Core Thread								{#coreThread}
===============
[TOC]

Banshee is a multi-threaded engine that has two primary threads. One is the main thread on which the application is started, this is where your game code runs and what majority of users will be working with, we call this the **simulation** thread. The second thread is the rendering thread, this is where all calls to render API (like DirectX/OpenGL) are made. This thread also deals with the OS (like the main message loop). We call this the **core** thread.

Various other operations can use threads other than the two primary ones (async resource loading, physics, etc.) in the form of worker threads or tasks. But we won't touch on those as they act as standard threads and the system using them has full control.

Dealing with the core thread on the other hand requires some knowledge of how it interacts with the simulation thread. The core threads deals with:
 - Render API calls (DirectX/OpenGL)
 - Renderer
 - OS functionality (window creation, main message loop)

Here are some examples of when the simulation thread needs to interact with the core:
 - You add a new renderable object to the scene. The simulation thread needs to notify the renderer.
 - You modify a property on a renderable/camera/light object. The simulation thread needs to notify the renderer.
 - You create a resource like a mesh or a texture. The simulation thread must schedule resource creation with the render API (i.e. creating vertex/pixel buffers for mesh, texture surfaces for texture).
 - You destroy a resource like a mesh or texture. The simulation thread must call into render API to free the GPU portions of those objects.
 - You create a new window and the simulation thread needs to schedule its creation on the core thread.
 - You resize/move a window and the simulation thread needs to notify the core thread.
 
As you can see the communication is one directional. Simulation thread calls into the core thread when it needs to notify it of a certain event. In rare cases the core thread needs to notify the simulation thread (e.g. when the user moves/resizes the window the simulation thread needs to be aware of the new position/size), but this is handled on a per-case basis using normal thread synchronization primitives, and we won't touch on it further.

# Command queue {#coreThread_a}
All the operations listed above happen with the help of a @ref BansheeEngine::CommandQueue<SyncPolicy> "CommandQueue". When simulation thread needs to notify the core thread about something it queues a command in the queue using @ref BansheeEngine::CommandQueue<SyncPolicy>::queue "CommandQueue::queue", which accepts a function to be executed. The core thread continually reads from the command queue and executes commands in the order of they are submitted.

The main command queue can be accessed using @ref BansheeEngine::CoreThread::queueCommand "CoreThread::queueCommand", which is just a wrapper for the @ref BansheeEngine::CommandQueue<SyncPolicy>::queue "CommandQueue::queue" method mentioned above.

## Returning values {#coreThread_a_a}
Sometimes a queued command needs to return a value to the simulation thread (for example, when reading pixels from a texture). This can be performed by calling @ref BansheeEngine::CoreThread::queueReturnCommand "CoreThread::queueReturnCommand", which internally calls @ref BansheeEngine::CommandQueue<SyncPolicy>::queueReturn "CommandQueue::queueReturn". 

Since the command was queued and we don't know when will the core thread execute, we have no guarantees when its return value will be available. Therefore this method will return an @ref BansheeEngine::AsyncOp "AsyncOp" object. This object can be used for checking if the return value is available by calling @ref BansheeEngine::AsyncOp::hasCompleted "AsyncOp::hasCompleted". If the return value is available you can retrieve it via @ref BansheeEngine::AsyncOp::getReturnValue<T> "AsyncOp::getReturnValue<T>".

## Blocking {#coreThread_a_b}
In certain cases you need to see the results of an operation right away. In such cases you can tell the simulation thread to wait until a certain command is done executing on the core thread. You can do this by setting the `blockUntilComplete` parameter of @ref BansheeEngine::CoreThread::queueCommand "CoreThread::queueCommand" and @ref BansheeEngine::CoreThread::queueReturnCommand "CoreThread::queueReturnCommand" to true.

You can also block by calling @ref BansheeEngine::AsyncOp::blockUntilComplete "AsyncOp::blockUntilComplete". This is similar to blocking directly on the @ref BansheeEngine::CoreThread::queueReturnCommand "CoreThread::queueReturnCommand" call, but can be more useful if you're not sure immediately if you need to wait for the result or not.

