Ability to force a theme on Windows

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Unreleased
 
 - On Android, calling `WindowEvent::Focused` now works properly instead of always returning false. 
+- On Windows, add `WindowBuilderExtWindows::with_theme(Theme::)` to set a preferred theme.
 - On Windows, fix alt-tab behaviour by removing borderless fullscreen "always on top" flag.
 - On Windows, fix bug preventing windows with transparency enabled from having fully-opaque regions.
 - **Breaking:** On Windows, include prefix byte in scancodes.

FEATURES.md

@@ -117,7 +117,7 @@ If your PR makes notable changes to Winit's features, please update this section
 * Setting the taskbar icon
 * Setting the parent window
 * `WS_EX_NOREDIRECTIONBITMAP` support
-* Theme the title bar according to Windows 10 Dark Mode setting
+* Theme the title bar according to Windows 10 Dark Mode setting or set a preferred theme
 
 ### macOS
 * Window activation policy

src/platform/windows.rs

@@ -13,7 +13,7 @@ use crate::{
     event_loop::EventLoop,
     monitor::MonitorHandle,
     platform_impl::{EventLoop as WindowsEventLoop, WinIcon},
-    window::{BadIcon, Icon, Window, WindowBuilder},
+    window::{BadIcon, Icon, Theme, Window, WindowBuilder},
 };
 
 /// Additional methods on `EventLoop` that are specific to Windows.
@@ -82,7 +82,7 @@ pub trait WindowExtWindows {
     fn set_taskbar_icon(&self, taskbar_icon: Option<Icon>);
 
     /// Whether the system theme is currently Windows 10's "Dark Mode".
-    fn is_dark_mode(&self) -> bool;
+    fn theme(&self) -> Theme;
 }
 
 impl WindowExtWindows for Window {
@@ -102,8 +102,8 @@ impl WindowExtWindows for Window {
     }
 
     #[inline]
-    fn is_dark_mode(&self) -> bool {
-        self.window.is_dark_mode()
+    fn theme(&self) -> Theme {
+        self.window.theme()
     }
 }
 
@@ -125,6 +125,9 @@ pub trait WindowBuilderExtWindows {
     /// If you need COM API with `COINIT_MULTITHREADED` you must initialize it before calling any winit functions.
     /// See https://docs.microsoft.com/en-us/windows/win32/api/objbase/nf-objbase-coinitialize#remarks for more information.
     fn with_drag_and_drop(self, flag: bool) -> WindowBuilder;
+
+    /// Forces a theme or uses the system settings if `None` was provided.
+    fn with_theme(self, theme: Option<Theme>) -> WindowBuilder;
 }
 
 impl WindowBuilderExtWindows for WindowBuilder {
@@ -151,6 +154,12 @@ impl WindowBuilderExtWindows for WindowBuilder {
         self.platform_specific.drag_and_drop = flag;
         self
     }
+
+    #[inline]
+    fn with_theme(mut self, theme: Option<Theme>) -> WindowBuilder {
+        self.platform_specific.preferred_theme = theme;
+        self
+    }
 }
 
 /// Additional methods on `MonitorHandle` that are specific to Windows.

src/platform_impl/windows/dark_mode.rs

@@ -14,6 +14,8 @@ use winapi::{
     um::{libloaderapi, uxtheme, winuser},
 };
 
+use crate::window::Theme;
+
 lazy_static! {
     static ref WIN10_BUILD_VERSION: Option<DWORD> = {
         // FIXME: RtlGetVersion is a documented windows API,
@@ -72,9 +74,12 @@ lazy_static! {
 
 /// Attempt to set dark mode on a window, if necessary.
 /// Returns true if dark mode was set, false if not.
-pub fn try_dark_mode(hwnd: HWND) -> bool {
+pub fn try_dark_mode(hwnd: HWND, preferred_theme: Option<Theme>) -> Theme {
     if *DARK_MODE_SUPPORTED {
-        let is_dark_mode = should_use_dark_mode();
+        let is_dark_mode = match preferred_theme {
+            Some(theme) => theme == Theme::Dark,
+            None => should_use_dark_mode(),
+        };
 
         let theme_name = if is_dark_mode {
             DARK_THEME_NAME.as_ptr()
@@ -84,10 +89,12 @@ pub fn try_dark_mode(hwnd: HWND) -> bool {
 
         let status = unsafe { uxtheme::SetWindowTheme(hwnd, theme_name as _, std::ptr::null()) };
 
-        status == S_OK && set_dark_mode_for_window(hwnd, is_dark_mode)
-    } else {
-        false
+        if status == S_OK && set_dark_mode_for_window(hwnd, is_dark_mode) {
+            return Theme::Dark;
+        }
     }
+
+    Theme::Light
 }
 
 fn set_dark_mode_for_window(hwnd: HWND, is_dark_mode: bool) -> bool {

src/platform_impl/windows/event_loop.rs

@@ -45,7 +45,7 @@ use crate::{
         window_state::{CursorFlags, WindowFlags, WindowState},
         wrap_device_id, WindowId, DEVICE_ID,
     },
-    window::{Fullscreen, WindowId as RootWindowId},
+    window::{Fullscreen, Theme, WindowId as RootWindowId},
 };
 use runner::{EventLoopRunner, EventLoopRunnerShared};
 
