DragDrop.cpp

#include <ole2.h>
#include <ShObjIdl.h>

#include <vector>
#include <string>

class CDropTarget : public IDropTarget
{
public:
  CDropTarget() {};
  ~CDropTarget() {};

  // IUnknown methods
  HRESULT __stdcall QueryInterface(REFIID iid, void** ppvObject);
  ULONG __stdcall AddRef();
  ULONG __stdcall Release();

  // IDropTarget methods
  HRESULT __stdcall DragEnter(IDataObject* pDataObj, DWORD grfKeyState, POINTL pt, DWORD* pdwEffect);
  HRESULT __stdcall DragOver(DWORD grfKeyState, POINTL pt, DWORD* pdwEffect);
  HRESULT __stdcall DragLeave();
  HRESULT __stdcall Drop(IDataObject* pDataObj, DWORD grfKeyState, POINTL pt, DWORD* pdwEffect);

  int m_RefCount = 0;
  DropHandler m_dropHandler;
  std::vector<std::vector<char> > m_filenames;
};

namespace lt {

  // This one is called externally
  bool RegisterDragDropWin (HWND__* in_windowHandle, const DropHandler& in_handler)
  {
    if (::OleInitialize (NULL) != S_OK)
    {
      return false;
    }

    CDropTarget* pDropTarget = new CDropTarget;
    pDropTarget->m_dropHandler = in_handler;
    return ::RegisterDragDrop (in_windowHandle, pDropTarget) == S_OK;
  }

} // namespace lt

HRESULT CDropTarget::QueryInterface (REFIID iid, void** ppvObject)
{
  if (iid == IID_IUnknown || iid == IID_IDropTarget)
  {
    *ppvObject = this;
    AddRef();
    return S_OK;
  }
  return E_NOINTERFACE;
}

ULONG CDropTarget::AddRef()
{
  return ++m_RefCount;
}

ULONG CDropTarget::Release()
{
  if (--m_RefCount == 0)
  {
    delete this;
    return 0;
  }
  return m_RefCount;
}

void FetchFileNames (IDataObject* pDataObj, std::vector<std::vector<char> >& out_filenames)
{
  out_filenames.clear();

  FORMATETC format = { CF_HDROP, nullptr, DVASPECT_CONTENT, -1, TYMED_HGLOBAL };
  STGMEDIUM stg;
  if (SUCCEEDED(pDataObj->GetData(&format, &stg)))
  {
    HDROP hDrop = (HDROP)stg.hGlobal;
    UINT count = ::DragQueryFileW(hDrop, 0xFFFFFFFF, nullptr, 0);
    for (UINT i = 0; i < count; i++)
    {
      UINT num_chars = ::DragQueryFileW(hDrop, i, nullptr, 0) + 1;
      WCHAR* buffer = new WCHAR[num_chars];
      ::DragQueryFileW (hDrop, i, buffer, num_chars);

      const int bytes_needed = WideCharToMultiByte (CP_UTF8, 0, buffer, -1, NULL, 0, NULL, NULL);

      out_filenames.emplace_back (bytes_needed, '\0');

      WideCharToMultiByte (CP_UTF8, 0, buffer, -1, out_filenames.back().data(), bytes_needed, NULL, NULL);

      delete[] buffer;
    }
    ::DragFinish (hDrop);
  }
  // release the data object
  ReleaseStgMedium (&stg);
}

HRESULT CDropTarget::DragEnter (IDataObject* pDataObj, DWORD grfKeyState, POINTL pt, DWORD* pdwEffect)
{
  GetGuiState().fileDropHovering = true;

  FetchFileNames (pDataObj, m_filenames);

  // Can use names of dragged files here as well

  return S_OK;
}

HRESULT CDropTarget::DragOver (DWORD grfKeyState, POINTL pt, DWORD* pdwEffect)
{
  GetGuiState().fileDropHovering = true;
  return S_OK;
}

HRESULT CDropTarget::DragLeave()
{
  GetGuiState().fileDropHovering = false;
  return S_OK;
}

HRESULT CDropTarget::Drop (IDataObject* pDataObj, DWORD grfKeyState, POINTL pt, DWORD* pdwEffect)
{
  FetchFileNames (pDataObj, m_filenames);

  if (m_filenames.size() > 0)
  {
    m_dropHandler (m_filenames[0].data());
  }

  return S_OK;
}

The code might have some bits from sokol_app.h