Add migrator

Author: ApolloZhu

README.md

@@ -73,3 +73,20 @@ var isNewUser: Bool
                             persistDefaultContent: true))
 var hasPaidBefore: Bool
 ```
+
+To migrate from another data type, you can use a migrator
+
+```swift
+@EFStorageComposition(
+    EFStorageUserDefaults<String>(forKey: "sameKey", 
+                                  defaultsTo: "Nah"),
+    EFStorageMigrate(
+        from: EFStorageUserDefaults<Int>(
+            forKey: "sameKey", 
+            defaultsTo: 1551, 
+            persistDefaultContent: true),
+        by: { number in String(number) }
+    )
+)
+var mixedType: String
+```

Sources/EFStorageCore/EFStorage.swift

@@ -64,3 +64,26 @@ public struct EFStorageComposition<A: EFStorage, B: EFStorage, Content>
         self.b = b
     }
 }
+
+/// Only for migration purposes. Setting the content/wrappedValue does nothing.
+public struct EFStorageMigrate<A: EFStorage, OldContent, Content>
+: EFStorage where A.Content == OldContent {
+    public typealias Migrator = (OldContent) -> Content
+    
+    public var wrappedValue: Content {
+        get { return content ?? migrator(a.wrappedValue) }
+        set { }
+    }
+    
+    public var content: Content? {
+        get { return a.content.map(migrator) }
+        set { }
+    }
+    
+    private var a: A
+    private var migrator: Migrator
+    public init(from a: A, by migrator: @escaping Migrator) {
+        self.a = a
+        self.migrator = migrator
+    }
+}

Tests/EFStorageTests/EFStorageTests.swift

@@ -43,6 +43,16 @@ final class EFStorageTests: XCTestCase {
                                 persistDefaultContent: true))
     var hasPaidBefore: Bool
 
+    
+    @EFStorageComposition(
+        EFStorageUserDefaults<String>(forKey: "sameKey", defaultsTo: "Nah"),
+        EFStorageMigrate(from: EFStorageUserDefaults<Int>(forKey: "sameKey",
+                                                          defaultsTo: 1551,
+                                                          persistDefaultContent: true),
+                         by: { number in String(number) })
+    )
+    var mixedType: String
+    
     var storageText: EFStorageUserDefaultsRef<String> = UserDefaults.efStorage.text
 
     func testExample() {
@@ -51,13 +61,19 @@ final class EFStorageTests: XCTestCase {
         XCTAssertEqual(_text.wrappedValue, "meow")
         _text.removeContentFromUnderlyingStorage()
         XCTAssertEqual(text, EFStorageTests.defaultText)
-        // XCTAssertEqual(text, UserDefaults.efStorageContents.text)
+        XCTAssertEqual(text, UserDefaults.efStorage.text)
         XCTAssertEqual(storageText.content, text)
         let hasPaidBeforeRef: EFStorageUserDefaultsRef<Bool> = UserDefaults.efStorage.oldHasPaidBeforeKey
         XCTAssertEqual(hasPaidBeforeRef.content, true)
         XCTAssertEqual(UserDefaults.standard.bool(forKey: "oldHasPaidBeforeKey"), true)
         debugPrint(efStorages)
         XCTAssertEqual(hasPaidBefore, true)
+        XCTAssertEqual(mixedType, "1551")
+        mixedType = "Brand New"
+        XCTAssertTrue(UserDefaults.standard.object(forKey: "sameKey") is String, "IS NOT STRING")
+        XCTAssertFalse(UserDefaults.standard.object(forKey: "sameKey") is Int, "IS NUMBER")
+        XCTAssertEqual(mixedType, "Brand New")
+        XCTAssertEqual(UserDefaults.efStorage.sameKey, "Brand New")
     }
 
     static var allTests = [