add action to get window id

native/src/window/action.rs

@@ -76,6 +76,8 @@ pub enum Action<T> {
     ///
     /// - **Web / Wayland:** Unsupported.
     ChangeAlwaysOnTop(bool),
+    /// Fetch an identifier unique to the window.
+    FetchId(Box<dyn FnOnce(u64) -> T + 'static>),
 }
 
 impl<T> Action<T> {
@@ -105,6 +107,7 @@ impl<T> Action<T> {
             Self::ChangeAlwaysOnTop(on_top) => {
                 Action::ChangeAlwaysOnTop(on_top)
             }
+            Self::FetchId(o) => Action::FetchId(Box::new(move |s| f(o(s)))),
         }
     }
 }
@@ -138,6 +141,7 @@ impl<T> fmt::Debug for Action<T> {
             Self::ChangeAlwaysOnTop(on_top) => {
                 write!(f, "Action::AlwaysOnTop({on_top})")
             }
+            Self::FetchId(_) => write!(f, "Action::FetchId"),
         }
     }
 }

winit/src/application.rs

@@ -794,6 +794,11 @@ pub fn run_command<A, E>(
                 window::Action::ChangeAlwaysOnTop(on_top) => {
                     window.set_always_on_top(on_top);
                 }
+                window::Action::FetchId(tag) => {
+                    proxy
+                        .send_event(tag(window.id().into()))
+                        .expect("Send message to event loop");
+                }
             },
             command::Action::System(action) => match action {
                 system::Action::QueryInformation(_tag) => {

winit/src/window.rs

@@ -95,3 +95,12 @@ pub fn change_always_on_top<Message>(on_top: bool) -> Command<Message> {
         on_top,
     )))
 }
+
+/// Fetches an identifier unique to the window.
+pub fn fetch_id<Message>(
+    f: impl FnOnce(u64) -> Message + 'static,
+) -> Command<Message> {
+    Command::single(command::Action::Window(window::Action::FetchId(Box::new(
+        f,
+    ))))
+}