Related articles:
Swift Advancement (Built-in Set Type) - A Record of the Growth of a Non-disciplinary Ban Nung

2017.08.09

sequence

Meeting the requirements of the Sequence protocol is very simple. All you need to do is provide a makeIterator() method that returns iterators:

protocol Sequence{
    
    associatedtype Iterator: IteratorProtocol
    func makeIterator() -> Iterator
    
}

iterator

protocol IteratorProtocol{
    
    associatedtype Element
    mutating func next() -> Element?
    
}

Essentially, the for loop is a short form of the following code:

var some = [1, 2, 3, 4]
var iterator = some.makeIterator()
//For loop is short for this code
while let element = iterator.next() {
    print(element)
}

You can also create a wireless, never-ending sequence. The simplest case is an iterator that keeps returning the same value.

struct ConstantIterator: IteratorProtocol{
    
    typealias Element = Int
    mutating func next() -> Int? {
        return 1
    }
}

var iter = ConstantIterator()
//while let x = iter.next() {
//    print(x)
//    // This is a dead cycle.
//}

Next, let's look at a more interesting example: FibsIterator iterator can produce a Fibonacci sequence.

struct FibsIterator:IteratorProtocol{
    
    var state = (0,1)
    //Adding mutating modifiers to modify your state
    mutating func next() -> Int?{
        
        let upNum = state.0
        state = (state.1, state.0 + state.1)
        return upNum
        
    }
}

var fibs = FibsIterator()
while let fib = fibs.next() {
    //Stop judging, or it will cycle to death.
    guard fib < 145 else {
        break
    }
    print(fib)
}

Compliance with Sequence Protocol

struct PrefixIterator: IteratorProtocol{
    
    let string: String
    var offset: String.Index
    
    init(string:String) {
        self.string = string
        offset = string.startIndex
    }
    
    mutating func next() -> String? {
    
        guard offset < string.endIndex else {
            return nil
        }
        offset = string.index(after: offset)
        return string[string.startIndex..<offset]
        
    }
}


//Remember to block the custom protocol at the beginning, otherwise error will be reported: type'PrefixSequence'does not conform to protocol'Sequence'
struct PrefixSequence: Sequence{
    
    let string: String
    
    func makeIterator() -> PrefixIterator{
        return PrefixIterator.init(string: string)
    }
    
}

var prefix = PrefixSequence.init(string: "Hello")
for char in prefix {
    print(char)
}

var pre = PrefixIterator.init(string: "World")
while let p = pre.next() {
    print(p)
}

//Replace all with capitals
let ma = prefix.map({ $0.uppercased() })
ma

Iterator and Value Semantics

let seq = stride(from: 0, to: 10, by: 1)
var i1 = seq.makeIterator()
i1.next()  //0
i1.next()  //1

var i2 = i1
i2.next()  //2
i2.next()  //3
i1.next()  //2
i1.next()  //3
//i1 and i2 are separate and independent.

//An Iterator Example without Value Semantics
var i3 = AnyIterator(i1)
var i4 = i3

i3.next()  //4
i4.next()  //5
i3.next()  //6
i4.next()  //7
//Sharing an iterator

Function-based iterators and sequences
Fibonacci iterator is defined by a function that returns AnyIterator:

func fibsIterator() -> AnyIterator<Int>{
    
    var state = (0, 1)
    return AnyIterator{
        let upNum = state.0
        state = (state.1, state.0 + state.1)
        return upNum
    }
    
}

let fibsSequence = AnySequence.init(fibsIterator())
Array.init(fibsSequence.prefix(10))  //[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
Array.init(fibsSequence.prefix(2))  //[55, 89]

let randomNumbers = sequence(first: 100) { (previous: UInt32) in
    
    let newValue = arc4random_uniform(previous)
    guard newValue > 0 else {
        return nil
    }
    return newValue
}
Array.init(randomNumbers)  //[100, 14, 8, 6, 2]

let fibs2 = sequence(state: (0, 1)) { (state: inout(Int, Int)) -> Int? in
    
    let upNum = state.0
    state = (state.1, state.0 + state.1)
    return upNum
    
}

Array.init(fibs2.prefix(10))  //[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
Array.init(fibs2.prefix(3))  //[0, 1, 1]

Reading makes me doze off. = Search found that the information conversion rate is not good, or indeed the code foundation is too poor!!!