Safe Swap

Swap an ImGui drawlist safely from one thread to another. Such as the Unreal Engine 4 game thread and the RHI (Rendering Hardware Interface) thread. I've uploaded this to github as I tend to get questions often on how I do this with ImGui.

⚠️ Please Note ⚠️

ImGui has since updated and now use their own Vector implementation rather than a C array. The code will need to be modified!

Now why would we want to do this?

A common approach to internals is having all your logic inside a DirectX function hook. While this is easy, it's a very poor way to go about it. The main reason being is that both the rendering thread (D3D) and the game thread are not in sync with each other. This leads to many issues such as accessing pointers that may no longer be valid. The engine may decide to free memory while your rendering thread is executing your logic which will lead to a crash.

How to use (Unreal Engine 4)

1. Create an instance of SafeDrawlistSwap

Game Thread

Call SafeDrawlistSwap::Swap(const ImDrawData* data) with the draw data from ImGui. For example in this case we'll use ImGui::GetDrawData(). Make sure this is called last and/or when you've done your drawing (at the end of the frame).

ImGui_ImplWin32_NewFrame();
ImGui::NewFrame();

// Drawing ...

ImGui::EndFrame();
ImGui::Render();

g_Swapper->Swap(ImGui::GetDrawData());

Rendering Thread

Call SafeDrawlistSwap::Render() at the end of the rendering thread frame (such as your PresentScene hook).

ImGui_ImplDX11_NewFrame(); // Make sure you create a new frame BEFORE you render the draw-list.
g_Swapper->Render();

return oPresent(pSwapChain, syncInterval, flags);