form_encode.swift

import Foundation

let queryItems = [
	NSURLQueryItem(name: "username", value: "so&ffes"),
	NSURLQueryItem(name: "password", value: "123456")
]

func formEncode(queryItems: [NSURLQueryItem]) -> String {
	let characterSet = NSMutableCharacterSet.alphanumericCharacterSet()
	characterSet.addCharactersInString("-._~")

	return queryItems.flatMap { item -> String? in
		guard var output = item.name.stringByAddingPercentEncodingWithAllowedCharacters(characterSet) else { return nil }

		output += "="

		if let value = item.value?.stringByAddingPercentEncodingWithAllowedCharacters(characterSet) {
			output += value
		}

		return output
	}.joinWithSeparator("&")
}

formEncode(queryItems) // username=so%26ffes&password=123456