Add extensions stored properties tip

codes/ExtensionStoredProperty.swift

@@ -0,0 +1,45 @@
+// Did you never asked how to add stored properties to extensions in Swift? ;)
+// One of the benfits of Objective C and key-value coding feature code 
+// is still valid in Swift is you can we could add stored values to an object at runtime.
+// Say welcome to associated object in Swift.
+
+import Foundation
+
+class MyClass {
+}
+
+// We declared the type of key as Void?, which means a pointer to anything
+private var key: Void?
+
+extension MyClass {
+    var title: String? {
+        get {
+            // Then & operator is used to get key address and send 
+            // into the method as UnsafePointer<Void> parameter.
+            // associated object API get method.
+            return objc_getAssociatedObject(self, &key) as? String
+        }
+        set {
+            // associated object API set method.
+            objc_setAssociatedObject(self, &key, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
+        }
+    }
+}
+
+// Test function
+func printTitle(input: MyClass) {
+    if let title = input.title {
+        print("Title: \(title)")
+    } else {
+        print("not set")
+    }
+}
+
+let myClassObject = MyClass()
+printTitle(input: myClassObject)
+
+myClassObject.title = "Swift Daily Tips 1"
+printTitle(input: myClassObject)
+
+myClassObject.title = "Swift Daily Tips 2"
+printTitle(input: myClassObject)