axyz/include/Engine/Core/ResourceManager.h

#ifndef ResourceManager_h__
#define ResourceManager_h__

#include <functional>
#include <set>
#include <fstream>

#include "../Common.h"
#include "Util/UnorderedMapPair.h"
#include "Util/FileWatcher.h"

/** Base Resource class.

	Implement this class for every resource to be handled by the resource manager.
	Implement Create() to return a new object of that type.
*/
class Resource
{
	friend class ResourceManager;

protected:
	Resource() { }

public:
	// Pretend that this is a pure virtual function that you have to implement
	// FIXME: Why did we do this again instead of just using the constructor?
	// static Resource* Create(std::string resourceName);

	virtual void Reload() {	}
	virtual void OnChildReloaded(Resource* child) { }

	unsigned int TypeID;
	unsigned int ResourceID;
};

/** Singleton resource manager to keep track of and cache any external engine assets */
class ResourceManager
{
private:
	ResourceManager();

public:
    //TODO: Check if this is ever used, and remove it if it isn't.
    //Why would a resource load another resource async. in the ctor?
    //Should be thrown in a Resource's constructor if it cannot complete because another resource is still loading.
    //Eg. If a Model loads a Texture asyncronously in the constructor, and it is not done yet.
    struct StillLoadingException : public std::exception
    {
        virtual const char* what() const throw()
        {
            return "Resource is still loading.";
        }
    };
    //Should be thrown in a Resource's constructor if certain work needs to be handled by 
    //the main thread, and not a parallel worker thread. Eg. opengl commands.
    struct WorkerCannotExecute : public std::exception
    {
        virtual const char* what() const throw()
        {
            return "Worker thread is unable to execute, main thread need to handle the code.";
        }
    };

    static void AssertIsMainThread();
    static bool IsMainThread();

	/*static ResourceManager& Instance()
	{
	static ResourceManager s;
	return s;
	}*/

	template <typename T>
	static void RegisterType(std::string typeName);

	/** Checks if a resource is in cache

		@param resourceType Resource type as string.
		@param resourceName Fully qualified name of the resource to preload.
	*/
	// TODO: Templateify
	static bool IsResourceLoaded(std::string resourceType, std::string resourceName);

    /** If the resource is not in cache, starts loading the resource and returns nullptr immediately.
        If the resource has been loaded already, return a pointer to it.

        @tparam T Resource type.
        @param resourceName Fully qualified name of the resource to load.
    */
    template <typename T>
    static T* LoadAsync(std::string resourceName, Resource* parent = nullptr);
	static Resource* LoadAsync(std::string resourceType, std::string resourceName, Resource* parent = nullptr);

	/** Hot-loads a resource and caches it for future use. 
        Fairly safe to assume that return value is a valid pointer, will only return nullptr on error.

		@tparam T Resource type.
		@param resourceName Fully qualified name of the resource to load.
	*/
	template <typename T>
	static T* Load(std::string resourceName, Resource* parent = nullptr);
    static Resource* Load(std::string resourceType, std::string resourceName, Resource* parent = nullptr);

	/** Reloads an already loaded resource, keeping its resource ID intact.

	 	@tparam T Resource type.
	 	@param resourceName Fully qualified name of the resource to reload.
	*/
    static void Reload(std::string resourceName);
    
    static void Release(std::string resourceType, std::string resourceName);

	static void Update();

private:
    //Represents pointer values to signify that a resource haven't failed, but is not fully loaded.
    class SpecialResourcePointer
    {
    public:
        SpecialResourcePointer()
            : m_Val(new Resource())
        {}
        ~SpecialResourcePointer()
        {
            delete m_Val;
        }
        //Make this class implicitly convertible to the Resource*.
        operator Resource* const() const { return m_Val; }
    private:
        Resource* const m_Val;
    };
    //TODO: Check if this is ever used, and remove it if it isn't.
    static SpecialResourcePointer m_StillLoading;
    static SpecialResourcePointer m_LoadWithMainThread;

	static std::unordered_map<std::string, std::string> m_CompilerTypenameToResourceType;
    static std::unordered_map<std::string, std::function<Resource*(std::string)>> m_FactoryFunctions; // type -> factory function
	static std::unordered_map<std::pair<std::string, std::string>, Resource*> m_ResourceCache; // (type, name) -> resource
	static std::unordered_map<std::string, Resource*> m_ResourceFromName; // name -> resource
	static std::unordered_map<Resource*, Resource*> m_ResourceParents; // resource -> parent resource

	static std::unordered_map<std::pair<std::string, std::string>, boost::thread> m_LoadingThreads; // (type, name) -> loading thread
    static boost::recursive_mutex m_Mutex;

	// TODO: Getters for IDs
	static unsigned int m_CurrentResourceTypeID;
	static std::unordered_map<std::string, unsigned int> m_ResourceTypeIDs;
	// Number of resources of a type. Doubles as local ID.
	static std::unordered_map<unsigned int, unsigned int> m_ResourceCount;

	static FileWatcher m_FileWatcher;
	static void fileWatcherCallback(std::string path, FileWatcher::FileEventFlags flags);

	static unsigned int GetTypeID(std::string resourceType);
	static unsigned int GetNewResourceID(unsigned int typeID);

	// Internal: Create a resource and cache it
	static Resource* createResource(std::string resourceType, std::string resourceName, Resource* parent);
};

template <typename T>
T* ResourceManager::Load(std::string resourceName, Resource* parent /* = nullptr */)
{
    auto resourceTypename = typeid(T).name();
    auto it = m_CompilerTypenameToResourceType.find(resourceTypename);
    if (it == m_CompilerTypenameToResourceType.end()) {
        LOG_ERROR("Failed to load resource \"%s\" of type \"%s\": type not registered", resourceName.c_str(), resourceTypename);
        return nullptr;
    }

    return static_cast<T*>(Load(it->second, resourceName, parent));
}

template <typename T>
void ResourceManager::RegisterType(std::string typeName)
{
	m_CompilerTypenameToResourceType[typeid(T).name()] = typeName;
	m_FactoryFunctions[typeName] = [](std::string resourceName) { return new T(resourceName); };
}

template <typename T>
T* ResourceManager::LoadAsync(std::string resourceName, Resource* parent /* = nullptr */)
{
	auto resourceTypename = typeid(T).name();
	auto it = m_CompilerTypenameToResourceType.find(resourceTypename);
	if (it == m_CompilerTypenameToResourceType.end()) {
		LOG_ERROR("Failed to load resource \"%s\" of type \"%s\": type not registered", resourceName.c_str(), resourceTypename);
		return nullptr;
	}

	return static_cast<T*>(LoadAsync(it->second, resourceName, parent));
}

#endif