Implement initial store-migration functionality

Shared/LocalStorageManager.swift

@@ -10,6 +10,17 @@ final class LocalStorageManager {
     public static var standard = LocalStorageManager()
     public let container: NSPersistentContainer
 
+    private var oldStoreURL: URL {
+        let appSupport = FileManager.default.urls(for: .applicationSupportDirectory, in: .userDomainMask).first!
+        return appSupport.appendingPathComponent("LocalStorageModel.sqlite")
+    }
+
+    private var sharedStoreURL: URL {
+        let id = "group.com.abunchtell.writefreely"
+        let groupContainer = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: id)!
+        return groupContainer.appendingPathComponent("LocalStorageModel.sqlite")
+    }
+
     init() {
         // Set up the persistent container.
         container = NSPersistentContainer(name: "LocalStorageModel")
@@ -56,6 +67,29 @@ final class LocalStorageManager {
             print("Error: Failed to purge cached collections.")
         }
     }
+
+    func migrateStore(for container: NSPersistentContainer) {
+        let coordinator = container.persistentStoreCoordinator
+
+        guard let oldStore = coordinator.persistentStore(for: oldStoreURL) else {
+            return
+        }
+
+        do {
+            try coordinator.migratePersistentStore(oldStore,
+                                                   to: sharedStoreURL,
+                                                   options: nil,
+                                                   withType: NSSQLiteStoreType)
+        } catch {
+            fatalError("Something went wrong migrating the store: \(error)")
+        }
+
+        do {
+            try FileManager.default.removeItem(at: oldStoreURL)
+        } catch {
+            fatalError("Something went wrong while deleting the old store: \(error)")
+        }
+    }
 }
 
 private extension LocalStorageManager {