Skip to main content
replaced http://gamedev.stackexchange.com/ with https://gamedev.stackexchange.com/
Source Link
Source Link
Nic Foster
  • 2.9k
  • 18
  • 23

What you're looking for is a messaging system that can communicate between components and objects, I've detailed such a system already, here:

How to implement the interaction between engine parts?

The structure here is a Scene Manager that contains all Objects in a scene. Objects have a list of components that can listen for messages. Both the object and its components have a chance to listen for each message. The following code sample is generic (engine-agnostic).

Here's the code:

#include <iostream>
#include <stdio.h>

#include <list>
#include <map>

using namespace std;

struct Vector3
{
public:
    Vector3() : x(0.0f), y(0.0f), z(0.0f)
    {}

    float x, y, z;
};

enum eMessageType
{
    SetPosition,
    GetPosition,    
};

class BaseMessage
{
protected: // Abstract class, constructor is protected
    BaseMessage(int destinationObjectID, eMessageType messageTypeID) 
        : m_destObjectID(destinationObjectID)
        , m_messageTypeID(messageTypeID)
    {}

public: // Normally this isn't public, just doing it to keep code small
    int m_destObjectID;
    eMessageType m_messageTypeID;
};

class PositionMessage : public BaseMessage
{
protected: // Abstract class, constructor is protected
    PositionMessage(int destinationObjectID, eMessageType messageTypeID, 
                    float X = 0.0f, float Y = 0.0f, float Z = 0.0f)
        : BaseMessage(destinationObjectID, messageTypeID)
        , x(X)
        , y(Y)
        , z(Z)
    {

    }

public:
    float x, y, z;
};

class MsgSetPosition : public PositionMessage
{
public:
    MsgSetPosition(int destinationObjectID, float X, float Y, float Z)
        : PositionMessage(destinationObjectID, SetPosition, X, Y, Z)
    {}
};

class MsgGetPosition : public PositionMessage
{
public:
    MsgGetPosition(int destinationObjectID)
        : PositionMessage(destinationObjectID, GetPosition)
    {}
};

class BaseComponent
{
public:
    virtual bool SendMessage(BaseMessage* msg) { return false; }
};

class RenderComponent : public BaseComponent
{
public:
    /*override*/ bool SendMessage(BaseMessage* msg)
    {
        // Object has a switch for any messages it cares about
        switch(msg->m_messageTypeID)
        {
        case SetPosition:
            {                   
                // Update render mesh position/translation

                cout << "RenderComponent handling SetPosition\n";
            }
            break;
        default:
            return BaseComponent::SendMessage(msg);
        }

        return true;
    }
};

class Object
{
public:
    Object(int uniqueID)
        : m_UniqueID(uniqueID)
    {
    }

    int GetObjectID() const { return m_UniqueID; }

    void AddComponent(BaseComponent* comp)
    {
        m_Components.push_back(comp);
    }

    bool SendMessage(BaseMessage* msg)
    {
        bool messageHandled = false;

        // Object has a switch for any messages it cares about
        switch(msg->m_messageTypeID)
        {
        case SetPosition:
            {               
                MsgSetPosition* msgSetPos = static_cast<MsgSetPosition*>(msg);
                m_Position.x = msgSetPos->x;
                m_Position.y = msgSetPos->y;
                m_Position.z = msgSetPos->z;

                messageHandled = true;
                cout << "Object handled SetPosition\n";
            }
            break;
        case GetPosition:
            {
                MsgGetPosition* msgSetPos = static_cast<MsgGetPosition*>(msg);
                msgSetPos->x = m_Position.x;
                msgSetPos->y = m_Position.y;
                msgSetPos->z = m_Position.z;

                messageHandled = true;
                cout << "Object handling GetPosition\n";
            }
            break;
        default:
            return PassMessageToComponents(msg);
        }

        // If the object didn't handle the message but the component
        // did, we return true to signify it was handled by something.
        messageHandled |= PassMessageToComponents(msg);

        return messageHandled;
    }

private: // Methods
    bool PassMessageToComponents(BaseMessage* msg)
    {
        bool messageHandled = false;

        std::list<BaseComponent*>::iterator compIt = m_Components.begin();
        for ( compIt; compIt != m_Components.end(); ++compIt )
        {
            messageHandled |= (*compIt)->SendMessage(msg);
        }

        return messageHandled;
    }

private: // Members
    int m_UniqueID;
    std::list<BaseComponent*> m_Components;
    Vector3 m_Position;
};

class SceneManager
{
public: 
    // Returns true if the object or any components handled the message
    bool SendMessage(BaseMessage* msg)
    {
        // We look for the object in the scene by its ID
        std::map<int, Object*>::iterator objIt = m_Objects.find(msg->m_destObjectID);       
        if ( objIt != m_Objects.end() )
        {           
            // Object was found, so send it the message
            return objIt->second->SendMessage(msg);
        }

        // Object with the specified ID wasn't found
        return false;
    }

    Object* CreateObject()
    {
        Object* newObj = new Object(nextObjectID++);
        m_Objects[newObj->GetObjectID()] = newObj;

        return newObj;
    }

private:
    std::map<int, Object*> m_Objects;
    static int nextObjectID;
};

// Initialize our static unique objectID generator
int SceneManager::nextObjectID = 0;

int main()
{
    // Create a scene manager
    SceneManager sceneMgr;

    // Have scene manager create an object for us, which
    // automatically puts the object into the scene as well
    Object* myObj = sceneMgr.CreateObject();

    // Create a render component
    RenderComponent* renderComp = new RenderComponent();

    // Attach render component to the object we made
    myObj->AddComponent(renderComp);

    // Set 'myObj' position to (1, 2, 3)
    MsgSetPosition msgSetPos(myObj->GetObjectID(), 1.0f, 2.0f, 3.0f);
    sceneMgr.SendMessage(&msgSetPos);
    cout << "Position set to (1, 2, 3) on object with ID: " << myObj->GetObjectID() << '\n';

    cout << "Retreiving position from object with ID: " << myObj->GetObjectID() << '\n';
    
    // Get 'myObj' position to verify it was set properly
    MsgGetPosition msgGetPos(myObj->GetObjectID());
    sceneMgr.SendMessage(&msgGetPos);
    cout << "X: " << msgGetPos.x << '\n';
    cout << "Y: " << msgGetPos.y << '\n';
    cout << "Z: " << msgGetPos.z << '\n';
}