These notes about ISwapChainPanelNative2 are VERY IMPORTANT

src/tools/ScratchWinRTClient/pch.h

@@ -12,5 +12,9 @@
 #include <winrt/Windows.Foundation.h>
 #include <winrt/Windows.Foundation.Collections.h>
 
+#include <winrt/Windows.UI.Xaml.Controls.h>
+#include <winrt/Windows.UI.Xaml.Media.h>
+#include <windows.ui.xaml.media.dxinterop.h>
+
 // Manually include til after we include Windows.Foundation to give it winrt superpowers
 #include "til.h"

src/tools/ScratchWinRTServer/HostClass.cpp

@@ -39,4 +39,54 @@ namespace winrt::ScratchWinRTServer::implementation
         _DoCount += 4;
         return S_OK;
     }
+
+    void HostClass::Attach(const Windows::UI::Xaml::Controls::SwapChainPanel& panel)
+    {
+        _panel = panel;
+    }
+
+    void HostClass::BeginRendering()
+    {
+        IDXGISwapChain1* swapChain; // = _getSwapchainFromMyRenderer();
+
+        // DANGER: I'm fairly certain that this needs to be called on the
+        // `SwapChainPanel`s UI thread. So we may be slightly out of luck here.
+        // Unless we can just
+        //   co_await winrt::resume_foreground(_panel.Dispatcher());
+        // But that's a thread in another process!
+
+        auto nativePanel = _panel.as<ISwapChainPanelNative>();
+        nativePanel->SetSwapChain(swapChain);
+
+        // Holy crap look at:
+        // ISwapChainPanelNative2::SetSwapChainHandle method
+        // https://docs.microsoft.com/en-us/windows/win32/api/windows.ui.xaml.media.dxinterop/nf-windows-ui-xaml-media-dxinterop-iswapchainpanelnative2-setswapchainhandle
+        //
+        // SetSwapChain(HANDLE swapChainHandle) allows a swap chain to be
+        // rendered by referencing a shared handle to the swap chain. This
+        // enables scenarios where a swap chain is created in one process and
+        // needs to be passed to another process.
+        //
+        // XAML supports setting a DXGI swap chain as the content of a
+        // SwapChainPanel element. Apps accomplish this by querying for the
+        // ISwapChainPanelNative interface from a SwapChainPanel instance and
+        // calling SetSwapChain(IDXGISwapChain *swapChain).
+        //
+        // This process works for pointers to in process swap chains. However,
+        // this doesn’t work for VoIP apps, which use a two-process model to
+        // enable continuing calls on a background process when a foreground
+        // process is suspended or shut down. This two-process implementation
+        // requires the ability to pass a shared handle to a swap chain, rather
+        // than a pointer, created on the background process to the foreground
+        // process to be rendered in a XAML SwapChainPanel in the foreground
+        // app.
+
+        // I _believe_ this will work something like:
+        //
+        // HANDLE hSwapChain;
+        // auto nativePanel2 = _panel.as<ISwapChainPanelNative2>();
+        // nativePanel2->SetSwapChainHandle(hSwapChain);
+        //
+        // But I'm not sure yet.
+    }
 }

src/tools/ScratchWinRTServer/HostClass.h

@@ -16,9 +16,14 @@ namespace winrt::ScratchWinRTServer::implementation
 
         HRESULT __stdcall Call() override;
 
+        void Attach(const Windows::UI::Xaml::Controls::SwapChainPanel& panel);
+        void BeginRendering();
+
     private:
         int _DoCount{ 0 };
         winrt::guid _id;
+
+        Windows::UI::Xaml::Controls::SwapChainPanel _panel{ nullptr };
     };
 }
 

src/tools/ScratchWinRTServer/HostClass.idl

@@ -9,6 +9,9 @@ namespace ScratchWinRTServer
         Int32 DoCount { get; };
         Guid Id { get; };
 
+        void Attach(Windows.UI.Xaml.Controls.SwapChainPanel panel);
+        void BeginRendering();
+
         // Adding a Windows.UI.Xaml element will crash the server when the
         // server tries to instantiate the element without a XAML host.
         //

src/tools/ScratchWinRTServer/pch.h

@@ -12,6 +12,10 @@
 #include <winrt/Windows.Foundation.h>
 #include <winrt/Windows.Foundation.Collections.h>
 
+#include <winrt/Windows.UI.Xaml.Controls.h>
+#include <winrt/Windows.UI.Xaml.Media.h>
+#include <windows.ui.xaml.media.dxinterop.h>
+
 // Manually include til after we include Windows.Foundation to give it winrt superpowers
 #include "til.h"