nicks_observer.cpp

// C++11 style observer pattern implementation using lambda for scoped 
// connection management.
//
//  Copyright (c) 2012 Nick C.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

#include <iostream>
#include <list>
#include <functional>

class EventBroadcaster;
class EventListener;

//Connection class
class Connection
{
public:
	Connection(std::function<void ()> _disconnect_handler) :disconnect_handler(_disconnect_handler) {}

	//Handle the disconnect housekeeping when the connection is deleted
	~Connection(){disconnect_handler();}	
	
private:
	std::function<void ()> disconnect_handler;
};

// Pure virtual listener base class declartion
class EventListener
{
public:
	virtual void eventCallback(EventBroadcaster*) =0;				
	void connect(EventBroadcaster& b); // Some nicer syntax	
	void disconnect();
	bool isConnected();
	
private:
	std::unique_ptr<Connection> c = nullptr; //C++11 allows these to be set without initializer list
	void connect(Connection* _c); // This connection syntax is just to ugly and is set to private because of it!!
};


// Broadcaster base class
class EventBroadcaster
{
public:
	~EventBroadcaster()
	{
		removeAllEventListeners(); //sever all connections on delete
	}
	
	Connection* addEventListener(EventListener* l_ptr)
	{	
		// Check to see if listener already added
		if( !binary_search (listenerList.begin(), listenerList.end(), l_ptr) )
		{
			listenerList.push_back(l_ptr);
			//Need to capture by making a copy [=] - wacky things happened with [&] - l_ptr gets munglerated
			return new Connection([=](){this->removeEventListener(l_ptr);});
		} else {
			return nullptr;
		}
		
	}
	
	void removeEventListener(EventListener* l_ptr)
	{
		// This fololwing command informs the connection object of the disconnect
		// We don't get into an infinite loops because the connection is already null after the first call
		l_ptr->disconnect();
		listenerList.remove(l_ptr);		
	}
	
	void removeAllEventListeners()
	{
		// Can't use clear because we need to tell each listener it has been disconnected
		// Can't use a range-based for on a container with a changing size!
		while(listenerList.size()>0){
			removeEventListener(listenerList.back());
		}
	}
	
	void notify()
	{
		for(auto l_ptr : listenerList) {
			if(l_ptr){l_ptr->eventCallback(this);}
		}
	}
	
private:
	std::list<EventListener*> listenerList;
};


//Event listener defninitions
void EventListener::connect(Connection* _c)
{
	if(_c) {// Need this if statement or we get a nullptr if this connection already registered
		disconnect(); // Sever any previous connection (these listeners can only observe one object)
		c.reset(_c);
	}			
}

void EventListener::connect(EventBroadcaster& b) // Some nicer syntax
{
	connect(b.addEventListener(this));
}

void EventListener::disconnect()
{		
	if(c) {c.reset(nullptr);} // No need to null a null							
}

bool EventListener::isConnected()
{
	return (c!=nullptr);
}
	

//========================================================
// Simple demo stuff from here down
//========================================================

// DEMO Notifier is an event broadcaster
class Notifier : public EventBroadcaster
{
public:
	int get() const {return i;};
	void set(int _i) {
		i=_i; 
		notify();
	};
	
private:
	int i = 0; //C++11 nicety
};

// DEMO Responder is an EventListener
class Responder : public EventListener
{
public:
	Responder(std::string _name = std::string()) : name(_name){}
	
	~Responder()
	{
		std::cout << "Responder "<< name <<" deleted" << std::endl;
	}		
	
	void eventCallback(EventBroadcaster* b_ptr)
	{
		auto Notifier_ptr = static_cast<Notifier*>(b_ptr);
		std::cout << name << " notified that value is " << Notifier_ptr->get() << std::endl;
	}
	
private:
	std::string name;	
};

// Main function purely for demonstration
int main (int argc, char const *argv[])
{	
	Notifier n;
	Responder r1("r1");
		
	// Test #1 connect and signal	
	r1.connect(n);
	n.set(5);
	
	// Test #2 disconnect at the responder end
	r1.disconnect();	
	n.set(7);
	
	// Test #3 reconnect multiple times
	r1.connect(n);
	r1.connect(n);
	r1.connect(n);
	n.set(9999999);
	
	// Test #4 disconnect at the broadcaster
	n.removeEventListener(&r1);	
	n.removeEventListener(&r1); //Check it can be done more than once	
	n.set(7);
	
	// Test #5 multiple listeners
	Responder  r2("r2"), r3("r3");
	r3.connect(n);
	r2.connect(n);
	r1.connect(n.addEventListener(&r1)); //Can still use this ugly syntax if you want
	n.set(321);
	
	
	n.removeAllEventListeners();
	n.set(2);
	
	// Test #6 multiple listeners again after a remove all
	r3.connect(n);
	r2.connect(n);
	r1.connect(n);
	n.set(4444);
	
	// Test #7 this is the cool one showing automatic scoped connection management
	{
		std::unique_ptr<Responder> r(  new Responder("r_heap")  );
		r->connect(n);
		n.set(11);
		n.set(55); 
	}
	
	n.set(88888888); //No heap listener (lambda magic at work)
			
	return 0;
}