How to enable buttons when scroll bar hits bottom

Question

I'm writing a license agreement dialog box with Win32 and I'm stumped. As usual with these things I want the "accept/don't accept" buttons to become enabled when the slider of the scroll bar of the richedit control hits bottom, but I can't find a way to get notified of that event. The earliest I've been able to learn about it is when the user releases the left mouse button.

Is there a way to do this?

Here's what I tried so far:

• WM_VSCROLL and WM_LBUTTONUP in richedit's wndproc
• EN_MSGFILTER notification in dlgproc (yes the filter mask is getting set)
• WM_VSCROLL and WM_LBUTTONUP in dlgproc.
• EN_VSCROLL notification in dlgproc

I got so desperate I tried polling but that didn't work either because apparently timer messages stop arriving while the mouse button is down on the slider. I tried both:

• timer callback (to poll) in dlgproc
• timer callback (to poll) in richedit's wndproc

Answer 1

Even though it is possible, I don't think you should do it that way - the user will have no clue why the buttons are disabled. This can be very confusing, and confusing the user should be avoided at all costs ;-)

That's why most license dialogs have radio buttons for accept/decline with decline enabled by default, so you actively have to enable accept.

Answer 2

You need to sub-class the edit box and intercept the messages to the edit box itself. Here's an artical on MSDN about subclassing controls.

EDIT: Some code to demonstrate the scroll bar enabling a button:

#include <windows.h>
#include <richedit.h>

LRESULT __stdcall RichEditSubclass
(
  HWND window,
  UINT message,
  WPARAM w_param,
  LPARAM l_param
)
{
  HWND
    parent = reinterpret_cast <HWND> (GetWindowLong (window, GWL_HWNDPARENT));

  WNDPROC
    proc = reinterpret_cast <WNDPROC> (GetWindowLong (parent, GWL_USERDATA));

  switch (message)
  {
  case WM_VSCROLL:
    {
      SCROLLINFO
        scroll_info = 
        {
          sizeof scroll_info,
          SIF_ALL
        };

      GetScrollInfo (window, SB_VERT, &scroll_info);

      if (scroll_info.nPos + static_cast <int> (scroll_info.nPage) >= scroll_info.nMax ||
          scroll_info.nTrackPos + static_cast <int> (scroll_info.nPage) >= scroll_info.nMax)
      {
        HWND
          button = reinterpret_cast <HWND> (GetWindowLong (parent, 0));

        EnableWindow (button, TRUE);
      }
    }
    break;
  }

  return CallWindowProc (proc, window, message, w_param, l_param);
}

LRESULT __stdcall ApplicationWindowProc
(
  HWND window,
  UINT message,
  WPARAM w_param,
  LPARAM l_param
)
{
  bool
    use_default_proc = false;

  LRESULT
    result = 0;

  switch (message)
  {
  case WM_CREATE:
    {
      CREATESTRUCT
        *creation_data = reinterpret_cast <CREATESTRUCT *> (l_param);

      RECT
        client;

      GetClientRect (window, &client);

      HWND
        child = CreateWindow (RICHEDIT_CLASS,
                              TEXT ("The\nQuick\nBrown\nFox\nJumped\nOver\nThe\nLazy\nDog\nThe\nQuick\nBrown\nFox\nJumped\nOver\nThe\nLazy\nDog"),
                              WS_CHILD | WS_VISIBLE | ES_MULTILINE | ES_AUTOVSCROLL | WS_VSCROLL | ES_DISABLENOSCROLL,
                              0, 0, client.right, client.bottom - 30,
                              window,
                              0,
                              creation_data->hInstance,
                              0);

      SetWindowLong (window, GWL_USERDATA, GetWindowLong (child, GWL_WNDPROC));
      SetWindowLong (child, GWL_WNDPROC, reinterpret_cast <LONG> (RichEditSubclass));
      SetWindowLong (child, GWL_ID, 0);

      child = CreateWindow (TEXT ("BUTTON"), TEXT ("Go Ahead!"), WS_CHILD | WS_VISIBLE | WS_DISABLED, 0, client.bottom - 30, client.right, 30, window, 0, creation_data->hInstance, 0);

      SetWindowLong (window, 0, reinterpret_cast <LONG> (child));
      SetWindowLong (child, GWL_ID, 1);
    }
    break;

  case WM_COMMAND:
    if (HIWORD (w_param) == BN_CLICKED && LOWORD (w_param) == 1)
    {
      DestroyWindow (window);
    }
    break;

  default:
    use_default_proc = true;
    break;
  }

  return use_default_proc ? DefWindowProc (window, message, w_param, l_param) : result;
}

int __stdcall WinMain
(
  HINSTANCE instance,
  HINSTANCE unused,
  LPSTR command_line,
  int show
)
{
  LoadLibrary (TEXT ("riched20.dll"));

  WNDCLASS
    window_class = 
    {
      0,
      ApplicationWindowProc,
      0,
      4,
      instance,
      0,
      LoadCursor (0, IDC_ARROW),
      reinterpret_cast <HBRUSH> (COLOR_BACKGROUND + 1),
      0,
      TEXT ("ApplicationWindowClass")
    };

  RegisterClass (&window_class);

  HWND
    window = CreateWindow (TEXT ("ApplicationWindowClass"),
                           TEXT ("Application"),
                           WS_VISIBLE | WS_OVERLAPPED | WS_SYSMENU,
                           CW_USEDEFAULT,
                           CW_USEDEFAULT,
                           400, 300, 0, 0,
                           instance,
                           0);

  MSG
    message;

  int
    success;

  while (success = GetMessage (&message, window, 0, 0))
  { 
    if (success == -1)
    {
      break;
    }
    else
    {
      TranslateMessage (&message);
      DispatchMessage (&message);
    }
  }

  return 0;
}

The above doesn't handle the user moving the cursor in the edit box.

Answer 3

Why not use the EM_GETTHUMB message. (Assuming Rich Edit 2.0 or later).

If you are lucky this bottom position will match EM_GETLINECOUNT.

Answer 4

I would recommend starting up Spy++ and seeing which windows messages are getting sent to where.

http://msdn.microsoft.com/en-us/library/aa264396(VS.60).aspx