add Observer pattern

Author: JakubVojvoda

observer/Observer.py

@@ -0,0 +1,97 @@
+#
+# Python Design Patterns: Observer
+# Author: Jakub Vojvoda [github.com/JakubVojvoda]
+# 2016
+#
+# Source code is licensed under MIT License
+# (for more details see LICENSE)
+# 
+
+import sys
+   
+#
+# Observer
+# defines an updating interface for objects that should be notified
+# of changes in a subject
+#
+class Observer:
+  def getState(self):
+    pass
+    
+  def update(self, subject):
+    pass
+  
+#
+# Concrete Observer
+# stores state of interest to ConcreteObserver objects and
+# sends a notification to its observers when its state changes
+#
+class ConcreteObserver(Observer):
+  def __init__(self, state):
+    Observer.__init__(self)
+    self._state = state
+  
+  def getState(self):
+    return self._state
+  
+  def update(self, subject):
+    self._state = subject.getState()
+    print("Observer state updated.")
+  
+#
+# Subject
+# knows its observers and provides an interface for attaching
+# and detaching observers
+#
+class Subject:
+  def __init__(self):
+    self._observers = []
+
+  def attach(self, observer):
+    self._observers.append(observer)
+
+  def detach(self, index):
+    self._observers.remove(index)
+
+  def notify(self):
+    for observer in self._observers:
+      observer.update(self)
+
+  def getState(self):
+    pass
+    
+  def setState(self, state):
+    pass
+
+#
+# Concrete Subject
+# stores state that should stay consistent with the subject's
+#
+class ConcreteSubject(Subject):
+  def __init__(self):
+    Subject.__init__(self)
+    self._state = 0
+  
+  def getState(self):
+    return self._state
+
+  def setState(self, state):
+    self._state = state
+
+
+if __name__ == "__main__":  
+  observer1 = ConcreteObserver(1)
+  observer2 = ConcreteObserver(2)
+
+  print("Observer 1 state: " + str(observer1.getState()))
+  print("Observer 2 state: " + str(observer2.getState()))
+
+  subject = ConcreteSubject()
+  subject.attach(observer1)
+  subject.attach(observer2)
+
+  subject.setState(10)
+  subject.notify()
+
+  print("Observer 1 state: " + str(observer1.getState()))
+  print("Observer 2 state: " + str(observer2.getState()))