Game Objects Talking To Each Other [closed]

Question

EDIT: Below I describe a basic event messaging system I have used over and over. And it occurred to me that both school projects are open source and on the web. You can find the second version of this messaging system (and quite a bit more) at http://sourceforge.net/projects/bpfat/ .. Enjoy, and read below for a more thorough description of the system!

I've written a generic messaging system and introduced it into a handful of games that have been released on the PSP as well as some enterprise level application software. The point of the messaging system is to pass only the data around that is needed for processing a message or event, depending on the terminology you want to use, so that objects do not have to know about each other.

A quick rundown of the list of objects used to accomplish this is something along the lines of:

struct TEventMessage
{
    int _iMessageID;
}

class IEventMessagingSystem
{
    Post(int iMessageId);
    Post(int iMessageId, float fData);
    Post(int iMessageId, int iData);
    // ...
    Post(TMessageEvent * pMessage);
    Post(int iMessageId, void * pData);
}

typedef float(*IEventMessagingSystem::Callback)(TEventMessage * pMessage);

class CEventMessagingSystem
{
    Init       ();
    DNit       ();
    Exec       (float fElapsedTime);

    Post       (TEventMessage * oMessage);

    Register   (int iMessageId, IEventMessagingSystem* pObject, FObjectCallback* fpMethod);
    Unregister (int iMessageId, IEventMessagingSystem* pObject, FObjectCallback * fpMethod);
}

#define MSG_Startup            (1)
#define MSG_Shutdown           (2)
#define MSG_PlaySound          (3)
#define MSG_HandlePlayerInput  (4)
#define MSG_NetworkMessage     (5)
#define MSG_PlayerDied         (6)
#define MSG_BeginCombat        (7)
#define MSG_EndCombat          (8)

And now a bit of an explanation. The first object, TEventMessage, is the base object to represent data sent by the messaging system. By default it will always have the Id of the message being sent so if you want to make sure you have received a message you were expecting you can (Generally I only do that in debug).

Next up is the Interface class that gives a generic object for the messaging system to use for casting while doing callbacks. Additionally this also provides an 'easy to use' interface for Post()ing different data types to the messaging system.

After that we have our Callback typedef, Simply put it expects an object of the type of the interface class and will pass along a TEventMessage pointer... Optionally you can make the parameter const but I've used trickle up processing before for things like stack debugging and such of the messaging system.

Last and at the core is the CEventMessagingSystem object. This object contains an array of callback object stacks (or linked lists or queues or however you want to store the data). The callback objects, not shown above, need to maintain (and are uniquely defined by) a pointer to the object as well as the method to call on that object. When you Register() you add an entry on the object stack under the message id's array position. When you Unregister() you remove that entry.

That is basically it. Now this does have the stipulation that everything needs to know about the IEventMessagingSystem and the TEventMessage object... but this object should Not be changing that often and only passes the parts of information that are vital to the logic dictated by the event being called. This way a player doesn't need to know about the map or the enemy directly for sending events off to it. A managed object can call an API to a larger system also, without needing to know anything about it.

For example: When an enemy dies you want it to play a sound effect. Assuming you have a sound manager that inherits the IEventMessagingSystem interface, you would set up a callback for the messaging system that would accept a TEventMessagePlaySoundEffect or something of that ilk. The Sound Manager would then register this callback when sound effects are enabled (or unregister the callback when you want to mute all sound effects for easy on/off abilities). Next, you would have the enemy object also inherit from the IEventMessagingSystem, put together a TEventMessagePlaySoundEffect object (would need the MSG_PlaySound for its Message ID and then the ID of the sound effect to play, be it an int ID or the name of the sound effect) and simply call Post(&oEventMessagePlaySoundEffect).

Now this is just a very simple design with no implementation. If you have immediate execution then you have no need to buffer the TEventMessage objects (What I used mostly in console games). If you are in a multi-threaded environment then this is a very well defined way for objects and systems running in separate threads to talk to each other, but you will want to preserve the TEventMessage objects so the data is available when processing.

Another alteration is for objects that only ever need to Post() data, you can create a static set of methods in the IEventMessagingSystem so they do not have to inherit from them (That is used for ease of access and callback abilities, not -directly- needed for Post() calls).

For all the people who mention MVC, it is a very good pattern, but you can implement it in so many different manners and at different levels. The current project I am working on professionally is an MVC setup about 3 times over, there is the global MVC of the entire application and then design wise each M V and C also is a self-contained MVC pattern. So what I have tried to do here is explain how to make a C that is generic enough to handle just about any type of M without the need to get into a View...

For example, an object when it 'dies' might want to play a sound effect.. You would make a struct for the Sound System like TEventMessageSoundEffect that inherits from the TEventMessage and adds in a sound effect ID (Be it a preloaded Int, or the name of the sfx file, however they are tracked in your system). Then all the object just needs to put together a TEventMessageSoundEffect object with the appropriate Death noise and call Post(&oEventMessageSoundEffect); object.. Assuming the sound is not muted (what you would want to Unregister the Sound Managers.

