EventBus

Simple and very fast event bus. The EventBus library is a convenient realization of the observer pattern. It works perfectly to supplement the implementation of MVC logic (model-view-controller) in event-driven UIs

EventBus was created because I want something easy to use and faster than CCNotificationCenter from cocos2d-x library. Of course C++11 support was mandatory.

EventBus main goals:

• Fast
• Easy to use
• Strongly typed
• Free
• tiny (~15 KB)
• Decouples notification senders and receivers

Sample

You can checkout sample/
If you want to play with sample online checkout this link: wandbox.org

Usage

0. Store bus

// store it in controller / singleton / std::sharted_ptr whenever you want
Dexode::EventBus bus;

1. Define events

namespace Event // optional namespace
{
	struct Gold
	{
		int goldReceived = 0;
	};
	
	struct OK {}; // Simple event when user press "OK" button
}

2. Subscribe

// ...
bus.listen<Event::Gold>
			([](const auto& event) // listen with lambda
			{
				std::cout << "I received gold: " << event.goldReceived << "!" << std::endl;
			});
			
HudLayer* hudLayer;
// Hud layer will receive info about gold
bus.listen<Event::Gold>(std::bind(&HudLayer::onGoldReceived,hudLayer,std::placeholders::_1));

3. Notify

//Inform listeners about event
bus.notify(Event::Gold{12}); // 1 way

bus.notify<Event::Gold>({12}); // 2 way

Event::Gold myGold{12};
bus.notify(myGold); // 3 way

Lambda listener:

struct SampleEvent {};
Dexode::EventBus bus;
//...
int token = bus.listen<SampleEvent>([](const SampleEvent& event) // register listener
{
});

//If we want unlisten exact listener we can use token for it
bus.unlistenAll(token);

Listener is identified by token. Token is returned from EventBus::listen methods. We can register multiple listeners on one token:

Dexode::EventBus bus;
struct SampleEvent {};
//...
int token = bus.listen<SimpleEvent>([](const auto& event) // register listener
{
});

bus.listen<SimpleEvent>(token, [](const auto& event) // another listener
{
});

bus.unlistenAll(token);//Now those two lambdas will be removed from listeners

If you don't want to handle manually with token you can use EventCollector class. It is useful when we want to have multiple listeners in one class. So above example could look like this:

Dexode::EventBus bus;
struct SampleEvent {};
Dexode::EventCollector collector{&bus};
//...
collector.listen<SampleEvent>([](const SampleEvent& event) // register listener
{
});

collector.listen<SampleEvent>([](const SampleEvent& event) // another listener
{
});

collector.unlistenAll();//Now those two lambdas will be removed from listeners

Or as component of class:

class Example
{
public:
	Example(const std::shared_ptr<Dexode::EventBus>& bus)
			: _collector{bus}
	{
		_collector.listen<SimpleEvent>(std::bind(&Example::onEvent1, this, std::placeholders::_1));
		_collector.listen<OtherEvent>(std::bind(&Example::onEvent2, this, std::placeholders::_1));
	}

	void onEvent1(const SimpleEvent& event)
	{
	}

	void onEvent2(const OtherEvent& event)
	{
	}

private:
	Dexode::EventCollector _collector;// use RAII
};

//EventCollector sample
std::shared_ptr<Dexode::EventBus> bus;
Example ex{bus};
//...
bus.notify<int>("event1", 2);

Add to your project

EventBus can be added as ADD_SUBDIRECTORY to your cmake file. Then simply link it via TARGET_LINK_LIBRARIES
Example:

ADD_SUBDIRECTORY(lib/EventBus)
ADD_EXECUTABLE(MyExecutable
		main.cpp
		)

TARGET_LINK_LIBRARIES(MyExecutable PUBLIC Dexode::EventBus)

Also if you want you can install library and add it at any way you want.
Eg.

mkdir Release && cd Release
cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=./install ..
make && make install

Now in Release/install library is placed.

Performance

I have prepared some performance results. You can read about them here
Small example:

check10NotificationsFor1kListeners                                     263 ns        263 ns    2668786 sum=-1.76281G
check10NotificationsFor1kListeners_CCNotificationCenter              11172 ns      11171 ns      62865 sum=54.023M

checkNotifyFor10kListenersWhenNoOneListens                              18 ns         18 ns   38976599 sum=0
checkNotifyFor10kListenersWhenNoOneListens_CCNotificationCenter     127388 ns     127378 ns       5460 sum=0

Future plans

• Write more and better tests
• Thread safe EventBus ?
• Verbose messages for easy debugging ?
• Generating graph flow ?
• ...

Thanks to

• staakk for fixing windows ;) 53d5026
• kuhar for his advice and suggestions for EventBus
• swietlana for english correction and support ;)
• ruslo for this great example: https://github.com/forexample/package-example

License

EventBus source code can be used according to the Apache License, Version 2.0.
For more information see LICENSE file