How can i construct multiple HTTP requests in Swift?

Question

I'm quite new to Swift and programming altogether. I am very keen on learning all the right ways. So any additional tips or remarks are always appreciated.

I'm doing a HTTP request to an api and that works fine. The problem is that it's limited to 100 results per request. Theres's an optional offset and a limit i can set. If i give a limit of 101 i get a server error saying: "Bad Request: Invalid value specified for limit. Maximum allowed value is 100." The total is 101, so i need to do at least two requests. Only after receiving the total data of all the requests i want to populate my tableview. This is what i have:

class Book {

var id: Int
var title: String
let description: String
var coverImage: String
var isbn: String
var publisherID: Int
var publisherName: String
var authorID: Int
var authorFirstName: String
var authorLastName: String

class func getDataFromJson(completionHandler: ([Book]) -> ()) {

    var books = [Book]()

    let session = NSURLSession(configuration: NSURLSessionConfiguration.defaultSessionConfiguration())

    let request = NSURLRequest(URL: NSURL(string: "http://example.website.nl/books/highlighted")!)

    let task: NSURLSessionDataTask = session.dataTaskWithRequest(request) { (data, response, error) -> Void in

        if let data = data {

            do {
                let json = try NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments)

                if let booksFromResult = json["books"] as? [[String: AnyObject]] {
                    for book in booksFromResult {
                        let bookID = book["id"] as! Int
                        let bookTitle = book["title"] as! String
                        let bookDescription = book["description"] as! String
                        let bookCoverImage = book["cover_url"] as! String
                        let bookISBN = book["isbn"] as! String
                        if let bookPublisher = book["publisher"] as? [String: AnyObject] {
                            let bookPublisherID = bookPublisher["id"] as! Int
                            let bookPublisherName = bookPublisher["name"] as! String
                            if let bookAuthor = book["author"] as? [String: AnyObject] {
                                let bookAuthorID = bookAuthor["id"] as! Int
                                let bookAuthorFirstname = bookAuthor["first_name"] as! String
                                let bookAuthorLastName = bookAuthor["last_name"] as! String
                                books.append(Book(id: bookID, title: bookTitle, description: bookDescription, coverImage: bookCoverImage, isbn: bookISBN, publisherID: bookPublisherID, publisherName: bookPublisherName, authorID: bookAuthorID, authorFirstName: bookAuthorFirstname, authorLastName: bookAuthorLastName))
                            }
                        }

                    }
                    print(books.count)

                }
                dispatch_async(dispatch_get_main_queue(),{
                    completionHandler(books)
                })
            } catch {
                print("error serializing JSON: \(error)")
            }

        }
    }
    task.resume()
}




init(id: Int, title: String, description: String, coverImage: String, isbn: String, publisherID: Int, publisherName: String, authorID: Int, authorFirstName: String, authorLastName: String) {

    self.id = id
    self.title = title
    self.description = description
    self.coverImage = coverImage
    self.isbn = isbn
    self.publisherID = publisherID
    self.publisherName = publisherName
    self.authorID = authorID
    self.authorFirstName = authorFirstName
    self.authorLastName = authorLastName
  }
}

I have been trying to solve this for more than 24 hours. I have really searched here and on the web for an example. The little i found here couldn't help me.

My thoughts are of how this should be done:

1. make first request -> store data somewhere
2. make second request -> add data to stored data
3. make last request -> add data to stored data
4. send data to populate tableview.

Should i use an array of urls and iterate through it and than append the data somewhere?

I hope someone can help me. I would really appreciate it.

Thanks in advance.

Answer 1

Direct Answer to the Question:

// Heavily based on the video I recommended. Watch it for a great explanation
struct Resource<A>{
    let url: NSURL
    let parse: (NSData) -> [A]?
}

extension Book {

    // You could figure a way to dynamically populate this based on limiting
    static let urls = [NSURL(string: "http://example.website.nl/books/highlighted")!,
                       NSURL(string: "http://example.website.nl/books/highlighted2")!]

    // Creates an array of Requests passing in each url for the url, but the same parse function
    static let requests = urls.map { Resource<Book>(url: $0, parse: Book.parse) }

    // Used by Webservice (from the Resource struct) to parse the data into a Book
    static let parse: (NSData?) -> [Book]? = { data in
        guard let data = data else { return nil }

        guard let json = try? NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments) else {
            print("Error deserializing json.")
            return nil
        }
        var books: [Book]? = nil
        guard let jsonBooks = json["books"] as? [[String: AnyObject]] else { return nil }
        for jsonBook in jsonBooks {
            guard let book = Book(fromJson: jsonBook) else { continue } // skips nil books from failable initializer, depends on how you want to handle that
            books = books ?? [Book]() // if nil create a new array, if not use the old one
            books!.append(book)
        }
        return books
    }
}

class Webservice {

    // A stands for a generic type. You could add a type called Publisher later and use the same function
    // This adopted from the video I showed you so it's a little more in depth
    func loadAll<A>(resources: [Resource<A>], completion: [A] -> ()) {
        let session = NSURLSession(configuration: NSURLSessionConfiguration.defaultSessionConfiguration())
        var currentRequest = 0 // used to keep track of asynchronous callback order
        var allA = [A]()
        for resource in resources {
            session.dataTaskWithURL(resource.url) { (data, _, _) in
                defer {
                    currentRequest += 1 // marks that we're done with one request

