Added Memento Example Code to behavioral patterns

Author: Sebastian Boldt

Behavioral.playground/Pages/Memento.xcplaygroundpage/Contents.swift

@@ -0,0 +1,47 @@
+//: [Previous](@previous)
+
+import Foundation
+
+typealias Memento = NSDictionary
+
+protocol MementoConvertible {
+    init?(memento: Memento)
+    var memento: Memento { get }
+}
+
+struct Student: MementoConvertible {
+    var memento: Memento {
+        return ["name" : self.name, "age" : self.age]
+    }
+    
+    private var name: String
+    private var age: Int
+    
+    init?(memento: Memento) {
+        guard let name = memento.object(forKey: "name") as? String else {
+            return nil
+        }
+        
+        guard let age = memento.object(forKey: "age") as? Int else {
+            return nil
+        }
+        
+        self.name = name
+        self.age = age
+    }
+}
+
+struct Store {
+    func store(name: String, memento: Memento) {
+        let defaults = UserDefaults.standard
+        defaults.set(memento, forKey: name)
+        defaults.synchronize()
+    }
+    
+    func retrieveStudent(name: String) -> Memento? {
+        let defaults = UserDefaults.standard
+        return defaults.object(forKey: name) as? Memento
+    }
+}
+
+//: [Next](@next)

Behavioral.playground/contents.xcplayground

@@ -8,5 +8,6 @@
         <page name='Mediator'/>
         <page name='Observer'/>
         <page name='Visitor'/>
+        <page name='Memento'/>
     </pages>
 </playground>

GOFSwift.xcworkspace/xcuserdata/sebastianboldt.xcuserdatad/UserInterfaceState.xcuserstate

@@ -14,9 +14,9 @@
 * ✅ Mediator
 * ✅ Visitor
 * ✅ Observer
-* ❌ Memento
-* ❌ Interpreter
+* ✅ Memento
 * ❌ State
+* ❌ Interpreter
 
 ## Creational