A REAPER extension that draws a red square on the arrange view.

reaper_square.cpp

#include <memory>

#ifdef _WIN32
#  include <windows.h>
#else
#  include <swell/swell.h>
#  include <wdltypes.h> // GWLP_WNDPROC
#endif

#define REAPERAPI_IMPLEMENT
#include "reaper_plugin_functions.h"

class WindowProcHook {
public:
  WindowProcHook(HWND window, WNDPROC proc)
    : m_window(window)
  {
    m_next = reinterpret_cast<WNDPROC>(SetWindowLongPtr(m_window,
      GWLP_WNDPROC, reinterpret_cast<LONG_PTR>(proc)));
  }

  LRESULT next(HWND window, UINT msg, WPARAM wParam, LPARAM lParam) const
  {
    return m_next(window, msg, wParam, lParam);
  }

  ~WindowProcHook()
  {
    SetWindowLongPtr(m_window, GWLP_WNDPROC, reinterpret_cast<LONG_PTR>(m_next));
  }

private:
  HWND m_window;
  WNDPROC m_next;
};

class Brush {
public:
  Brush(COLORREF color) : m_brush(CreateSolidBrush(color)) {}
  ~Brush() { DeleteObject(m_brush); }

  operator HBRUSH() const { return m_brush; }

private:
  HBRUSH m_brush;
};

static std::unique_ptr<WindowProcHook> g_mainHook;

static void paint(HWND window)
{
  static RECT rect{42, 42, 84, 84};
  static Brush brush(RGB(255, 0, 0));

  HDC hdc = GetDC(window);
  FillRect(hdc, &rect, brush);
  ReleaseDC(window, hdc);
}

static LRESULT CALLBACK paintProc(HWND window, UINT msg, WPARAM wParam, LPARAM lParam)
{
  LRESULT ret = g_mainHook->next(window, msg, wParam, lParam);

  switch(msg) {
  case WM_PAINT:
    paint(window);
    break;
  }

  return ret;
}

extern "C" REAPER_PLUGIN_DLL_EXPORT int REAPER_PLUGIN_ENTRYPOINT(
  REAPER_PLUGIN_HINSTANCE instance, reaper_plugin_info_t *rec)
{
  if(!rec) {
    g_mainHook.release();
    return 0;
  }

  if(rec->caller_version != REAPER_PLUGIN_VERSION)
    return 0;

  GetMainHwnd = reinterpret_cast<decltype(GetMainHwnd)>(rec->GetFunc("GetMainHwnd"));

  if(!GetMainHwnd)
    return 0;

  HWND arrange = GetDlgItem(GetMainHwnd(), 1000);
  g_mainHook = std::make_unique<WindowProcHook>(arrange, &paintProc);

  return 1;
}