Chapter 14 : Series On Design Patterns – Observer Pattern

Topics at a glance:

Let us observe a subject closely
how we can improvise our Document editor application with observer pattern
Document editor application is now 2.0 !

In previous chapter we saw command behavioral pattern. In this chapter we will improvise our Application – Document editor using Observer pattern.

Observer pattern in C++

Observer in simple terms is a contract between two entities called Subject and Observer. Its core philosophy is created on a subscription-based notification model, where the observers are the subscribers who are interested in the changes happening in a subject.

In the Document editor, you might have seen that whenever a document is closed, it will notify the application. But in the previous version presented in Chapter 13, I used basic object pointer (pointer to Application) and a function call back using member function pointer. The purpose was to notify the application about a document close. What all things to do when the subscriber receives such a notification depends on the use-case. Say in our Application class, Application can prompt the user to re-open the closed document or open another new document for editing and so on.

This use-case where observers are notified by the subject about any of its specific updates could be generalized and designed with the help of Observer Design pattern. So, here we are making subjects and its observers as objects. Subject instances must implement the interfaces such as notify(), attach(Observer *) and detach(Observer *) , while the Observer instances must implement the interface update(current_state val). This is like a contract between the subject and the observers and unlike chapter 13’s implementation of a similar functionality, here we can subscribe, unsubscribe and get updates. Also, in Observer pattern we build this framework on top of object instances rather than isolated call back functions.

Now, let us see how we can improvise our Document editor application with the help of Observer pattern. Here the Observer is Application instance and is interested only when, the state of it subject i.e. Document instance changes to closed. When Application instance is notified about a document close, it prompts the user to open a new document.

#include <iostream>
#include <string>
#include <memory>
#include <vector>

using namespace std;

class Command
{
public:
    virtual ~Command(){}
    virtual void execute() = 0;
};

class Menu_Item
{
private:
    unique_ptr<Command> command_object_;
    string name_;
public:
    Menu_Item(const char* const name) : 
        name_{name},
        command_object_{nullptr}
    {
    }
    
    ~Menu_Item()
    {
        cout << "Menu_Item : " << name_ << " destroyed" << endl;
        command_object_.reset();        
    }
    
    void add_command_object(Command *command_object)
    {
        command_object_.reset(command_object);
    }
    
    int click()
    {
        int status = -1;
        if(command_object_ != nullptr)
        {
            command_object_->execute();
            status = 0;
        }
        
        return status;
    }
    
    string& get_name()
    {
        return name_;
    }
};

class Menu
{
private:
    string name_;
    vector<unique_ptr<Menu_Item>> menu_items;
public:
    Menu(const char* const name): 
        name_{name}
    {
        
    }
    
    ~Menu()
    {
        cout << "Menu " << name_ << " destroyed" << endl;
        for (auto &item : menu_items)
        {
            item.reset();
        }
    }
    
    void add_menu_item(unique_ptr<Menu_Item> menu_item)
    {
        menu_items.push_back(std::move(menu_item));
    }
    
    int hover()
    {
        int choice;
        int count = 0;
        
        for (auto &item : menu_items)
        {
            cout << item->get_name() << " : " << count++ << endl;
        }
        
        cout << "Enter your Menu Item choice : ";
        cin >> choice;
        
        if(   (choice >= 0) 
            &&(choice <= menu_items.size()) )
        {
            return menu_items[choice]->click();
        }
        
    }
    
    // overloaded version 
    // used for mimicking a menu item click by the application  
    int hover(int choice)
    {
        return menu_items[choice]->click();
    }
    
    string& get_name()
    {
        return name_;
    }
    
};

class Subject;

enum class doc_states{created = 0, opened, closed, changed, moved};

class Observer
{
public:
    ~Observer(){}
    virtual void update(doc_states current_state) = 0;
};

class Subject
{
public:
    ~Subject(){}
    virtual void notify() = 0;
    virtual void attach(Observer *this_observer) = 0;
    virtual void dettach(Observer *this_observer) = 0;
};

class Document : public Subject 
{
private:
    string name_;
    doc_states current_state;
    vector<Observer*> observers;    
public:
    typedef void (Document::*doc_function)();
    Document(): current_state{doc_states::created}
    { 
        /* do nothing */ 
    }
    
    ~Document()
    {
        if(   (current_state != doc_states::closed)
            &&(current_state != doc_states::created) )
        {
            // i.e. a document is open. So close it.
            close();
        }
    }
    
    void open()
    {
        cout << "Enter the file name to open" << endl;
        cin >> name_;
        current_state = doc_states::opened;
        cout << "Document : " << name_ << " is open" << endl;
    }
    
    void close()
    {
       if(   (current_state != doc_states::closed)
            &&(current_state != doc_states::created) )
        {
            current_state = doc_states::closed;
            cout << "Document : " << name_ << " is closed" << endl;
            name_.clear();
            // notify the subscribed observers 
            notify();
        }
        else
        {
            cout << "No documents are open" << endl;
        }
    }
    