EDIT: To clarify this a bit in regards to the comment below: Any object to send or receive a message just needs to know about the IEventMessagingSystem interface, and this is the only object the EventMessagingSystem needs to know of all the other objects. This is what gives you the detachment. Any object who wants to receive a message simply Register(MSG, Object, Callback)s for it. Then when an object calls Post(MSG,Data) it sends that to the EventMessagingSystem via the interface it knows about, the EMS will then notify each registered object of the event. You could do a MSG_PlayerDied that other systems handle, or the player can call MSG_PlaySound, MSG_Respawn, etc to let things listening for those messages to act upon them. Think of the Post(MSG,Data) as an abstracted API to the different systems within a game engine.

Oh! One other thing that was pointed out to me. The system I describe above fits the Observer pattern in the other answer given. So if you want a more general description to make mine make a bit more sense, that is a short article that gives it a good description.

Hope this helps and Enjoy!

the generic solutions for communication between objects avoiding tight coupling:

1. Mediator pattern
2. Observer pattern

Here is a neat event system written for C++11 you can use. It uses templates and smart pointers as well as lambdas for the delegates. It's very flexible. Below you will also find an example. Email me at [email protected] if you have questions about this.

What these classes gives you is a way to send events with arbitrary data attached to them and an easy way to directly bind functions that accept already converted argument types that the system casts and checks for correct conversion prior to calling your delegate.

Basically, every event is derived from IEventData class (you can call it IEvent if you want). Each "frame" you call ProcessEvents() at which point the event system loops through all the delegates and calls the delegates that have been supplied by other systems that have subscribed to each event type. Anyone can pick which events they would like to subscribe to, as each event type has a unique ID. You can also use lambdas to subscribe to events like this: AddListener(MyEvent::ID(), [&](shared_ptr ev){ do your thing }..

Anyway, here is the class with all the implementation:

#pragma once

#include <list>
#include <memory>
#include <map>
#include <vector>
#include <functional>

class IEventData {
public:
    typedef size_t id_t; 
    virtual id_t GetID() = 0; 
}; 

typedef std::shared_ptr<IEventData> IEventDataPtr; 
typedef std::function<void(IEventDataPtr&)> EventDelegate; 

class IEventManager {
public:
    virtual bool AddListener(IEventData::id_t id, EventDelegate proc) = 0;
    virtual bool RemoveListener(IEventData::id_t id, EventDelegate proc) = 0; 
    virtual void QueueEvent(IEventDataPtr ev) = 0; 
    virtual void ProcessEvents() = 0; 
}; 


#define DECLARE_EVENT(type) \
    static IEventData::id_t ID(){ \
        return reinterpret_cast<IEventData::id_t>(&ID); \
    } \
    IEventData::id_t GetID() override { \
        return ID(); \
    }\

class EventManager : public IEventManager {
public:
    typedef std::list<EventDelegate> EventDelegateList; 

    ~EventManager(){
    } 
    //! Adds a listener to the event. The listener should invalidate itself when it needs to be removed. 
    virtual bool AddListener(IEventData::id_t id, EventDelegate proc) override; 

    //! Removes the specified delegate from the list
    virtual bool RemoveListener(IEventData::id_t id, EventDelegate proc) override; 

    //! Queues an event to be processed during the next update
    virtual void QueueEvent(IEventDataPtr ev) override; 

    //! Processes all events
    virtual void ProcessEvents() override; 
private:
    std::list<std::shared_ptr<IEventData>> mEventQueue; 
    std::map<IEventData::id_t, EventDelegateList> mEventListeners; 

}; 

//! Helper class that automatically handles removal of individual event listeners registered using OnEvent() member function upon destruction of an object derived from this class. 
class EventListener {
public:
    //! Template function that also converts the event into the right data type before calling the event listener. 
    template<class T>
    bool OnEvent(std::function<void(std::shared_ptr<T>)> proc){
        return OnEvent(T::ID(), [&, proc](IEventDataPtr data){
            auto ev = std::dynamic_pointer_cast<T>(data); 
            if(ev) proc(ev); 
        }); 
    }
protected:
    typedef std::pair<IEventData::id_t, EventDelegate> _EvPair; 
    EventListener(std::weak_ptr<IEventManager> mgr):_els_mEventManager(mgr){

    }
    virtual ~EventListener(){
        if(_els_mEventManager.expired()) return; 
        auto em = _els_mEventManager.lock(); 
        for(auto i : _els_mLocalEvents){
            em->RemoveListener(i.first, i.second); 
        }
    }

    bool OnEvent(IEventData::id_t id, EventDelegate proc){
        if(_els_mEventManager.expired()) return false; 
        auto em = _els_mEventManager.lock(); 
        if(em->AddListener(id, proc)){
            _els_mLocalEvents.push_back(_EvPair(id, proc)); 
        }
    }
private:
    std::weak_ptr<IEventManager> _els_mEventManager; 
    std::vector<_EvPair>        _els_mLocalEvents; 
    //std::vector<_DynEvPair> mDynamicLocalEvents; 
}; 

And the Cpp file:

