NativeRenderPlugin是Unity直接接触原生图形API，组织渲染指令的方法，本文通过UnityC#层、插件层、Unity源码层角度，解析代码编写方式，给读者提供编写参考。
  本文代码解析路径Gfx后端基于DirectX12。工程路径：NativeRenderingPlugin。

Plugin编译

  工程ReadMe中说过，DX12编译需要打开宏，在PlatformBase.h中，将#define SUPPORT_D3D12 0改为1，此时编译会缺少d3dx12.h，这个很多人本地都有，没有去Github找个也行，直接拷贝到本地目录然后项目添加现有项。
  编译后，将PluginSource\build\x64\Debug\RenderingPlugin.dll及同目录下的pdb文件拷贝到UnityPlugin\Assets\Plugins\x86_64中覆盖源文件。
  用UnityEditor启动UnityPlugin下的Unity项目，默认可能是DX11后端，运行能看到像波浪一样的面片和不断旋转的三角形。
  此时如果切换成DX12后端重启编辑器后运行，波浪和三角形不会运动，可以打开UseRenderingPlugin.cs，CallPluginAtEndOfFrames方法中，判断如果是DX12或Switch平台就不会设置时间，所以直接把判断注释掉，在DX12平台下也可以有相同的效果了。

调度流程

  对于C#脚本来说，大部分方法是[DllImport("RenderingPlugin")]，将dll动态链接到程序中，部分平台只能靠静态链接（例如IOS），所以Attribute是[DllImport("__Internal")]。

初始化

  插件会在第一次调用DllImport的方法时通过名称寻找Plugin(重新Start会重新找)，然后通过调用(PluginLoadFunc)LoadPluginFunction(pluginHandle, "UnityPluginLoad")，寻找初始化方法UnityPluginLoad，假如dll中有这个方法，会调用并将Interfaces作为参数传入。
  UnityPluginLoad需要插件编写者自己编写，在这个示例中，首先获取图形接口IUnityGraphics：

static IUnityInterfaces* s_UnityInterfaces = NULL;
static IUnityGraphics* s_Graphics = NULL;

extern "C" void UNITY_INTERFACE_EXPORT UNITY_INTERFACE_API UnityPluginLoad(IUnityInterfaces* unityInterfaces)
{
    s_UnityInterfaces = unityInterfaces;
    s_Graphics = s_UnityInterfaces->Get<IUnityGraphics>();
    s_Graphics->RegisterDeviceEventCallback(OnGraphicsDeviceEvent);
    
    OnGraphicsDeviceEvent(kUnityGfxDeviceEventInitialize);
}

  这里调用RegisterDeviceEventCallback向IUnityGraphics注册了一个回调，这个回调的作用是接收DeviceEvent，执行特定逻辑，Event包括：

typedef enum UnityGfxDeviceEventType
{
    kUnityGfxDeviceEventInitialize     = 0,
    kUnityGfxDeviceEventShutdown       = 1,
    kUnityGfxDeviceEventBeforeReset    = 2,
    kUnityGfxDeviceEventAfterReset     = 3,
} UnityGfxDeviceEventType;

  这个示例只用到初始化kUnityGfxDeviceEventInitialize和销毁kUnityGfxDeviceEventShutdown。
  这里主动调用了一次OnGraphicsDeviceEvent(kUnityGfxDeviceEventInitialize)，此外，Unity底层GfxDevice的实现类在构造方法会调用回调方法并传入Event kUnityGfxDeviceEventInitialize，相应的，析构方法调用回调并传入Event kUnityGfxDeviceEventShutdown。
  回调中，初始化事件创建抽象渲染接口，销毁事件delete掉接口对象，然后将事件分发给底层不同的渲染接口执行逻辑：

static void UNITY_INTERFACE_API OnGraphicsDeviceEvent(UnityGfxDeviceEventType eventType)
{
    // Create graphics API implementation upon initialization
    if (eventType == kUnityGfxDeviceEventInitialize)
    {
        assert(s_CurrentAPI == NULL);
        s_DeviceType = s_Graphics->GetRenderer();
        s_CurrentAPI = CreateRenderAPI(s_DeviceType);
    }

    // Let the implementation process the device related events
    if (s_CurrentAPI)
    {
        s_CurrentAPI->ProcessDeviceEvent(eventType, s_UnityInterfaces);
    }

    // Cleanup graphics API implementation upon shutdown
    if (eventType == kUnityGfxDeviceEventShutdown)
    {
        delete s_CurrentAPI;
        s_CurrentAPI = NULL;
        s_DeviceType = kUnityGfxRendererNull;
    }
}

