window: introduce interface for determining if dark mode is active

This is a baby step: it introduces the Appearance concept
and provides an accessor at the Connection level.

Only macos implements this at this time, and nothing else
makes use of it.

refs: https://github.com/wez/wezterm/issues/894
refs: https://github.com/wez/wezterm/issues/806

window/src/connection.rs

@@ -1,4 +1,4 @@
-use crate::Connection;
+use crate::{Appearance, Connection};
 use anyhow::Result as Fallible;
 use std::cell::RefCell;
 use std::rc::Rc;
@@ -36,6 +36,11 @@ pub trait ConnectionOps {
     fn terminate_message_loop(&self);
     fn run_message_loop(&self) -> Fallible<()>;
 
+    /// Retrieve the current appearance for the application.
+    fn get_appearance(&self) -> Appearance {
+        Appearance::Light
+    }
+
     /// Hide the application.
     /// This actions hides all of the windows of the application and switches
     /// focus away from it.

window/src/lib.rs

@@ -61,6 +61,34 @@ pub enum MouseCursor {
     SizeLeftRight,
 }
 
+/// Represents the preferred appearance of the windowing
+/// environment.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum Appearance {
+    /// Standard dark-text-on-light-background presentation
+    Light,
+    /// Dark mode, with predominantly dark or muted colors
+    Dark,
+    /// dark-text-on-light-background, but in a higher contrast
+    /// more accesible palette
+    LightHighContrast,
+    /// darker background but with higher contrast than regular
+    /// dark mode
+    DarkHighContrast,
+}
+
+impl std::string::ToString for Appearance {
+    fn to_string(&self) -> String {
+        match self {
+            Self::Light => "Light",
+            Self::Dark => "Dark",
+            Self::LightHighContrast => "LightHighContrast",
+            Self::DarkHighContrast => "DarkHighContrast",
+        }
+        .to_string()
+    }
+}
+
 #[derive(Debug)]
 pub enum WindowEvent {
     /// Called when the window close button is clicked.

window/src/os/macos/connection.rs

@@ -1,9 +1,11 @@
 // let () = msg_send! is a common pattern for objc
 #![allow(clippy::let_unit_value)]
 
+use super::nsstring_to_str;
 use super::window::WindowInner;
 use crate::connection::ConnectionOps;
 use crate::spawn::*;
+use crate::Appearance;
 use cocoa::appkit::{NSApp, NSApplication, NSApplicationActivationPolicyRegular};
 use cocoa::base::{id, nil};
 use objc::*;
@@ -81,6 +83,22 @@ impl ConnectionOps for Connection {
         }
     }
 
+    fn get_appearance(&self) -> Appearance {
+        let name = unsafe {
+            let appearance: id = msg_send![self.ns_app, effectiveAppearance];
+            nsstring_to_str(msg_send![appearance, name])
+        };
+        match name {
+            "NSAppearanceNameVibrantDark" | "NSAppearanceNameDarkAqua" => Appearance::Dark,
+            "NSAppearanceNameVibrantLight" | "NSAppearanceNameAqua" => Appearance::Light,
+            "NSAppearanceNameAccessibilityHighContrastVibrantLight"
+            | "NSAppearanceNameAccessibilityHighContrastAqua" => Appearance::LightHighContrast,
+            "NSAppearanceNameAccessibilityHighContrastVibrantDark"
+            | "NSAppearanceNameAccessibilityHighContrastDarkAqua" => Appearance::DarkHighContrast,
+            _ => Appearance::Light,
+        }
+    }
+
     fn run_message_loop(&self) -> anyhow::Result<()> {
         unsafe {
             self.ns_app.run();