fix struct copy on write issue

Author: winddpan

CodableWrapper.podspec

@@ -8,7 +8,7 @@
 
 Pod::Spec.new do |spec|
   spec.name         = "CodableWrapper"
-  spec.version      = "0.3.2"
+  spec.version      = "0.3.3"
   spec.requires_arc = true
   spec.summary      = "Codable + PropertyWrapper"
   spec.description  = "Codable + PropertyWrapper = ☕"

Sources/CodableWrapper/Codec.swift

@@ -9,13 +9,18 @@ import Foundation
 
 @propertyWrapper
 public struct Codec<Value>: Codable {
-    let construct: CodecConstruct<Value>
+    var construct: CodecConstruct<Value>
 
     public var wrappedValue: Value {
         get {
-            construct.storedValue!
+            return construct.storedValue!
         }
         set {
+            if !isKnownUniquelyReferenced(&construct) {
+                let newConstruct = CodecConstruct<Value>(unsafed: ())
+                newConstruct.transferFrom(construct)
+                construct = newConstruct
+            }
             construct.storedValue = newValue
         }
     }

Tests/CodableWrapperTests/ExampleTest.swift

@@ -64,6 +64,14 @@ class ExampleTest: XCTestCase {
 
     override class func setUp() {
     }
+    
+    func testStructCopyOnWrite() {
+        let a = ExampleModel()
+        let valueInA = a.stringVal
+        var b = a
+        b.stringVal = "changed!"
+        XCTAssertEqual(a.stringVal, valueInA)
+    }
 
     func testBasicUsage() throws {
         let json = #"{"stringVal": "pan", "intVal": "233", "bool": "1", "animal": "cat"}"#