Fetcher.Swift

//                 .-'
//            '--./ /     _.---.
//            '-,  (__..-`       \
//               \          .     |
//                `,.__.   ,__.--/
//                  '._/_.'___.-`

import Foundation
import ReactiveCocoa
import Orcinus
import RealmSwift

/// Uses download signal to fetch objects from API endpoint
/// and adds them to the realm. It sends a notification through
/// its update signal every time new items are downloaded so
/// observers know to refresh their realm query
public class Fetcher<T: Object> {
    /// Give the fetcher a name so it can be identified
    public let name: String?
    /// Subscribe to this signal to be notified of updates.
    /// The signal will always send `true`. It is just so
    /// you know when you might refresh relevant realm queries.
    public let updateSignal: Signal<Bool, NoError>
    
    private let downloadSignal: Signal<T, Error>
    private let sink: SinkOf<Event<Bool, NoError>>
    private let namePrefix: String
    
    public var debug = false
    
    public init(name: String?=nil, downloadSignal: Signal<T, Error>) {
        self.name = name
        self.namePrefix = name.map { "\($0) - " } ?? "" // either "MyFetcherName - " or ""
        
        (updateSignal, sink) = Signal<Bool, NoError>.pipe()
        self.downloadSignal = downloadSignal
        self.downloadSignal.observe(error: handleError, completed: handleCompleted, interrupted: handleInterrupted, next: handleNext)
    }
    
    private func handleError(error: Error) {
        if debug {
            ("\(namePrefix)Fetching error: \(error.nsError.localizedDescription)")
        }
    }
    
    private func handleCompleted() {
        if debug {
            print("\(namePrefix)Completed.")
        }
        sendCompleted(sink)
    }
    
    private func handleInterrupted() {
        if debug {
            print("\(namePrefix)Fetching interrupted")
        }
    }
    
    /// Adds the object to the realm and forwards the value down
    /// through the publically exposed signal
    private func handleNext(value: T) {
        if debug {
            print("\(namePrefix)Fetched: \(value)")
        }
        
        let realm = Realm()
        realm.write {
            realm.add(value, update: true)
        }
        
        sendNext(self.sink, true)
    }
}