    //void copy(int start, int end, string &copied_text)
    void copy()
    {
        if(current_state != doc_states::closed)
        {
            cout << "copying..." << endl;
            // copy from start to end to copied_text
            cout << "copy complete" << endl;
        }
        else
        {
            cout << "No documents are open" << endl;
        }
    }
    
    //void paste(int position, string& insert_text)
    void paste()
    {
        if(current_state != doc_states::closed)
        {
            cout << "pasting..." << endl;
            cout << "pasting complete" << endl;
        }
        else
        {
            cout << "No documents are open" << endl;
        }
    }
    
    //void cut(int start, int end, string &extracted_text)
    void cut()
    {
        if(current_state != doc_states::closed)
        {
            cout << "cutting..." << endl;
            // copy from start to end to copied_text
            cout << "cut complete" << endl;
        }
        else
        {
            cout << "No documents are open" << endl;
        }
    }
    
    // support for Observer pattern
    void notify() override
    {
        for( auto &observer : observers )
        {
            observer->update(current_state);
        }
    }
    
    void attach(Observer *this_observer) override
    {
        observers.push_back(this_observer);
    }
    
    void dettach(Observer *this_observer) override
    {
        int position = 0;
        auto begin = observers.begin();
        auto end = observers.end();
        
        for( auto itr = begin; itr < end; ++itr )
        {
            auto obs = *itr;
            if(this_observer == obs)
            {
                observers.erase(itr);
            }
        }
    }
    
};

class Open_Document_Command : public Command
{
private:
    shared_ptr<Document> document_;
    Document::doc_function function;

public:
    Open_Document_Command(shared_ptr<Document> document, Document::doc_function call_back):
        document_{document}, function{call_back}
    {
        
    }
    void execute() override
    {
        (document_.get()->*function)();
    }
};

class Close_Document_Command : public Command
{
private:
    shared_ptr<Document> document_;
    Document::doc_function function;

public:
    Close_Document_Command(shared_ptr<Document> document, Document::doc_function call_back):
        document_{document}, function{call_back}
    {
        
    }
    void execute() override
    {
        (document_.get()->*function)();
    }
};


class Copy_Document_Command : public Command
{
private:
    shared_ptr<Document> document_;
    Document::doc_function function;

public:
    Copy_Document_Command(shared_ptr<Document> document, Document::doc_function call_back):
        document_{document}, function{call_back}
    {
        
    }
    void execute() override
    {
        (document_.get()->*function)();
    }
};

class Paste_Document_Command : public Command
{
private:
    shared_ptr<Document> document_;
    Document::doc_function function;

public:
    Paste_Document_Command(shared_ptr<Document> document, Document::doc_function call_back):
        document_{document}, function{call_back}
    {
        
    }
    void execute() override
    {
        (document_.get()->*function)();
    }
};

class Cut_Document_Command : public Command
{
private:
    shared_ptr<Document> document_;
    Document::doc_function function;

public:
    Cut_Document_Command(shared_ptr<Document> document, Document::doc_function call_back):
        document_{document}, function{call_back}
    {
        
    }
    
    void execute() override
    {
        (document_.get()->*function)();
    }
};


class Application : public Observer
{
private:
    vector<unique_ptr<Menu>> menus;
    shared_ptr<Document> current_doc;
    enum class signal{app_launched = 0, app_exit, user_interrupt, other, no_trigger};
    signal trigger;
    
    void prompt_to_open_new_document()
    {
        // prompt the user 
        cout << "Open a document for editing" << endl;        
        return;
    }
    
public:
    Application()
    {
        string doc_name;
        trigger = signal::app_launched;
        
        cout << "Document Editor App, Version : 2.0" << endl;
        
        shared_ptr<Document> document = make_shared<Document>();
        current_doc = document;

        // Attach this 'Application' as an Observer of document 
        (document.get())->attach(this);
        
        // 2. add Menus "File" and "Edit"
        unique_ptr<Menu> file_menu = make_unique<Menu>("File");
        unique_ptr<Menu_Item> open_item = make_unique<Menu_Item>("Open");
        Open_Document_Command *open_cmd_object = new Open_Document_Command(document, &Document::open);
        open_item->add_command_object(open_cmd_object);
        
        file_menu->add_menu_item(std::move(open_item));
        
        unique_ptr<Menu_Item> close_item = make_unique<Menu_Item> ("Close");
        Close_Document_Command *close_cmd_object = new Close_Document_Command(document, &Document::close);
        close_item->add_command_object(close_cmd_object);
        
        file_menu->add_menu_item(std::move(close_item));
        
        unique_ptr<Menu> edit_menu = make_unique<Menu>("Edit");
        unique_ptr<Menu_Item> copy_item = make_unique<Menu_Item>("Copy");
        Copy_Document_Command *copy_cmd_object = new Copy_Document_Command(document, &Document::copy);
        copy_item->add_command_object(copy_cmd_object);
        
