I have written an event system for my game. It works fine but has one big flaw - it's not typesafe thus requiring the subscribed callback function to cast the received base event manually. Now, I am trying to implement a typesafe version using templates. I am generally ok with the topic but clearly not an expert.
Firstly, here is some code demonstrating how I want to use the event:
Define the derived events
// Derived Events
class ClickEvent : public Event
{
public:
float x;
float y;
};
class RenderNodeCreatedEvent : public Event
{
public:
unsigned long long int renderNodeId;
};
Create them and use them (e.g. for test purposes in the main program)
// Create the on event functions
std::function<void(const ClickEvent &)> onClickFunction = [](const ClickEvent & event)
{
std::cout << std::endl << "Mouse clicked at position: " << event.x << event.y;
};
std::function<void(const RenderNodeCreatedEvent &)> onRenderNodeCreatedFunction = [](const RenderNodeCreatedEvent & event)
{
std::cout << std::endl << "Render node created with id: " << event.renderNodeId;
};
// Create the events
ClickEvent clickEvent;
clickEvent.x = 300.f;
clickEvent.y = 255.5f;
RenderNodeCreatedEvent renderNodeCreatedEvent;
renderNodeCreatedEvent.renderNodeId = 26234628374324;
// Create the event manager and subscribe the event functions
EventManager eventManager;
eventManager.Subscribe(onClickFunction);
eventManager.Subscribe(onRenderNodeCreatedFunction);
// Raise the events
eventManager.Raise(clickEvent);
eventManager.Raise(renderNodeCreatedEvent);
Here is what I have tried to generate a unique id of type std::size_t
for each derived event class:
class BaseEvent
{
public:
typedef std::size_t ID;
BaseEvent() = default;
virtual ~BaseEvent() = default;
protected:
static ID GetNextID()
{
static ID id = 0;
return id++;
}
};
class Event : public BaseEvent
{
public:
Event() = default;
virtual ~Event() = default;
// Sets a unique id for the event the first time this function is called
ID GetID()
{
static ID id = BaseEvent::GetNextID();
return id;
}
};
Finally, this is my (terrible) try at an event manager that can provide the functionality described above. I am really struggling to manage correct casting and/or store different types of callback functions. The callback wrapper doesn't work - its just a basic idea I had to get around this problem, so I have included it in the post.
class EventManager
{
public:
// Define the template callback type
template <class DerivedEvent>
using TCallback = std::function<void(const DerivedEvent &)>;
public:
EventManager() = default;
~EventManager() = default;
template<class DerivedEvent>
void Subscribe(TCallback<DerivedEvent> callback)
{
// Get the index of the callback list this callback will be added to
Event::ID id = DerivedEvent::GetID();
// This won't work sinve TCallback is a different type than TCallback<Event>
callbackListList[id].push_back(callback);
}
template <class DerivedEvent>
void Raise(DerivedEvent event)
{
// Get the type of the event and therefore the index in the callbackListList
Event::ID = DerivedEvent::GetID();
/// How to cast the events back
// Get the respective list of callback functions
std::vector<TCallback<DerivedEvent>> /*&*/ callbackList;
// Create a callback wrapper of with the type 'derived event' ?????
CallbackWrapper<DerivedEvent> callbackWrapper(/*derived callback*/);
// Call the callback wrapper using the base event ?????
}
template <typename DerivedEvent>
class CallbackWrapper
{
public:
CallbackWrapper(TCallback<DerivedEvent> callback) : callback(callback) {}
void operator () (const Event & event)
{
callback(static_cast<const Event<DerivedEvent> &>(event).event);
}
private:
TCallback<DerivedEvent> callback;
};
private:
std::vector<std::vector<TCallback<Event>>> callbackListList;
};
I know this is a lot of code, but I felt like this would be the easiest way to demonstrate what I am talking about.
Thanks for your help, Adrian
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