#include "Events.hpp"

using namespace std; 

bool EventManager::AddListener(IEventData::id_t id, EventDelegate proc){
    auto i = mEventListeners.find(id); 
    if(i == mEventListeners.end()){
        mEventListeners[id] = list<EventDelegate>(); 
    }
    auto &list = mEventListeners[id]; 
    for(auto i = list.begin(); i != list.end(); i++){
        EventDelegate &func = *i; 
        if(func.target<EventDelegate>() == proc.target<EventDelegate>()) 
            return false; 
    }
    list.push_back(proc); 
}

bool EventManager::RemoveListener(IEventData::id_t id, EventDelegate proc){
    auto j = mEventListeners.find(id); 
    if(j == mEventListeners.end()) return false; 
    auto &list = j->second; 
    for(auto i = list.begin(); i != list.end(); ++i){
        EventDelegate &func = *i; 
        if(func.target<EventDelegate>() == proc.target<EventDelegate>()) {
            list.erase(i); 
            return true; 
        }
    }
    return false; 
}

void EventManager::QueueEvent(IEventDataPtr ev) {
    mEventQueue.push_back(ev); 
}

void EventManager::ProcessEvents(){
    size_t count = mEventQueue.size(); 
    for(auto it = mEventQueue.begin(); it != mEventQueue.end(); ++it){
        printf("Processing event..\n"); 
        if(!count) break; 
        auto &i = *it; 
        auto listeners = mEventListeners.find(i->GetID()); 
        if(listeners != mEventListeners.end()){
            // Call listeners
            for(auto l : listeners->second){
                l(i); 
            }
        }
        // remove event
        it = mEventQueue.erase(it); 
        count--; 
    }
}

I use an EventListener class for the sake of convenience as base class for any class that would like to listen to events. If you derive your listening class from this class and supply it with your event manager, you can use the very convenient function OnEvent(..) to register your events. And the base class will automatically unsubscribe your derived class from all events when it is destroyed. This is very convenient since forgetting to remove a delegate from event manager when your class is destroyed will almost certainly cause your program to crash.

A neat way to get a unique type id for an event by simply declaring a static function in the class and then casting it's address into an int. Since every class will have this method on different addresses, it can be used for unique identification of class events. You can also cast typename() to an int to get a unique id if you want. There are different ways to do this.

So here is an example on how to use this:

#include <functional>
#include <memory>
#include <stdio.h>
#include <list>
#include <map>

#include "Events.hpp"
#include "Events.cpp"

using namespace std; 

class DisplayTextEvent : public IEventData {
public:
    DECLARE_EVENT(DisplayTextEvent); 

    DisplayTextEvent(const string &text){
        mStr = text; 
    }
    ~DisplayTextEvent(){
        printf("Deleted event data\n"); 
    }
    const string &GetText(){
        return mStr; 
    }
private:
    string mStr; 
}; 

class Emitter { 
public:
    Emitter(shared_ptr<IEventManager> em){
        mEmgr = em; 
    }
    void EmitEvent(){
        mEmgr->QueueEvent(shared_ptr<IEventData>(
            new DisplayTextEvent("Hello World!"))); 
    }
private:
    shared_ptr<IEventManager> mEmgr; 
}; 

class Receiver : public EventListener{
public:
    Receiver(shared_ptr<IEventManager> em) : EventListener(em){
        mEmgr = em; 

        OnEvent<DisplayTextEvent>([&](shared_ptr<DisplayTextEvent> data){
            printf("It's working: %s\n", data->GetText().c_str()); 
        }); 
    }
    ~Receiver(){
        mEmgr->RemoveListener(DisplayTextEvent::ID(), std::bind(&Receiver::OnExampleEvent, this, placeholders::_1)); 
    }
    void OnExampleEvent(IEventDataPtr &data){
        auto ev = dynamic_pointer_cast<DisplayTextEvent>(data); 
        if(!ev) return; 
        printf("Received event: %s\n", ev->GetText().c_str()); 
    }
private:
    shared_ptr<IEventManager> mEmgr; 
}; 

int main(){
    auto emgr = shared_ptr<IEventManager>(new EventManager()); 


    Emitter emit(emgr); 
    {
        Receiver receive(emgr); 

        emit.EmitEvent(); 
        emgr->ProcessEvents(); 
    }
    emit.EmitEvent(); 
    emgr->ProcessEvents(); 
    emgr = 0; 

    return 0; 
}

This probably does not only apply to game classes but to classes in the general sense. the MVC (model-view-controller) pattern together with your suggested message pump is all you need.

"Enemy" and "Player" will probably fit into the Model part of MVC, it does not matter much, but the rule of thumb is have all models and views interact via the controller. So, you would want to keep references (better than pointers) to (almost) all other class instances from this 'controller' class, let's name it ControlDispatcher. Add a message pump to it (varies depending on what platform you are coding for), instantiate it firstly (before any other classes and have the other objects part of it) or lastly (and have the other objects stored as references in ControlDispatcher).

Of course, the ControlDispatcher class will probably have to be split down further into more specialized controllers just to keep the code per file at around 700-800 lines (this is the limit for me at least) and it may even have more threads pumping and processing messages depending on your needs.

Cheers