add Observer pattern

observer/Observer.cpp

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+/*
+ * C++ Design Patterns: Observer
+ * Author: Jakub Vojvoda [github.com/JakubVojvoda]
+ * 2016
+ *
+ * Source code is licensed under MIT License
+ * (for more details see LICENSE)
+ *
+ */
+
+#include <iostream>
+#include <vector>
+
+class Subject;
+
+/*
+ * Observer
+ * defines an updating interface for objects that should be notified
+ * of changes in a subject
+ */
+class Observer {
+public:
+  virtual int getState() = 0;
+  virtual void update(Subject *subject) = 0;
+  // ...
+};
+
+/*
+ * Concrete Observer
+ * stores state of interest to ConcreteObserver objects and
+ * sends a notification to its observers when its state changes
+ */
+class ConcreteObserver : public Observer {
+public:
+  ConcreteObserver(int state)
+    : observer_state(state) {}
+
+  int getState() {
+    return observer_state;
+  }
+
+  void update(Subject *subject);
+  // ...
+
+private:
+  int observer_state;
+  // ...
+};
+
+/*
+ * Subject
+ * knows its observers and provides an interface for attaching
+ * and detaching observers
+ */
+class Subject {
+public:
+  void attach(Observer *observer) {
+    observers.push_back(observer);
+  }
+
+  void detach(int index) {
+    observers.erase(observers.begin() + index);
+  }
+
+  void notify() {
+    for (unsigned int i = 0; i < observers.size(); i++) {
+      observers.at(i)->update(this);
+    }
+  }
+
+  virtual int getState() = 0;
+  virtual void setState(int s) = 0;
+  // ...
+
+private:
+  std::vector<Observer *> observers;
+  // ...
+};
+
+/*
+ * Concrete Subject
+ * stores state that should stay consistent with the subject's
+ */
+class ConcreteSubject : public Subject {
+public:
+  int getState() {
+    return subject_state;
+  }
+
+  void setState(int s) {
+    subject_state = s;
+  }
+  // ...
+
+private:
+  int subject_state;
+  // ...
+};
+
+void ConcreteObserver::update(Subject *subject) {
+  observer_state = subject->getState();
+  std::cout << "Observer state updated." << std::endl;
+}
+
+
+int main()
+{
+  ConcreteObserver observer1(1);
+  ConcreteObserver observer2(2);
+
+  std::cout << "Observer 1 state: " << observer1.getState() << std::endl;
+  std::cout << "Observer 2 state: " << observer2.getState() << std::endl;
+
+  Subject *subject = new ConcreteSubject;
+  subject->attach(&observer1);
+  subject->attach(&observer2);
+
+  subject->setState(10);
+  subject->notify();
+
+  std::cout << "Observer 1 state: " << observer1.getState() << std::endl;
+  std::cout << "Observer 2 state: " << observer2.getState() << std::endl;
+
+  return 0;
+}

observer/README.md

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+## Observer
+
+Observer defines a one-to-many dependency between objects so that when one object 
+changes state, all its dependents are notified and updated automatically. The pattern
+has behavioral purpose and applies to the objects.
+
+### When to use
+
+* when an abstraction has two aspects, one dependent on the other
+* when a change to one object requires changing others, and you don't know how many objects need to be changed
+* when an object should be able to notify other objects without making assumptions about who these objects are