Ability to force a theme on Windows (#1666)

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,
@@ -70,11 +72,14 @@ lazy_static! {
     static ref LIGHT_THEME_NAME: Vec<u16> = widestring("");
 }
 
-/// 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 {
+/// Attempt to set a theme on a window, if necessary.
+/// Returns the theme that was picked
+pub fn try_theme(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

@@ -36,7 +36,7 @@ use crate::{
     event_loop::{ControlFlow, EventLoopClosed, EventLoopWindowTarget as RootELW},
     monitor::MonitorHandle as RootMonitorHandle,
     platform_impl::platform::{
-        dark_mode::try_dark_mode,
+        dark_mode::try_theme,
         dpi::{become_dpi_aware, dpi_to_scale_factor, enable_non_client_dpi_scaling},
         drop_handler::FileDropHandler,
         event::{self, handle_extended_keys, process_key_params, vkey_to_winit_vkey},
@@ -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_theme(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

@@ -13,6 +13,7 @@ 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 {
@@ -20,6 +21,7 @@ pub struct PlatformSpecificWindowBuilderAttributes {
     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

@@ -34,7 +34,7 @@ use crate::{
     icon::Icon,
     monitor::MonitorHandle as RootMonitorHandle,
     platform_impl::platform::{
-        dark_mode::try_dark_mode,
+        dark_mode::try_theme,
         dpi::{dpi_to_scale_factor, hwnd_dpi},
         drop_handler::FileDropHandler,
         event_loop::{self, EventLoopWindowTarget, DESTROY_MSG_ID},
@@ -43,7 +43,7 @@ use crate::{
         window_state::{CursorFlags, SavedWindow, WindowFlags, WindowState},
         PlatformSpecificWindowBuilderAttributes, WindowId,
     },
-    window::{CursorIcon, Fullscreen, UserAttentionType, WindowAttributes},
+    window::{CursorIcon, Fullscreen, Theme, UserAttentionType, WindowAttributes},
 };
 
 /// The Win32 implementation of the main `Window` object.
@@ -653,8 +653,8 @@ 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
     }
 }
 
@@ -764,14 +764,15 @@ 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_theme(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(),
         }