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Support MS-Windows light/dark mode theme change during runtime. #2

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Support MS-Windows light/dark mode theme change during runtime. #2

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114 changes: 93 additions & 21 deletions src/w32fns.c
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Original file line number Diff line number Diff line change
Expand Up @@ -92,6 +92,7 @@ typedef enum _WTS_VIRTUAL_CLASS {
See: https://github.com/microsoft/WindowsAppSDK/issues/41
*/
#define DARK_MODE_APP_NAME L"DarkMode_Explorer"
#define LIGHT_MODE_APP_NAME L"Explorer"
/* For Windows 10 version 1809, 1903, 1909. */
#ifndef DWMWA_USE_IMMERSIVE_DARK_MODE_OLD
#define DWMWA_USE_IMMERSIVE_DARK_MODE_OLD 19
Expand Down Expand Up @@ -311,9 +312,25 @@ int w32_major_version;
int w32_minor_version;
int w32_build_number;

/* If the OS supports light/dark mode. */
BOOL w32_supports_darkmode = FALSE;
/* If the OS is set to use dark mode. */
BOOL w32_darkmode = FALSE;

/* Track ALL window handles so they can be updated if the Windows
light/dark mode theme is changed. Each frame could have somehwere
between 1-6 HWNDs depending on which GUI features are enabled by
the user.

TODO: Convert this to something more dynamic:
* Remove upper limit (256) of HWNDs.
* When a HWND is destroyed it should be removed from this list
(just for sake of memory management cleanliness; it does not
actually cause a problem to make w32 calls to dead HWNDs).
*/
HWND g_hwnds[256];
int g_hwnds_idx = -1;

/* Distinguish between Windows NT and Windows 95. */
int os_subtype;

Expand Down Expand Up @@ -2413,29 +2430,77 @@ w32_init_class (HINSTANCE hinst)
}
}


/* Gets the preferred Windows app mode:
* FALSE = Light mode (this is equivalent to the user specifying
Light, or the absence of any setting)
* TRUE = Dark mode (added in Windows 10 1809). */
static BOOL
w32_querydarkmode (void)
{
if (w32_supports_darkmode && w32_follow_system_dark_mode)
{
/* Check Windows Registry for system theme.
TODO: "Nice to have" would be to create a lisp setting (which
defaults to this Windows Registry value), then read that lisp
value here instead. This would allow the user to forcibly
override the system theme (which is also user-configurable in
Windows settings; see MS-Windows section in Emacs manual). */
LPBYTE val =
w32_get_resource ("Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
"AppsUseLightTheme",
NULL);
return val && *val == 0;
}
return FALSE;
}

/* Applies the Windows system theme (light or dark) to the window
handle HWND. */
handle HWND. `track` should generally be TRUE to keep a reference
to this HWND for future use. */
static void
w32_applytheme (HWND hwnd)
w32_applytheme (HWND hwnd, bool track)
{
if (w32_darkmode && w32_follow_system_dark_mode)
if (w32_supports_darkmode && w32_follow_system_dark_mode)
{
/* Set window theme to that of a built-in Windows app (Explorer),
because it has dark scroll bars and other UI elements. */
if (SetWindowTheme_fn)
SetWindowTheme_fn (hwnd, DARK_MODE_APP_NAME, NULL);
{
if (w32_darkmode)
SetWindowTheme_fn (hwnd, DARK_MODE_APP_NAME, NULL);
else
SetWindowTheme_fn (hwnd, LIGHT_MODE_APP_NAME, NULL);
}

/* Set the titlebar to system dark mode. */
/* Toggle darkmode titlebar on or off. */
if (DwmSetWindowAttribute_fn)
{
/* Windows 10 version 2004 and up, Windows 11. */
DWORD attr = DWMWA_USE_IMMERSIVE_DARK_MODE;
/* Windows 10 older than 2004. */
if (w32_build_number < 19041)
attr = DWMWA_USE_IMMERSIVE_DARK_MODE_OLD;
/* Toggle dark mode flag based on value of `w32_darkmode` */
DwmSetWindowAttribute_fn (hwnd, attr,
&w32_darkmode, sizeof (w32_darkmode));
}