You should be very careful about blocking as it wastes CPU cycles since one thread is just sitting there doing nothing until the core thread reaches the command (which could be as long as a few dozen milliseconds).

## Core accessors {#coreThread_a_c}
Core accessors can be considered as light-weight command queues. They perform the same operations as @ref BansheeEngine::CoreThread::queueCommand "CoreThread::queueCommand" and @ref BansheeEngine::CoreThread::queueReturnCommand "CoreThread::queueReturnCommand". However core accessors can only be accessed from a single thread, while the methods just listed can be accessed from any thread. Accessors also don't submit their commands to the core thread immediately, instead they are submitted all together using a separate method.

Because of these two reasons core accessors are much faster than the primary command queue, and you should always prefer to use them instead of directly queuing commands on the primary queue.

Accessor for the current thread can be retrieved with @ref BansheeEngine::CoreThread::getAccessor "CoreThread::getAccessor". You queue the commands in the accessor by calling @ref BansheeEngine::CoreThreadAccessorBase::queueCommand "CoreThreadAccessor::queueCommand" and @ref BansheeEngine::CoreThreadAccessorBase::queueReturnCommand "CoreThreadAccessor::queueReturnCommand". Once you are done queuing commands you can submit them to the core thread by calling @ref BansheeEngine::CoreThread::submitAccessors "CoreThread::submitAccessors".

Internally @ref BansheeEngine::CoreThread::submitAccessors "CoreThread::submitAccessors" uses the primary queue to submit a command that reads all the commands from the accessor and executes them in order. So esentially they are just built on top of the primary command queue, and if fact most of the threading functionality is.

### Core accessor APIs {#coreThread_a_c_a}
Although you can queue your own commands to the core accessor, many systems provide methods that automatically queue commands on the core accessor. For example take a look at @ref BansheeEngine::RenderAPI "RenderAPI" which allows you to interact directly with the render API from the simulation thread (something that is normally reserved for the core thread). Most of the methods accept a @ref BansheeEngine::CoreThreadAccessor<CommandQueueSyncPolicy> "CoreThreadAccessor", and internally just queue commands on it. This allows you to perform low level rendering operations from the simulation thread.

@ref BansheeEngine::Mesh "Mesh", @ref BansheeEngine::Texture "Texture" and @ref BansheeEngine::RenderTarget "RenderTarget" also provide several methods for performing core thread operations from the simulation thread via the core accessors.

It's important to remember these methods are just a convenience, and internally they use the same command queuing methods we described in the previous section.

# Core objects {#coreThread_b}
Core objects are objects that need to exist on both simulation and core threads. Although you could technically handle such cases manually by using the command queue, it is useful to provide an interface that allows the user to work normally with an object without needing to know about the threading internals, and this is where core objects come in.

For example, a @ref BansheeEngine::Mesh "Mesh" is a core object because we want to allow the user to intuitively work with it on the simulation thread (without having to know about command queue or the core thread), but we also want to use it on the core thread (it needs to create index/vertex buffers on the GPU, and have a DirectX/OpenGL representation).

Every core object is split into two interfaces:
 - @ref BansheeEngine::CoreObject "CoreObject" - Implementations of this interface represents the simulation thread counterpart of the object.
 - @ref BansheeEngine::CoreObjectCore "CoreObjectCore" - Implementations of this interface represents the core thread counterpart of the object.
 
When a @ref BansheeEngine::CoreObject "CoreObject" is created it internally queues the creation of its @ref BansheeEngine::CoreObjectCore "CoreObjectCore" counterpart on the main command queue. Similar thing happens when it is destroyed, a destroy operation is queued and sent to the core thread.

Aside from initialization/destruction, core objects also support synchronization of data between the two threads (e.g. a @ref BansheeEngine::Light "Light" is a core object, and when the user changes light radius, it is automatically synchronized to its core thread counterpart @ref BansheeEngine::LightCore "LightCore").

