Add NetworkService class

Because:
- Abstracting collection classes to this file
- Includes LocalCollection class now

Req partial for #4

AnnotateAR/NetworkService.swift

@@ -0,0 +1,82 @@
+//
+//  NetworkService.swift
+//  AnnotateAR
+//
+//  Created by Tyler Franklin on 3/30/20.
+//  Copyright © 2020 Tyler Franklin. All rights reserved.
+//
+
+import Firebase
+import FirebaseFirestore
+import Foundation
+
+protocol DocumentSerializable {
+    init?(dictionary: [String: Any])
+}
+
+final class LocalCollection<T: DocumentSerializable> {
+    private(set) var items: [T]
+    private(set) var documents: [DocumentSnapshot] = []
+    let query: Query
+
+    private let updateHandler: ([DocumentChange]) -> Void
+
+    private var listener: ListenerRegistration? {
+        didSet {
+            oldValue?.remove()
+        }
+    }
+
+    var count: Int {
+        return items.count
+    }
+
+    subscript(index: Int) -> T {
+        return items[index]
+    }
+
+    init(query: Query, updateHandler: @escaping ([DocumentChange]) -> Void) {
+        items = []
+        self.query = query
+        self.updateHandler = updateHandler
+    }
+
+    func index(of document: DocumentSnapshot) -> Int? {
+        for i in 0 ..< documents.count {
+            if documents[i].documentID == document.documentID {
+                return i
+            }
+        }
+
+        return nil
+    }
+
+    func listen() {
+        guard listener == nil else { return }
+        listener = query.addSnapshotListener { [unowned self] querySnapshot, error in
+            guard let snapshot = querySnapshot else {
+                print("Error fetching snapshot results: \(error!)")
+                return
+            }
+            let models = snapshot.documents.map { (document) -> T in
+                if let model = T(dictionary: document.data()) {
+                    return model
+                } else {
+                    // handle error
+                    fatalError("Unable to initialize type \(T.self) with dictionary \(document.data())")
+                }
+            }
+            self.items = models
+            self.documents = snapshot.documents
+            self.updateHandler(snapshot.documentChanges)
+        }
+    }
+
+    func stopListening() {
+        listener = nil
+    }
+
+    deinit {
+        stopListening()
+    }
+}