        edit_menu->add_menu_item(std::move(copy_item));
        
        unique_ptr<Menu_Item> paste_item = make_unique<Menu_Item>("Paste");
        Paste_Document_Command *paste_cmd_object = new Paste_Document_Command(document, &Document::paste);
        paste_item->add_command_object(paste_cmd_object);
        
        edit_menu->add_menu_item(std::move(paste_item));
        
        unique_ptr<Menu_Item> cut_item = make_unique<Menu_Item>("Cut");
        Cut_Document_Command *cut_cmd_object = new Cut_Document_Command(document, &Document::cut);
        cut_item->add_command_object(cut_cmd_object);
        
        edit_menu->add_menu_item(std::move(cut_item));
        
        menus.push_back(std::move(file_menu));
        menus.push_back(std::move(edit_menu));
    }
    
    int select_menu()
    {
        int choice = 0;
        int end_app = 0;
        cout << "Enter your Menu choice" << endl;
        for(auto &menu : menus)
        {
            cout << menu->get_name() << " : " << choice++ << endl;
        }
        end_app = choice;
        cout << "Exit : " << end_app << endl;
        
        cin >> choice;
        
        if(  (choice >= 0)
           &&(choice < menus.size()))
        {
            menus[choice]->hover();
        }
        else if(choice == end_app)
        {
            // set the trigger 
            trigger = signal::app_exit;
            end_application();
            return 1;
        }
        
        return 0;
    }
    // To support Observer pattern
    void update(doc_states current_state)
    {
        cout << "Application : document close() detected" << endl;
        switch(current_state)
        {
            case (doc_states::closed):
            {
                // open new document for editing by prompting the user
                if(trigger != signal::app_exit)
                {
                    prompt_to_open_new_document();
                }
                break;
            }
            default:
            {
                // do nothing
                break;
            }
        }
        return;
    }
    
    void end_application()
    {
        // do clean up
        // release back the memory resources to free store 
        for(auto &menu : menus)
        {
            menu.reset();
        }
        // close the current_doc
        current_doc.reset();
        
        cout << "Application exiting..." << endl;
        return;
    }
    
    ~Application()
    {
        cout << "Inside Application destructor" << endl;
    }
    
};

int main()
{
    Application new_app;
    int end_app = 0;
    
    while(end_app == 0)
    {
        end_app = new_app.select_menu();
    }
    
    cout << "Main Ends" << endl; 
}

Want to see this code in action?

Document Editor App, Version : 2.0
Enter your Menu choice
File : 0
Edit : 1
Exit : 2
0
Open : 0
Close : 1
Enter your Menu Item choice : 0
Enter the file name to open
hello
Document : hello is open
Enter your Menu choice
File : 0
Edit : 1
Exit : 2
0
Open : 0
Close : 1
Enter your Menu Item choice : 1
Document : hello is closed
Application : document close() detected
Open a document for editing
Enter your Menu choice
File : 0
Edit : 1
Exit : 2
2
Menu File destroyed
Menu_Item : Open destroyed
Menu_Item : Close destroyed
Menu Edit destroyed
Menu_Item : Copy destroyed
Menu_Item : Paste destroyed
Menu_Item : Cut destroyed
Application exiting...
Main Ends
Inside Application destructor

Observer in action:

Here you can see that when the User choose to close the document, application gets notified and prompts the user to open a file to edit. Finally, when User chooses to exit, Application does the clean-up as usual.

Other improvisations.

This document editor app (version 2.0) also fixed the issue with user selecting an operation when no documents are open or try to close a document which is not open. Want to see these features?

Document Editor App, Version : 2.0
Enter your Menu choice
File : 0
Edit : 1
Exit : 2
0
Open : 0
Close : 1
Enter your Menu Item choice : 1
No documents are open
Enter your Menu choice
File : 0
Edit : 1
Exit : 2
0
Open : 0
Close : 1
Enter your Menu Item choice : 0
Enter the file name to open
hello
Document : hello is open
Enter your Menu choice
File : 0
Edit : 1
Exit : 2
0
Open : 0
Close : 1
Enter your Menu Item choice : 1
Document : hello is closed
Application : document close() detected
Open a document for editing
Enter your Menu choice
File : 0
Edit : 1
Exit : 2
1
Copy : 0
Paste : 1
Cut : 2
Enter your Menu Item choice : 0
No documents are open
Enter your Menu choice
File : 0
Edit : 1
Exit : 2
2
Menu File destroyed
Menu_Item : Open destroyed
Menu_Item : Close destroyed
Menu Edit destroyed
Menu_Item : Copy destroyed
Menu_Item : Paste destroyed
Menu_Item : Cut destroyed
Application exiting...
Main Ends
Inside Application destructor

You can see that the Application tells the User that “No documents are open” when any operation is invoked when no document is already open.

Enjoyed the chapter. Let me know in the comments below. Thanks 🙂

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Chapter 14 : Series On Design Patterns – Observer Pattern

Observer pattern in C++

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Observer pattern in C++

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