observer pattern

Author: xiao cheng

CMakeLists.txt

@@ -32,4 +32,5 @@ if(Boost_FOUND)
     add_subdirectory(Interpreter)
     add_subdirectory(IteratorPattern)
     add_subdirectory(Visitor)
+    add_subdirectory(Observer)
 endif()

Observer/CMakeLists.txt

@@ -0,0 +1,21 @@
+cmake_minimum_required(VERSION 3.8)
+
+project(Observer)
+
+include_directories(${Boost_INCLUDE_DIRS})
+
+add_executable(Observer_example1 example1.cpp)
+target_link_libraries(Observer_example1 PUBLIC gtest_main ${Boost_LIBRARIES})
+install(TARGETS Observer_example1 RUNTIME DESTINATION ../bin)
+
+add_executable(Observer_example2 example2.cpp)
+target_link_libraries(Observer_example2 PUBLIC gtest_main ${Boost_LIBRARIES})
+install(TARGETS Observer_example2 RUNTIME DESTINATION ../bin)
+
+
+
+
+
+add_executable(Observer_test observer_test.cpp)
+target_link_libraries(Observer_test PUBLIC gtest_main ${Boost_LIBRARIES})
+install(TARGETS Observer_test RUNTIME DESTINATION ../bin)

Observer/example1.cpp

@@ -0,0 +1,73 @@
+#include <iostream>
+#include <vector>
+
+using namespace std;
+
+class Observer
+{
+  public:
+    virtual void update(int value) = 0;
+};
+
+class Subject
+{
+    int m_value;
+    vector<Observer*> m_views;
+
+  public:
+    void attach(Observer *obs)
+    {
+        m_views.push_back(obs);
+    }
+    void set_val(int value)
+    {
+        m_value = value;
+        notify();
+    }
+    void notify()
+    {
+        for (int i = 0; i < m_views.size(); ++i)
+            m_views[i]->update(m_value);
+    }
+};
+
+class DivObserver : public Observer
+{
+    int m_div;
+
+  public:
+    DivObserver(Subject *model, int div)
+    {
+        model->attach(this);
+        m_div = div;
+    }
+    /* virtual */ void update(int v)
+    {
+        cout << v << " div " << m_div << " is " << v / m_div << '\n';
+    }
+};
+
+class ModObserver : public Observer
+{
+    int m_mod;
+
+  public:
+    ModObserver(Subject *model, int mod)
+    {
+        model->attach(this);
+        m_mod = mod;
+    }
+    /* virtual */ void update(int v)
+    {
+        cout << v << " mod " << m_mod << " is " << v % m_mod << '\n';
+    }
+};
+
+int main()
+{
+    Subject subj;
+    DivObserver divObs1(&subj, 4);
+    DivObserver divObs2(&subj, 3);
+    ModObserver modObs3(&subj, 3);
+    subj.set_val(14);
+}

Observer/example2.cpp

@@ -0,0 +1,93 @@
+#include <iostream>
+#include <vector>
+using namespace std;
+
+// independent subject, only coupled with base class: Observer
+class Subject
+{
+    vector<class Observer *> views;
+    int value;
+
+  public:
+    void attach(Observer *obs)
+    {
+        views.push_back(obs);
+    }
+    void setVal(int val)
+    {
+        value = val;
+        notify();
+    }
+    int getVal()
+    {
+        return value;
+    }
+    void notify();
+};
+
+class Observer
+{
+    //dependent functionality
+    Subject *model;
+    int denom;
+
+  public:
+    Observer(Subject *mod, int div)
+    {
+        model = mod;
+        denom = div;
+        //Observers register themselves with the Subject
+        model->attach(this);
+    }
+    virtual void update() = 0;
+
+  protected:
+    Subject *getSubject()
+    {
+        return model;
+    }
+    int getDivisor()
+    {
+        return denom;
+    }
+};
+
+void Subject::notify()
+{
+    //broadcasts
+    for (int i = 0; i < views.size(); i++)
+        views[i]->update();
+}
+
+// View
+class DivObserver : public Observer
+{
+  public:
+    DivObserver(Subject *mod, int div) : Observer(mod, div) {}
+    void update()
+    {
+        // 6. "Pull" information of interest
+        int v = getSubject()->getVal(), d = getDivisor();
+        cout << v << " div " << d << " is " << v / d << '\n';
+    }
+};
+
+class ModObserver : public Observer
+{
+  public:
+    ModObserver(Subject *mod, int div) : Observer(mod, div) {}
+    void update()
+    {
+        int v = getSubject()->getVal(), d = getDivisor();
+        cout << v << " mod " << d << " is " << v % d << '\n';
+    }
+};
+
+int main()
+{
+    Subject subj;
+    DivObserver divObs1(&subj, 4); // 7. Client configures the number and
+    DivObserver divObs2(&subj, 3); //    type of Observers
+    ModObserver modObs3(&subj, 3);
+    subj.setVal(14);
+}

Observer/example3.cpp

@@ -0,0 +1,111 @@
+#include <string>
+#include <vector>
+#include <iostream>
+
+
+
+using namespace std;
+
+
+#include <boost/any.hpp>
+using namespace boost;
+
+namespace
+{
+
+struct Person; // subject
+
+struct PersonListener // base observer
+{
+    virtual ~PersonListener() = default;
+
+    virtual void person_changed(Person &p, const string &property_name, const any new_value) = 0;
+};
+
+struct Person
+{
+    explicit Person(int age)
+    
+        : age(age)
+    {
+    }
+
+    int get_age() const
+    {
+        return age;
+    }
+
+    void set_age(const int age);
+
+    bool get_can_vote() const
+    {
+        return age >= 16;
+    }
+
+    void subscribe(PersonListener *pl);
+
+    void notify(const string &property_name, const any new_value);
+
+  private:
+    int age;
+    vector<PersonListener *> listeners;
+};
+
+void Person::set_age(const int age)
+{
+    if (this->age == age)
+        return;
+
+    auto old_c_v = get_can_vote();
+
+    this->age = age;
+    notify("age", this->age);
+
+    auto new_c_v = get_can_vote();
+    if (old_c_v != new_c_v)
+    {
+        notify("can_vote", new_c_v);
+    }
+}
+
+void Person::subscribe(PersonListener *pl)
+{
+    listeners.push_back(pl);
+}
+
+void Person::notify(const string &property_name, const any new_value)
+{
+    for (const auto listener : listeners)
+        listener->person_changed(*this, property_name, new_value);
+}
+
+struct ConsoleListener : PersonListener
+{
+    void person_changed(Person &p, const string &property_name, const any new_value) override
+    {
+        cout << "person's " << property_name << " has been changed to ";
+        if (property_name == "age")
+        {
+            cout << any_cast<int>(new_value);
+        }
+        else if (property_name == "can_vote")
+        {
+            cout << any_cast<bool>(new_value);
+        }
+        cout << "\n";
+    }
+};
+
+int main()
+{
+    Person p{14};
+    ConsoleListener cl;
+    p.subscribe(&cl);
+    p.set_age(15);
+    p.set_age(16);
+
+    getchar();
+    return 0;
+}
+
+} // namespace