iOS: Add support for dark/light theme detection

This is here until rust-windowing/winit#4570
lands in a winit release we can use.

ChangeLog: iOS: Added detection of system dark/light theme

Author: tronical

internal/backends/winit/Cargo.toml

@@ -130,7 +130,7 @@ i-slint-renderer-skia = { workspace = true, features = ["default"] }
 [target.'cfg(target_os = "ios")'.dependencies]
 objc2 = { workspace = true }
 objc2-foundation = { version = "0.3.2", default-features = false, features = ["std", "block2", "NSString", "NSNotification", "NSOperation", "NSGeometry", "objc2-core-foundation"] }
-objc2-ui-kit = { version = "0.3.2", default-features = false, features = ["UIScreen", "UIWindow", "UIView", "UIViewAnimating", "UIViewPropertyAnimator", "UIResponder", "objc2-core-foundation", "objc2-quartz-core", "block2"] }
+objc2-ui-kit = { version = "0.3.2", default-features = false, features = ["UIScreen", "UIWindow", "UIView", "UIViewAnimating", "UIViewPropertyAnimator", "UIResponder", "UIInterface", "UITrait", "UITraitCollection", "objc2-core-foundation", "objc2-quartz-core", "block2"] }
 objc2-quartz-core = { version = "0.3.2", default-features = false, features = ["CADisplayLink", "CATransaction"] }
 # Match version in objc2
 block2 = "0.6.2"

internal/backends/winit/ios.rs

@@ -1,8 +1,12 @@
 // Copyright © SixtyFPS GmbH <info@slint.dev>
 // SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
 
+mod color_scheme;
 mod keyboard_animator;
 mod virtual_keyboard;
 
+pub(crate) use color_scheme::{
+    ColorSchemeObserver, current_color_scheme, install_color_scheme_observer,
+};
 pub(crate) use keyboard_animator::KeyboardCurveSampler;
 pub(crate) use virtual_keyboard::{KeyboardNotifications, register_keyboard_notifications};

internal/backends/winit/ios/color_scheme.rs

@@ -0,0 +1,84 @@
+// Copyright © SixtyFPS GmbH <info@slint.dev>
+// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
+
+// Stopgap until winit ships iOS appearance support (rust-windowing/winit#4570).
+// Once that lands, `winit_window.theme()` and `WindowEvent::ThemeChanged` will
+// work on iOS and this whole module — plus the `ios_color_scheme_observer`
+// wiring in winitwindowadapter.rs — can be deleted.
+
+use std::ptr::NonNull;
+use std::rc::Weak;
+
+use block2::RcBlock;
+use objc2::{
+    ClassType, Message, available, msg_send,
+    rc::Retained,
+    runtime::{AnyClass, ProtocolObject},
+};
+use objc2_foundation::NSArray;
+use objc2_ui_kit::{
+    UITraitChangeObservable, UITraitChangeRegistration, UITraitCollection, UITraitEnvironment,
+    UITraitUserInterfaceStyle, UIUserInterfaceStyle, UIView,
+};
+
+use i_slint_core::items::ColorScheme;
+
+use crate::winitwindowadapter::WinitWindowAdapter;
+
+fn style_to_color_scheme(style: UIUserInterfaceStyle) -> ColorScheme {
+    match style {
+        UIUserInterfaceStyle::Dark => ColorScheme::Dark,
+        UIUserInterfaceStyle::Light => ColorScheme::Light,
+        _ => ColorScheme::Unknown,
+    }
+}
+
+pub(crate) fn current_color_scheme(view: &UIView) -> ColorScheme {
+    style_to_color_scheme(unsafe { view.traitCollection().userInterfaceStyle() })
+}
+
+pub(crate) struct ColorSchemeObserver {
+    view: Retained<UIView>,
+    registration: Retained<ProtocolObject<dyn UITraitChangeRegistration>>,
+}
+
+impl Drop for ColorSchemeObserver {
+    fn drop(&mut self) {
+        self.view.unregisterForTraitChanges(&self.registration);
+    }
+}
+
+pub(crate) fn install_color_scheme_observer(
+    view: &UIView,
+    adapter: Weak<WinitWindowAdapter>,
+) -> Option<ColorSchemeObserver> {
+    // `registerForTraitChanges:withHandler:` is iOS 17+. Older iOS still gets the
+    // initial scheme via `current_color_scheme`, but no live updates.
+    if !available!(ios = 17.0) {
+        return None;
+    }
+
+    let handler = RcBlock::new(
+        move |env: NonNull<ProtocolObject<dyn UITraitEnvironment>>,
+              _prev: NonNull<UITraitCollection>| {
+            let Some(adapter) = adapter.upgrade() else { return };
+            let env = unsafe { env.as_ref() };
+            let scheme =
+                style_to_color_scheme(unsafe { env.traitCollection().userInterfaceStyle() });
+            adapter.set_color_scheme(scheme);
+        },
+    );
+
+    let traits: Retained<NSArray<AnyClass>> =
+        NSArray::from_slice(&[UITraitUserInterfaceStyle::class()]);
+
+    let registration: Retained<ProtocolObject<dyn UITraitChangeRegistration>> = unsafe {
+        msg_send![
+            view,
+            registerForTraitChanges: &*traits,
+            withHandler: &*handler,
+        ]
+    };
+
+    Some(ColorSchemeObserver { view: view.retain(), registration })
+}

internal/backends/winit/winitwindowadapter.rs

@@ -173,6 +173,8 @@ enum WinitWindowOrNone {
         context_menu_muda_adapter: RefCell<Option<crate::muda::MudaAdapter>>,
         #[cfg(target_os = "ios")]
         keyboard_curve_sampler: super::ios::KeyboardCurveSampler,
+        #[cfg(target_os = "ios")]
+        _color_scheme_observer: Option<super::ios::ColorSchemeObserver>,
     },
     None(RefCell<WindowAttributes>),
 }
@@ -516,6 +518,12 @@ impl WinitWindowAdapter {
             (view, self.self_weak.clone())
         };
 
+        // winit doesn't surface iOS appearance, so query the view's trait
+        // collection directly; the matching live observer is installed below as
+        // part of the `HasWindow` variant so its lifetime is tied to the window.
+        #[cfg(target_os = "ios")]
+        self.set_color_scheme(crate::ios::current_color_scheme(&content_view));
+
         let frame_throttle = crate::frame_throttle::create_frame_throttle(
             self.self_weak.clone(),
             &winit_window,
@@ -552,6 +560,11 @@ impl WinitWindowAdapter {
                     }
                 },
             ),
+            #[cfg(target_os = "ios")]
+            _color_scheme_observer: crate::ios::install_color_scheme_observer(
+                &content_view,
+                self.self_weak.clone(),
+            ),
         };
 
         #[cfg(muda)]