added observer example

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

@@ -0,0 +1,41 @@
+//: [Previous](@previous)
+
+import Foundation
+
+protocol Observer : class {
+    func willChange(propertyName: String, newPropertyValue: Any?)
+    func didChange(propertyName: String, oldPropertyValue: Any?)
+}
+
+final class Observable {
+
+    weak var observer: Observer?
+    private let observablePropertyName = "observableProperty"
+
+    var observableProperty: Int = 0 {
+        willSet(newValue) {
+            observer?.willChange(propertyName: observablePropertyName, newPropertyValue: newValue)
+        }
+        didSet {
+            observer?.didChange(propertyName: observablePropertyName, oldPropertyValue: oldValue)
+        }
+    }
+}
+
+final class Spy : Observer {
+    func willChange(propertyName: String, newPropertyValue: Any?) {
+       print("will change to value \(newPropertyValue)")
+    }
+
+    func didChange(propertyName: String, oldPropertyValue: Any?) {
+        print("changed from oldValue \(oldPropertyValue)")
+    }
+}
+
+let observable = Observable()
+let observer = Spy()
+observable.observer = observer
+
+observable.observableProperty = 200
+
+//: [Next](@next)

Behavioral.playground/contents.xcplayground

@@ -6,5 +6,7 @@
         <page name='Iterator'/>
         <page name='Strategy'/>
         <page name='Mediator'/>
+        <page name='Observer'/>
+        <page name='Visitor'/>
     </pages>
 </playground>

GOFSwift.xcworkspace/xcshareddata/IDEWorkspaceChecks.plist

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>IDEDidComputeMac32BitWarning</key>
+	<true/>
+</dict>
+</plist>

README.md

@@ -13,9 +13,9 @@
 * ✅ Strategy
 * ✅ Mediator
 * ✅ Visitor
+* ✅ Observer
 * ❌ Memento
 * ❌ Interpreter
-* ❌ Observer
 * ❌ State
 
 ## Creational