Map function explanation - arrays

Question

Someone can explain me that piece of code because I can't understand well. I find this code and I can't understand notably this line : Room(dict: $0)
var rooms: [Room] = [] // The globale variable
func refresh() {
let request = URLRequest(url: URL(string: "\(Config.serverUrl)/rooms")!)
NSURLConnection.sendAsynchronousRequest(request, queue: OperationQueue.main, completionHandler: { resp, data, err in
guard err == nil else {
return
}
let rooms = try! JSONSerialization.jsonObject(with: data!, options: JSONSerialization.ReadingOptions()) as! [[String: AnyObject]]
self.rooms = rooms.map {
Room(dict: $0) // I can't understand this line
}
self.tableView.reloadData()
})
}
My Room struct:
struct Room {
var key: String
var title: String
var cat: String!
init(dict: [String: AnyObject]) {
title = dict["title"] as! String
key = dict["key"] as! String
cat = dict["cat"] as! String
}
init(key: String, title: String, cat: String) {
self.key = key
self.title = title
self.cat = cat
}
func toDict() -> [String: AnyObject] {
return [
"title": title as AnyObject,
"key": key as AnyObject,
"cat": cat as AnyObject
]
}
}
If someone can help me to understand and explain it, thank you

Answer 1

The map function loops over every item in a collection, and applies an operation to each element in the collection.
This piece of code
self.rooms = rooms.map {
Room(dict: $0)
}
is a short form of this.
// `dict` paramater is `$0` in shorter form
self.rooms = rooms.map { (dict : [String: AnyObject]) -> Room in
return Room(dict: dict)
}