## Creating your own core objects {#coreThread_b_a}
To create a custom core object, you need to implement the @ref BansheeEngine::CoreObject "CoreObject" class, which by default requires no methods to be implemented. 

When creating your core object its important to note they are only allowed to be referenced using specialized shared pointers, and require specific initialization steps. The shared pointer needs to be created using the @ref BansheeEngine::bs_core_ptr_new<T> "bs_core_ptr_new<T>" method, followed by calls to @ref BansheeEngine::CoreObject::_setThisPtr "CoreObject::_setThisPtr" and @ref BansheeEngine::CoreObject::initialize "CoreObject::initialized". Due to this complex initialization procedure it is highly suggested that you create a static `create` method for your core object, which does every step automatically. For example:
~~~~~~~~~~~~~{.cpp}
SPtr<MyCoreObject> MyCoreObject::create()
{
	SPtr<MyCoreObject> myCoreObj = bs_core_ptr_new<MyCoreObject>();
	myCoreObj->_setThisPtr(myCoreObj);
	myCoreObj->initialize();
	
	return myCoreObj;
}
~~~~~~~~~~~~~

You will also want to override @ref BansheeEngine::CoreObject::createCore "CoreObject::createCore", which creates the core thread counterpart of your object. It should return a normal shared pointer to your implementation of @ref BansheeEngine::CoreObjectCore "CoreObjectCore".

Once a core thread object is created you can use it as a normal object, while you can retrieve its core thread counterpart by calling @ref BansheeEngine::CoreObject::getCore "CoreObject::getCore", which you can use on the core thread. Object creation/destruction will happen automatically on the valid thread, and you also get the ability to synchronize information between the two (see below).

### CoreObjectCore {#coreThread_b_a_a}
To create the core thread counterpart of a @ref BansheeEngine::CoreObject "CoreObject" you must implement the @ref BansheeEngine::CoreObjectCore "CoreObjectCore" class. 

This object provides an @ref BansheeEngine::CoreObject::initialize "CoreObject::initialize" method. You should perform any initialization in this method instead of the constructor, as it is guaranteed to be run on the core thread.

The destructor is always assumed to be executed on the core thread. For this reason you must ensure never to store references to CoreObjectCore on the simulation thread, because if they go out of scope there it will trigger an error. Similar rule applies to @ref BansheeEngine::CoreObject "CoreObjects" as they shouldn't be stored on the core thread.

### Synchronization {#coreThread_b_a_b}
Earlier we mentioned that aside from handling construction/destruction the core objects also provide a way to synchronize between the two threads. The synchronization is always one way, from @ref BansheeEngine::CoreObject "CoreObject" to @ref BansheeEngine::CoreObjectCore "CoreObjectCore". 

Synchronization should happen whenever some property on the @ref BansheeEngine::CoreObject "CoreObject" changes, that you would wish to make available on the core thread. To synchronize implement the @ref BansheeEngine::CoreObject::syncToCore(FrameAlloc*) "CoreObject::syncToCore" method, which generates the data for synchronization, and @ref BansheeEngine::CoreObjectCore::syncToCore "CoreObjectCore::syncToCore" which accepts it.