  这里直接看RenderAPI_D3D12::ProcessDeviceEvent:

void RenderAPI_D3D12::ProcessDeviceEvent(UnityGfxDeviceEventType type, IUnityInterfaces* interfaces)
{
    switch (type)
    {
    case kUnityGfxDeviceEventInitialize:
        s_d3d12 = interfaces->Get<IUnityGraphicsD3D12v7>();

        UnityD3D12PluginEventConfig config_1;
        config_1.graphicsQueueAccess = kUnityD3D12GraphicsQueueAccess_DontCare;
        config_1.flags = kUnityD3D12EventConfigFlag_SyncWorkerThreads | kUnityD3D12EventConfigFlag_ModifiesCommandBuffersState | kUnityD3D12EventConfigFlag_EnsurePreviousFrameSubmission;
        config_1.ensureActiveRenderTextureIsBound = true;
        s_d3d12->ConfigureEvent(1, &config_1);

        UnityD3D12PluginEventConfig config_2;
        config_2.graphicsQueueAccess = kUnityD3D12GraphicsQueueAccess_Allow;
        config_2.flags = kUnityD3D12EventConfigFlag_SyncWorkerThreads | kUnityD3D12EventConfigFlag_ModifiesCommandBuffersState | kUnityD3D12EventConfigFlag_EnsurePreviousFrameSubmission;
        config_2.ensureActiveRenderTextureIsBound = false;
        s_d3d12->ConfigureEvent(2, &config_2);

        initialize_and_create_resources();
        break;
    case kUnityGfxDeviceEventShutdown:
        release_resources();
        break;
    }
}

  initialize_and_create_resources和release_resources都很简单，是申请和释放当前插件所需的资源，例如顶点索引缓冲区、PSO、RTV、RootSignature等等。
  这里说下s_d3d12->ConfigureEvent的作用，s_d3d12是IUnityGraphicsD3D12v7，作用类似D3D12的接口，例如ID3D12Device7，作用是用于兼容接口，高版本拥有低版本所有的方法，这些方法能获取当前API的一些能力，例如IUnityGraphicsD3D12v7包含如下方法：

UNITY_DECLARE_INTERFACE(IUnityGraphicsD3D12v7)
{
    ID3D12Device* (UNITY_INTERFACE_API * GetDevice)();

    IDXGISwapChain* (UNITY_INTERFACE_API * GetSwapChain)();
    UINT32(UNITY_INTERFACE_API * GetSyncInterval)();
    UINT(UNITY_INTERFACE_API * GetPresentFlags)();

    ID3D12Fence* (UNITY_INTERFACE_API * GetFrameFence)();
    // Returns the value set on the frame fence once the current frame completes or the GPU is flushed
    UINT64(UNITY_INTERFACE_API * GetNextFrameFenceValue)();

    //     Executes a given command list on a worker thread. The command list type must be D3D12_COMMAND_LIST_TYPE_DIRECT.
    //    [Optional] Declares expected and post-execution resource states.
    //     Returns the fence value. The value will be set once the current frame completes or the GPU is flushed.
    UINT64(UNITY_INTERFACE_API * ExecuteCommandList)(ID3D12GraphicsCommandList * commandList, int stateCount, UnityGraphicsD3D12ResourceState * states);

    void(UNITY_INTERFACE_API * SetPhysicalVideoMemoryControlValues)(const UnityGraphicsD3D12PhysicalVideoMemoryControlValues * memInfo);

    ID3D12CommandQueue* (UNITY_INTERFACE_API * GetCommandQueue)();

    ID3D12Resource* (UNITY_INTERFACE_API * TextureFromRenderBuffer)(UnityRenderBuffer rb);
    ID3D12Resource* (UNITY_INTERFACE_API * TextureFromNativeTexture)(UnityTextureID texture);

    // Change the precondition for a specific user-defined event
    // Should be called during initialization
    void(UNITY_INTERFACE_API * ConfigureEvent)(int eventID, const UnityD3D12PluginEventConfig * pluginEventConfig);

    bool(UNITY_INTERFACE_API * CommandRecordingState)(UnityGraphicsD3D12RecordingState * outCommandRecordingState);
};