/* After applying the theme, add the HWND to our global list so
it can be changed later if the OS light/dark mode theme is
changed. */
if(track)
{
if(g_hwnds_idx < 256)
{
g_hwnds_idx++;
g_hwnds[g_hwnds_idx] = hwnd;
}
else
{
printf("Number of window handles has exceeded capacity!");
}
}
}
}

Expand All @@ -2452,7 +2517,7 @@ w32_createvscrollbar (struct frame *f, struct scroll_bar * bar)
bar->left, bar->top, bar->width, bar->height,
FRAME_W32_WINDOW (f), NULL, hinst, NULL);
if (hwnd)
w32_applytheme (hwnd);
w32_applytheme (hwnd, TRUE);
return hwnd;
}

Expand All @@ -2469,7 +2534,7 @@ w32_createhscrollbar (struct frame *f, struct scroll_bar * bar)
bar->left, bar->top, bar->width, bar->height,
FRAME_W32_WINDOW (f), NULL, hinst, NULL);
if (hwnd)
w32_applytheme (hwnd);
w32_applytheme (hwnd, TRUE);
return hwnd;
}

Expand Down Expand Up @@ -2792,7 +2857,7 @@ w32_createwindow (struct frame *f, int *coords)
}

/* Enable system light/dark theme. */
w32_applytheme (hwnd);
w32_applytheme (hwnd, TRUE);

/* Do this to discard the default setting specified by our parent. */
ShowWindow (hwnd, SW_HIDE);
Expand Down Expand Up @@ -5638,6 +5703,23 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
changed, so if Emacs is interested in some of them, it could
update its internal values. */
my_post_msg (&wmsg, hwnd, msg, wParam, lParam);

/* Check if settings changed Light/Dark mode.
Re-lookup the setting and update the HWNDs accordingly. */
if(w32_supports_darkmode)
{
BOOL new_darkmode = w32_querydarkmode();
if (w32_darkmode != new_darkmode)
{
w32_darkmode = new_darkmode;
/* Loop through all known HWNDs and apply theme */
for(int i=0; i<=g_hwnds_idx; i++)
{
w32_applytheme(g_hwnds[i], false);
}
}
}

goto dflt;

case WM_SETFOCUS:
Expand Down Expand Up @@ -11774,6 +11856,7 @@ globals_of_w32fns (void)
if (os_subtype == OS_SUBTYPE_NT
&& w32_major_version >= 10 && w32_build_number >= 17763)
{
w32_supports_darkmode = TRUE;
/* Load dwmapi.dll and uxtheme.dll, which will be needed to set
window themes. */
HMODULE dwmapi_lib = LoadLibrary("dwmapi.dll");
Expand All @@ -11782,19 +11865,8 @@ globals_of_w32fns (void)
HMODULE uxtheme_lib = LoadLibrary("uxtheme.dll");
SetWindowTheme_fn = (SetWindowTheme_Proc)
get_proc_addr (uxtheme_lib, "SetWindowTheme");

/* Check Windows Registry for system theme and set w32_darkmode.
TODO: "Nice to have" would be to create a lisp setting (which
defaults to this Windows Registry value), then read that lisp
value here instead. This would allow the user to forcibly
override the system theme (which is also user-configurable in
Windows settings; see MS-Windows section in Emacs manual). */
LPBYTE val =
w32_get_resource ("Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
"AppsUseLightTheme",
NULL);
if (val && *val == 0)
w32_darkmode = TRUE;
/* Set the preferred mode from OS settings. */
w32_darkmode = w32_querydarkmode();
}

except_code = 0;
Expand Down