested to work in the latest Xcode version (6.1 at the time of writing), and iOS 8.1.2. This code should be stable through future versions, though.
P.S. With generics, you can easily do your own Array replica by following this pattern, and only implement the methods which you want.

Answer 3

#Matt Gibson is correct. However, I would like to add more information for future reference.
From Advanced Swift:
This code:
for x in someSequence {
...
}
Is converted into this:
var __g = someSequence.generate()
while let x = __g.next() {
...
}
Therefore, one must adopt Sequence, which gives the class generate() and next(). Here are these protocols:
protocol Generator {
typealias Element
mutating func next() -> Element?
}
protocol Sequence {
typealias GeneratorType : Generator
func generate() -> GeneratorType
}

Answer 4

That would be the SequenceType protocol, and its related Generator protocol.
The SequenceType protocol says that the class must implement generate(), which returns something that conforms to the Generator protocol, which is the bit that does the actual work; the Generator protocol is the one with the all-important next() method.
There's an example of implementing it to allow for..in in the WWDC 2014 video "Advanced Swift" (in the generics example "A Simple Generic Stack", starting around slide 183.)
The basic info on which protocol to implement for for..in is in the Statements section of the documentation, which gives a brief overview of SequenceType and Generator

Answer 5

NOTE AnyGenerator and SequenceType are old types that do not exist in recent versions. You need to implement Sequence protocol now.
There are two ways to implement Sequence.
Conform to Sequence, IteratorProtocol at the same time by just implementing next() method. This approach is the simplest and there is an example in the headers. See Sequence.swift. Implementing both protocols at the same time could be non-realistic or not fulfill your needs. (It might prevent two different instances to be iterated at the same time, etc)
Conform to Sequence and return an object that implements IteratorProtocol. I think this is the most common case in real world classes (when things become a bit complicated, not Hello Worlds). There is also an example in the Sequence.swift
Below is an example of approach 2. An custom class (Linked List) which is iterable:
/// Linked List Node:
public class LinkedListNode <T> {
public internal(set) var value: T
public internal(set) var next: LinkedListNode<T>?
internal init(_ value: T) {
self.value = value
self.next = nil
}
}
/// Linked List with append method only.
public class LinkedList<T> {
public internal(set) var first: LinkedListNode<T>? = nil
public internal(set) var last: LinkedListNode<T>? = nil
/// Appends a new node.
public func append(_ value: T) {
if first == nil {
first = LinkedListNode(value)
last = first
} else {
last.next = LinkedListNode(value)
last = last.next
}
}
}
Finally the Sequence implementation
/// Sequence protocol adoption. It allows `for ... in` and a bunch of other methods too.
extension LinkedList: Sequence {
/// Iterator implementation
public class Iterator<T>: IteratorProtocol {
/// Maintain a ref to current element so next element can be reached
var cur: LinkedListNode<T>?
/// IteratorProtocol protocol requirement
public func next() -> T? {
let res = cur?.value
cur = cur?.next
return res
}
}
/// Sequence protocol requirement
public func makeIterator() -> Iterator<T> {
let g = LinkedList.Iterator()
g.cur = first
return g
}
}
Usage:
let linkedList = LinkedList<Int>()
linkedList.append(3)
linkedList.append(6)
linkedList.append(9)
linkedList.append(12)
for element in linkedList {
print(element)
}
let odds = linkedList.filter { return $0 % 2 == 0 }
print(odds)

Answer 6

The accepted answer is correct, and up until recently was the accepted way to address this. However, given the introduction of Protocol Extensions in Swift 2.0, rather than conformance to SequenceTypeand implementing func generate() -> GeneratorOf<Car> there is now an abstract base class that handles the implementation of this functionality for you called AnyGenerator<T> (see Apple docs) since GeneratorOf<T> no longer exists.
What this means is that you can simply subclass this abstract base class, and by doing do, inherit all of the functionality of the aforementioned protocol conformance:
class Cars: AnyGenerator<Car> {
private var carList = [Car]()
private var currentIndex:Int
...
}
One then need only override the next() method declared by the GeneratorType protocol (which AnyGenerator<T> also conforms to) in order to define the desired iteration behavior:
class Cars: AnyGenerator<Car> {
private var carList = [Car]()
private var currentIndex:Int
override func next() -> Car? {
if (currentIndex < self.carList.count) {
currentIndex++
return self.carList[currentIndex-1]
} else {
currentIndex = 0;
return nil
}
}
}