Add state pattern

closes #10

README.md

@@ -14,6 +14,7 @@ The goal is to have a set of [GOF patterns](http://www.blackwasp.co.uk/gofpatter
   - [Iterator](behavioral/iterator.cr)
   - [Memento](behavioral/memento.cr)
   - [Observer](behavioral/observer.cr)
+  - [State](behavioral/state.cr)
   - [Strategy](behavioral/strategy.cr)
   - [Template](behavioral/template.cr)
   - [Visitor](behavioral/visitor.cr)

behavioral/state.cr

@@ -0,0 +1,38 @@
+# Defines a manner for controlling communication between classes or entities.
+# The state pattern is used to alter the behaviour of an object as its internal
+# state changes.
+
+class GameContext
+  property state : State
+
+  def initialize(@state)
+  end
+
+  def toggle
+    @state.handle(self)
+  end
+end
+
+abstract class State
+  abstract def handle(context : GameContext)
+end
+
+class PauseState < State
+  def handle(context : GameContext)
+    puts "Pause -> Play"
+    context.state = PlayState.new
+  end
+end
+
+class PlayState < State
+  def handle(context : GameContext)
+    puts "Play -> Pause"
+    context.state = PauseState.new
+  end
+end
+
+initial_state = PlayState.new
+context = GameContext.new initial_state
+
+context.toggle # Play -> Pause
+context.toggle # Pause -> Play