                    // This check ensures that we only call the completion handler
                    // after we're done with the last request
                    if currentRequest == resources.count {
                        completion(allA)
                    }
                }
                guard let data = data else { return }

                // this parse function comes from the resource struct passed in.
                // It converts the data we get back from one request into an array of books.
                guard let manyA = resource.parse(data) else { return }

                // This is the total running tally of books from all our requests.
                allA.appendContentsOf(manyA)
            }
        }
    }
}

class TableViewController: UITableViewController {

    var books = [Book]() {
        didSet { tableView.reloadData() }
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        // Call site
        Webservice().loadAll(Book.requests) { [weak self] (books) in
            dispatch_async(dispatch_get_main_queue()) {
                self?.books.appendContentsOf(books)
            }
        }

    }

    //... all your normal methods for cells and stuff
}

Extra

I made a failable initializer for your Book class based on a JSON object so that your class method doesn't have to do the bulk of the parsing. You probably want your Book type to be a struct to gain the memberwise initializer and pass by value semantics)

Take advantage of guard let else control statements to avoid the pyramid of doom from optional unwrapping.

defer statements are useful to call a completion handler once no matter how you exit your scope (avoid duplication of code).

I highly recommend this video showing a webservice api design. It's a bit advanced but it shows you a great way initialize model objects from a webservice.

class Book {

    var id: Int
    var title: String
    let description: String
    var coverImage: String
    var isbn: String
    var publisherID: Int
    var publisherName: String
    var authorID: Int
    var authorFirstName: String
    var authorLastName: String

    init(id: Int, title: String, description: String, coverImage: String, isbn: String, publisherID: Int, publisherName: String, authorID: Int, authorFirstName: String, authorLastName: String) {

        self.id = id
        self.title = title
        self.description = description
        self.coverImage = coverImage
        self.isbn = isbn
        self.publisherID = publisherID
        self.publisherName = publisherName
        self.authorID = authorID
        self.authorFirstName = authorFirstName
        self.authorLastName = authorLastName
    }

    typealias JSONDictionary = [String: AnyObject] // syntactic sugar, makes it clearer

    convenience init?(fromJson json: JSONDictionary) {
        let bookID = json["id"] as! Int
        let bookTitle = json["title"] as! String
        let bookDescription = json["description"] as! String
        let bookCoverImage = json["cover_url"] as! String
        let bookISBN = json["isbn"] as! String

        // I would use guard let else statements here to avoid the pyramid of doom but it's stylistic
        if let bookPublisher = json["publisher"] as? [String: AnyObject] {
            let bookPublisherID = bookPublisher["id"] as! Int
            let bookPublisherName = bookPublisher["name"] as! String
            if let bookAuthor = json["author"] as? [String: AnyObject] {
                let bookAuthorID = bookAuthor["id"] as! Int
                let bookAuthorFirstname = bookAuthor["first_name"] as! String
                let bookAuthorLastName = bookAuthor["last_name"] as! String
                self.init(id: bookID, title: bookTitle, description: bookDescription, coverImage: bookCoverImage, isbn: bookISBN, publisherID: bookPublisherID, publisherName: bookPublisherName, authorID: bookAuthorID, authorFirstName: bookAuthorFirstname, authorLastName: bookAuthorLastName)
                return

            }
        }
        return nil
    }

}

extension Book {

    class func getDataFromJson(completionHandler: ([Book]) -> ()) {

        var books = [Book]()

        let session = NSURLSession(configuration: NSURLSessionConfiguration.defaultSessionConfiguration())

        let request = NSURLRequest(URL: NSURL(string: "http://example.website.nl/books/highlighted")!)

        let task: NSURLSessionDataTask = session.dataTaskWithRequest(request) { (data, response, error) -> Void in

            defer { // no matter how you exit the scope this will be called
                dispatch_async(dispatch_get_main_queue()) {
                    completionHandler(books)
                }
            }

            guard let data = data else { return } // still will call the deferred completion handler

            guard let json = try? NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments) else {
                print("Error deserializing json.")
                return // still will call the deferred completion handler
            }

            if let jsonBooks = json["books"] as? [[String: AnyObject]] {
                for jsonBook in jsonBooks {
                    guard let book = Book(fromJson: jsonBook) else { continue } // skips nil books from failable initializer, depends on how you want to handle that
                    books.append(book)
                }
                print(books.count)
            }
        }
        task.resume()
        // call the deferred completion handler after leaving scope
    }
}