  上述接口中包含获取Device、Swapchain、垂直同步、PresentFlag、执行CommandList，获取CommandQueue等方法。
  而ConfigureEvent用于指定事件对当前图形管线的影响，这对不同图形API是不同的，例如对Unity DX12封装来说，kUnityD3D12GraphicsQueueAccess可以标识当前事件是否需要访问CommandQueue，如果设置为DontCare，代表不会直接接触CommandQueue，CommandList交给Unity代为上传，这样可以在Gfx工作线程直接提交CmdList；另一个选择是Allow，对于插件callback来说，会直接获取底层CommandQueue提交CmdList，根据Unity选项的不同，如果有NativeJob，则启动另一个线程执行，否则也是在工作线程现场执行。
  此外，其他flag也各有作用，kUnityD3D12EventConfigFlag_SyncWorkerThreads表示，执行当前Event的callback前，是否需要同步TaskExecutor所在的线程；
  kUnityD3D12EventConfigFlag_ModifiesCommandBuffersState标识用于声明，当前Event表示的callback，可能会改变CommandList/CommandBuffer的状态，需要让Unity内部无效化对管线状态的记录，防止后续添加命令时，误以为管线状态没变化，导致没有覆盖新状态；
  kUnityD3D12EventConfigFlag_EnsurePreviousFrameSubmission为是否同步上一帧的present；
  ensureActiveRenderTextureIsBound表示是否在执行callback前，自动绑定设置好当前active的RT。
  通过上述调用，我们注册了两种event类型以供后续调度。

渲染组织

  C#脚本在Start开启了一个协程，在每帧结束时调用：

GL.IssuePluginEvent(GetRenderEventFunc(), 1);

if (SystemInfo.graphicsDeviceType == GraphicsDeviceType.Direct3D12)
{
    GL.IssuePluginEvent(GetRenderEventFunc(), 2);
}

  这里GetRenderEventFunc()返回渲染函数OnRenderEvent的指针，作为执行函数的callback，第二个传给IssuePluginEvent的参数就是上文的eventId，看看这个函数的实现：

static void UNITY_INTERFACE_API OnRenderEvent(int eventID)
{
    // Unknown / unsupported graphics device type? Do nothing
    if (s_CurrentAPI == NULL)
        return;

    if (eventID == 1)
    {
        drawToRenderTexture();
        DrawColoredTriangle();
        ModifyTexturePixels();
        ModifyVertexBuffer();
    }

    if (eventID == 2)
    {
        drawToPluginTexture();
    }
}

  drawToRenderTexture和drawToPluginTexture大致逻辑相同，甚至可以说干的是同一件事，区别在于前者的RT是在Unity中申请的，后者是在插件中申请的，于是调度上就有不同eventId对应的小差别。
  例如Unity的流程，Fence是由Unity内部返回的：

UINT64 RenderAPI_D3D12::submit_cmd_to_unity_worker(ID3D12GraphicsCommandList* cmd, UnityGraphicsD3D12ResourceState* resource_states, int state_count)
{
    return s_d3d12->ExecuteCommandList(cmd, state_count, resource_states);
}

void RenderAPI_D3D12::wait_for_unity_frame_fence(UINT64 fence_value)
{
    ID3D12Fence* unity_fence = s_d3d12->GetFrameFence();
    UINT64 current_fence_value = unity_fence->GetCompletedValue();

    if (current_fence_value < fence_value)
    {
        handle_hr(unity_fence->SetEventOnCompletion(fence_value, m_fence_event), "Failed to set fence event on completion\n");
        WaitForSingleObject(m_fence_event, INFINITE);
    }
}

void RenderAPI_D3D12::drawToRenderTexture()
{
    wait_for_unity_frame_fence(m_render_texture_draw_fence);

    // ...

    m_render_texture_draw_fence = submit_cmd_to_unity_worker(m_render_texture_cmd_list, &resource_states, 1);
}

  而偏向插件的流程，Fence是自己维护的：

void RenderAPI_D3D12::wait_on_fence(UINT64 fence_value, ID3D12Fence* fence, HANDLE fence_event)
{
    UINT64 current_fence_value = fence->GetCompletedValue();

    if (current_fence_value < fence_value)
    {
        handle_hr(fence->SetEventOnCompletion(fence_value, fence_event));
        WaitForSingleObject(fence_event, INFINITE);
    }
}

void RenderAPI_D3D12::drawToPluginTexture()
{
    wait_on_fence(m_plugin_texture_fence_value, m_plugin_texture_fence, m_plugin_texture_fence_event);

    // ...

    ID3D12CommandQueue* unity_command_queue = s_d3d12->GetCommandQueue();
    unity_command_queue->ExecuteCommandLists(1, (ID3D12CommandList**)&m_plugin_texture_cmd_list);
    unity_command_queue->Signal(m_plugin_texture_fence, ++m_plugin_texture_fence_value);
}