More efficently resize at break point

Uses iced `events_with()` to only subscribe to window resize
events. The window width is stored in a static AtomicU32 so
that the subscription can compare to an old window width as to
only send messages when the width crosses the break point.

fix up

examples/cosmic/src/window.rs

@@ -2,17 +2,17 @@
 // SPDX-License-Identifier: MPL-2.0
 
 use cosmic::{
+    iced_native,
     iced_native::window,
     iced::widget::{column, container, horizontal_space, row, text},
-    iced::{self, Application, Command, Length},
+    iced::{self, Application, Command, Length, Subscription},
-    iced_lazy::responsive,
     iced_winit::window::{close, drag, toggle_maximize, minimize},
     theme::{self, Theme},
     widget::{icon, list, nav_bar, nav_button, header_bar, settings, scrollable, spin_button::{SpinButtonModel, SpinMessage}},
     Element,
     ElementExt,
 };
-use std::vec;
+use std::{vec, sync::atomic::{AtomicU32, Ordering}};
 
 mod bluetooth;
 
@@ -115,6 +115,9 @@ impl Default for Page {
     }
 }
 
+static WINDOW_WIDTH: AtomicU32 = AtomicU32::new(0);
+const BREAK_POINT: u32 = 900;
+
 #[derive(Default)]
 pub struct Window {
     title: String,
@@ -168,7 +171,8 @@ pub enum Message {
     Minimize,
     Maximize,
     InputChanged,
-    SpinButton(SpinMessage)
+    SpinButton(SpinMessage),
+    WindowWidthChanged(u32),
 }
 
 impl Window {
@@ -179,6 +183,10 @@ impl Window {
         ).into()
     }
 
+    fn is_condensed(&self) -> bool {
+      WINDOW_WIDTH.load(Ordering::Relaxed) < BREAK_POINT
+    }
+
     fn parent_page_button(&self, sub_page: impl SubPage) -> Element<Message> {
         let page = sub_page.parent_page();
         column!(
@@ -258,6 +266,22 @@ impl Application for Window {
         self.title.clone()
     }
 
+    fn subscription(&self) -> Subscription<Message> {
+      iced_native::subscription::events_with(|event, _| match event {
+        cosmic::iced::Event::Window(_window_id, window::Event::Resized{width, height}) => {
+          let old_width = WINDOW_WIDTH.load(Ordering::Relaxed);
+          if old_width == 0
+            || old_width < BREAK_POINT && width > BREAK_POINT
+              || old_width > BREAK_POINT && width < BREAK_POINT {
+            Some(width)
+          } else {
+            None
+          }
+        }
+        _ => None
+      }).map(Message::WindowWidthChanged)
+    }
+
     fn update(&mut self, message: Message) -> iced::Command<Self::Message> {
         match message {
             Message::Page(page) => {
@@ -282,6 +306,7 @@ impl Application for Window {
             Message::RowSelected(row) => println!("Selected row {row}"),
             Message::InputChanged => {},
             Message::SpinButton(msg) => self.spin_button.update(msg),
+            Message::WindowWidthChanged(new_width) => WINDOW_WIDTH.store(new_width, Ordering::Relaxed),
 
         }
 
@@ -289,14 +314,7 @@ impl Application for Window {
     }
 
     fn view(&self) -> Element<Message> {
-        // TODO: Adding responsive makes this regenerate on every size change, and regeneration
+          let (sidebar_message, sidebar_toggled) = if self.is_condensed() {
-        // involves allocations for many different items. Ideally, we could only make the nav bar
-        // responsive and leave the content to be sized normally.
-        responsive(|size| {
-            //TODO: send a message when this happens instead of having everything be recalculated on resize
-            let condensed = size.width < 900.0;
-
-            let (sidebar_message, sidebar_toggled) = if condensed {
               (Message::ToggleSidebarCondensed, self.sidebar_toggled_condensed)
           } else {
               (Message::ToggleSidebar, self.sidebar_toggled)
@@ -361,7 +379,7 @@ impl Application for Window {
               .padding(8)
               .style(theme::Container::Custom(nav_bar::nav_bar_sections_style));
 
-                if ! condensed {
+              if ! self.is_condensed() {
                   sidebar = sidebar.max_width(300)
               }
 
@@ -369,7 +387,7 @@ impl Application for Window {
               widgets.push(sidebar.debug(self.debug));
           }
 
-            if ! (condensed && sidebar_toggled) {
+          if ! (self.is_condensed() && sidebar_toggled) {
               let content: Element<_> = match self.page {
                   Page::Demo => self.view_demo(),
                   Page::Networking(None) => settings::view_column(vec![
@@ -429,8 +447,6 @@ impl Application for Window {
               .into();
 
           column(vec![header, content]).into()
-        })
-        .into()
     }
 
     fn theme(&self) -> Theme {