@@ -1874,20 +1874,20 @@ unsafe extern "system" fn public_window_callback<T: 'static>(
         winuser::WM_SETTINGCHANGE => {
             use crate::event::WindowEvent::ThemeChanged;
 
-            let is_dark_mode = try_dark_mode(window);
-            let mut window_state = subclass_input.window_state.lock();
-            let changed = window_state.is_dark_mode != is_dark_mode;
+            let preferred_theme = subclass_input.window_state.lock().preferred_theme;
 
-            if changed {
-                use crate::window::Theme::*;
-                let theme = if is_dark_mode { Dark } else { Light };
+            if preferred_theme == None {
+                let new_theme = try_dark_mode(window, preferred_theme);
+                let mut window_state = subclass_input.window_state.lock();
 
-                window_state.is_dark_mode = is_dark_mode;
-                mem::drop(window_state);
-                subclass_input.send_event(Event::WindowEvent {
-                    window_id: RootWindowId(WindowId(window)),
-                    event: ThemeChanged(theme),
-                });
+                if window_state.current_theme != new_theme {
+                    window_state.current_theme = new_theme;
+                    mem::drop(window_state);
+                    subclass_input.send_event(Event::WindowEvent {
+                        window_id: RootWindowId(WindowId(window)),
+                        event: ThemeChanged(new_theme),
+                    });
+                }
             }
 
             commctrl::DefSubclassProc(window, msg, wparam, lparam)

src/platform_impl/windows/mod.rs

@@ -13,13 +13,15 @@ pub use self::icon::WinIcon as PlatformIcon;
 
 use crate::event::DeviceId as RootDeviceId;
 use crate::icon::Icon;
+use crate::window::Theme;
 
 #[derive(Clone)]
 pub struct PlatformSpecificWindowBuilderAttributes {
     pub parent: Option<HWND>,
     pub taskbar_icon: Option<Icon>,
     pub no_redirection_bitmap: bool,
     pub drag_and_drop: bool,
+    pub preferred_theme: Option<Theme>,
 }
 
 impl Default for PlatformSpecificWindowBuilderAttributes {
@@ -29,6 +31,7 @@ impl Default for PlatformSpecificWindowBuilderAttributes {
             taskbar_icon: None,
             no_redirection_bitmap: false,
             drag_and_drop: true,
+            preferred_theme: None,
         }
     }
 }

src/platform_impl/windows/window.rs

@@ -43,6 +43,7 @@ use crate::{
         window_state::{CursorFlags, SavedWindow, WindowFlags, WindowState},
         PlatformSpecificWindowBuilderAttributes, WindowId,
     },
+    window::Theme,
     window::{CursorIcon, Fullscreen, WindowAttributes},
 };
 
@@ -622,8 +623,8 @@ impl Window {
     }
 
     #[inline]
-    pub fn is_dark_mode(&self) -> bool {
-        self.window_state.lock().is_dark_mode
+    pub fn theme(&self) -> Theme {
+        self.window_state.lock().current_theme
     }
 }
 
@@ -733,14 +734,15 @@ unsafe fn init<T: 'static>(
     // If the system theme is dark, we need to set the window theme now
     // before we update the window flags (and possibly show the
     // window for the first time).
-    let dark_mode = try_dark_mode(real_window.0);
+    let current_theme = try_dark_mode(real_window.0, pl_attribs.preferred_theme);
 
     let window_state = {
         let window_state = WindowState::new(
             &attributes,
             pl_attribs.taskbar_icon,
             scale_factor,
-            dark_mode,
+            current_theme,
+            pl_attribs.preferred_theme,
         );
         let window_state = Arc::new(Mutex::new(window_state));
         WindowState::set_window_flags(window_state.lock(), real_window.0, |f| *f = window_flags);

src/platform_impl/windows/window_state.rs

@@ -3,7 +3,7 @@ use crate::{
     event::ModifiersState,
     icon::Icon,
     platform_impl::platform::{event_loop, util},
-    window::{CursorIcon, Fullscreen, WindowAttributes},
+    window::{CursorIcon, Fullscreen, Theme, WindowAttributes},
 };
 use parking_lot::MutexGuard;
 use std::{io, ptr};
@@ -31,7 +31,8 @@ pub struct WindowState {
 
     pub modifiers_state: ModifiersState,
     pub fullscreen: Option<Fullscreen>,
-    pub is_dark_mode: bool,
+    pub current_theme: Theme,
+    pub preferred_theme: Option<Theme>,
     pub high_surrogate: Option<u16>,
     window_flags: WindowFlags,
 }
@@ -101,7 +102,8 @@ impl WindowState {
         attributes: &WindowAttributes,
         taskbar_icon: Option<Icon>,
         scale_factor: f64,
-        is_dark_mode: bool,
+        current_theme: Theme,
+        preferred_theme: Option<Theme>,
     ) -> WindowState {
         WindowState {
             mouse: MouseProperties {
@@ -122,7 +124,8 @@ impl WindowState {
 
             modifiers_state: ModifiersState::default(),
             fullscreen: None,
-            is_dark_mode,
+            current_theme,
+            preferred_theme,
             high_surrogate: None,
             window_flags: WindowFlags::empty(),
         }

src/window.rs

@@ -869,7 +869,7 @@ pub enum Fullscreen {
     Borderless(Option<MonitorHandle>),
 }
 
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Copy, Debug, PartialEq)]
 pub enum Theme {
     Light,
     Dark,