Exemplo how to use GeneratorType

PrefixGenerator.swift

/// Prefix Class Generator
class StringPrefixGenerator: GeneratorType {
	typealias Element = String
	let string: String
	var offset: String.Index
	
	init(string:String) {
		self.string = string
		self.offset = string.startIndex
	}
	
	/**
	Get always the next letter at the sentence and return new prefix based on previous one
	- returns: returns new prefix
	*/
	func next() -> Element? {
		guard offset < string.endIndex else {
			return nil
		}
		
		offset = offset.successor()
		return string[ string.startIndex..<offset ]
	}
}

let prefixGenerator = StringPrefixGenerator(string: "Douglas Mendes Barreto")

prefixGenerator.next() //print D
prefixGenerator.next() //print Do
prefixGenerator.next() //print Dou