Change some types from class to struct to avoid data racing (#33)

* Change the type of ChangePack from class to struct
* Change ``Change`` and types confirming ``Operation``  from class to struct

Sources/Document/Change/Change.swift

@@ -19,11 +19,11 @@ import Foundation
 /**
  * `Change` represents a unit of modification in the document.
  */
-class Change {
+struct Change {
     private var id: ChangeID
 
     // `operations` represent a series of user edits.
-    private let operations: [Operation]
+    private var operations: [Operation]
 
     // `message` is used to save a description of the change.
     private let message: String?
@@ -58,11 +58,14 @@ class Change {
     /**
      * `setActor` sets the given actor.
      */
-    func setActor(_ actorID: ActorID) {
-        self.operations.forEach {
-            $0.setActor(actorID)
+    mutating func setActor(_ actorID: ActorID) {
+        let operations = self.operations.map {
+            var new = $0
+            new.setActor(actorID)
+            return new
         }
 
+        self.operations = operations
         self.id.setActor(actorID)
     }
 

Sources/Document/Change/ChangePack.swift

@@ -19,7 +19,7 @@ import Foundation
 /**
  * `ChangePack` is a unit for delivering changes in a document to the remote.
  */
-class ChangePack {
+struct ChangePack {
     /**
      * `documentKey` is the key of the document.
      */
@@ -41,7 +41,7 @@ class ChangePack {
      * `minSyncedTicket` is the minimum logical time taken by clients who attach
      * the document. It used to collect garbage on the replica on the client.
      */
-    private var minSyncedTicket: TimeTicket?
+    private let minSyncedTicket: TimeTicket?
 
     init(key: String, checkpoint: Checkpoint, changes: [Change], snapshot: Data? = nil, minSyncedTicket: TimeTicket? = nil) {
         self.documentKey = key

Sources/Document/Document.swift

@@ -148,9 +148,14 @@ public actor Document {
      *
      */
     func setActor(_ actorID: ActorID) {
-        for change in self.localChanges {
-            change.setActor(actorID)
+        let changes = self.localChanges.map {
+            var new = $0
+            new.setActor(actorID)
+            return new
         }
+
+        self.localChanges = changes
+
         self.changeID.setActor(actorID)
 
         // TODOs also apply into root.

Sources/Document/Operation/AddOperation.swift

@@ -19,7 +19,7 @@ import Foundation
 /**
  * `AddOperation` is an operation representing adding an element to an Array.
  */
-class AddOperation: Operation {
+struct AddOperation: Operation {
     let parentCreatedAt: TimeTicket
     var executedAt: TimeTicket
     private var previousCreatedAt: TimeTicket

Sources/Document/Operation/MoveOperation.swift

@@ -19,7 +19,7 @@ import Foundation
 /**
  * `MoveOperation` is an operation representing moving an element to an Array.
  */
-class MoveOperation: Operation {
+struct MoveOperation: Operation {
     let parentCreatedAt: TimeTicket
     var executedAt: TimeTicket
     private var previousCreatedAt: TimeTicket

Sources/Document/Operation/Operation.swift

@@ -18,8 +18,9 @@ import Foundation
 
 /**
  * `Operation` represents an operation to be executed on a document.
+ *  Types confiming ``Operation`` must be struct to avoid data racing.
  */
-protocol Operation: AnyObject {
+protocol Operation {
     var parentCreatedAt: TimeTicket { get }
     var executedAt: TimeTicket { get set }
 
@@ -58,7 +59,7 @@ extension Operation {
     /**
      * `setActor` sets the given actor to this operation.
      */
-    func setActor(_ actorID: ActorID) {
+    mutating func setActor(_ actorID: ActorID) {
         self.executedAt.setActor(actorID)
     }
 }

Sources/Document/Operation/RemoveOperation.swift

@@ -19,7 +19,7 @@ import Foundation
 /**
  * `RemoveOperation` is an operation representing removes an element from Container.
  */
-class RemoveOperation: Operation {
+struct RemoveOperation: Operation {
     let parentCreatedAt: TimeTicket
     var executedAt: TimeTicket
     private var createdAt: TimeTicket

Sources/Document/Operation/SetOperation.swift

@@ -20,7 +20,7 @@ import Foundation
  * `SetOperation` represents an operation that stores the value corresponding to the
  * given key in the Object.
  */
-class SetOperation: Operation {
+struct SetOperation: Operation {
     let parentCreatedAt: TimeTicket
     var executedAt: TimeTicket
     private let key: String

Tests/Document/Change/ChangeTests.swift

@@ -42,7 +42,7 @@ class ChangeTests: XCTestCase {
 
         let changeID = ChangeID(clientSeq: 1, lamport: 2, actor: self.actorId)
 
-        let target = Change(id: changeID, operations: [setOperation])
+        var target = Change(id: changeID, operations: [setOperation])
 
         XCTAssertEqual(target.getOperations()[0].executedAt.getStructureAsString(), "8:actor-1:0")