The synchronized data is transfered between the objects in the form of raw bytes, within the @ref BansheeEngine::CoreSyncData "CoreSyncData" structure. You can use @ref BansheeEngine::rttiGetElemSize "rttiGetElemSize" and @ref BansheeEngine::rttiWriteElem "rttiWriteElem" to encode fields into raw memory, and @ref BansheeEngine::rttiReadElem "rttiReadElem" to decode them. See the manual about [rtti](@ref rtti) for more information about serialization. @ref BansheeEngine::CoreObject::syncToCore(FrameAlloc*) "CoreObject::syncToCore" is provided an instance of @ref BansheeEngine::FrameAlloc "FrameAlloc" which should be used for allocating the serialization buffer. This is an allocator that is fast and doesn't require explicit memory deallocation making it perfect for synchronization. A simple synchronization example would look like so:
~~~~~~~~~~~~~{.cpp}
// CoreObject (creates the synchronization data)
CoreSyncData MyCoreObject::syncToCore(FrameAlloc* allocator) 
{
	UINT32 size = 0;
	size += rttiGetElemSize(mField1);
	size += rttiGetElemSize(mField2);

	UINT8* buffer = allocator->alloc(size);

	char* dataPtr = (char*)buffer;
	dataPtr = rttiWriteElem(mField1, dataPtr);
	dataPtr = rttiWriteElem(mField2, dataPtr);

	return CoreSyncData(buffer, size);
}

// CoreObjectCore (receives the synchronization data)
void MyCoreObjectCore::syncToCore(const CoreSyncData& data) 
{
	char* dataPtr = (char*)data.getBuffer();

	dataPtr = rttiReadElem(mField1, dataPtr);
	dataPtr = rttiReadElem(mField2, dataPtr); 
	
	// Potentially trigger something depending on new data
}
~~~~~~~~~~~~~

Whenever you need to trigger synchronization you must call @ref BansheeEngine::CoreObject::markCoreDirty "CoreObject::markCoreDirty" which notifies the system that synchronization is required. This will in turn trigger a call to @ref BansheeEngine::CoreObject::syncToCore(FrameAlloc*) "CoreObject::syncToCore" method you implemented earlier. Synchronization happens automatically for all dirty core objects once per frame. Optionally you may call @ref BansheeEngine::CoreObject::syncToCore(CoreAccessor&) "CoreObject::syncToCore" to manually queue the synchronization.

See implementation of @ref BansheeEngine::Light "Light" and @ref BansheeEngine::LightCore "LightCore" in "BsLight.cpp" for a simple example of synchronization.

### Dependencies {#coreThread_b_a_c}
Core objects might be dependant on other core objects. For example a @ref BansheeEngine::Material "Material" is dependant on a @ref BansheeEngine::Shader "Shader". Whenever the shader's object is marked as dirty the material might want need to perform synchronization as well. In general whenever a dependancy core object is marked as dirty, its dependant will be synchronized as well.

To add dependencies implement the @ref BansheeEngine::CoreObject::getCoreDependencies "CoreObject::getCoreDependencies" method, which returns all currently valid dependencies. Whenever the dependencies change call @ref BansheeEngine::CoreObject::markDependenciesDirty "CoreObject::markDependenciesDirty" so the system can refresh its dependency list.

## Other features {#coreThread_b_b}
Core objects also have some other potentially useful features:
 - @ref BansheeEngine::CoreObject::getInternalID "CoreObject::getInternalID" will return a globally unique ID for the core object
 - @ref BansheeEngine::CoreObject::destroy "CoreObject::destroy" will destroy the core object and its core thread counterpart. You do not need to call this manually as it will be automatically called when the object goes out of scope (is no longer referenced). The core thread counterpart (@ref BansheeEngine::CoreObjectCore "CoreObjectCore") will not be destroyed if something on the core thread is still holding a reference to it.
 - Override @ref BansheeEngine::CoreObject::initialize "CoreObject::initialize" or @ref BansheeEngine::CoreObject::destroy "CoreObject::destroy" methods if you need to perform some initialization/destruction operations not suitable for the constructor/destructor (e.g. calling virtual methods).
 - You can construct a core object without a core thread counterpart. Simply don't override @ref BansheeEngine::CoreObject::createCore "CoreObject::createCore".
 - You can construct a core object with a @ref BansheeEngine::CoreObjectCore "CoreObjectCore" that isn't initialized on the core thread by setting the @ref BansheeEngine::CoreObject "CoreObject" constructor parameter `requiresCoreInit` to false.
 - Core objects always hold a shared pointer to themselves. Use @ref BansheeEngine::CoreObject::getThisPtr "CoreObject::getThisPtr" to access it.