chore: reduce component overhead (#3)

Reviewed-on: #3

crates/core/tests/field_bench.rs

@@ -1,3 +1,5 @@
+use std::{cell::Cell, rc::Rc, time::Duration};
+
 use gpui::{
     AppContext as _, Context, Entity, IntoElement, ParentElement as _, Render, SharedString,
     Styled as _, TestAppContext, VisualTestContext, Window, div,
@@ -9,10 +11,12 @@ use gpui_native_core::input::{Input, InputState};
 
 struct FieldHost {
     url: Entity<InputState>,
+    renders: Rc<Cell<usize>>,
 }
 
 impl Render for FieldHost {
     fn render(&mut self, _window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        self.renders.set(self.renders.get() + 1);
         let url_error = self
             .url
             .read(cx)
@@ -34,15 +38,27 @@ impl Render for FieldHost {
 
 /// A field page in a real window; the url input is focused unless
 /// `focus` is false, which leaves the page truly idle.
-fn field_host(focus: bool) -> (TestAppContext, Entity<InputState>, VisualTestContext) {
+fn field_host(
+    focus: bool,
+) -> (
+    TestAppContext,
+    Entity<InputState>,
+    VisualTestContext,
+    Rc<Cell<usize>>,
+) {
     let cx = TestAppContext::single();
     cx.update(init);
+    let renders = Rc::new(Cell::new(0));
+    let renders_for_host = renders.clone();
     let (url, window) = cx.update(|cx| {
         let url = InputState::new(cx);
         let url_for_host = url.clone();
         let window = cx
             .open_window(gpui::WindowOptions::default(), |_, cx| {
-                cx.new(|_| FieldHost { url: url_for_host })
+                cx.new(|_| FieldHost {
+                    url: url_for_host,
+                    renders: renders_for_host.clone(),
+                })
             })
             .unwrap();
         (url, window)
@@ -55,12 +71,12 @@ fn field_host(focus: bool) -> (TestAppContext, Entity<InputState>, VisualTestCon
         }
         window.draw(cx).clear(cx);
     });
-    (cx, url, visual)
+    (cx, url, visual, renders)
 }
 
 #[test]
 fn an_idle_field_page_schedules_no_frames() {
-    let (_cx, _url, mut visual) = field_host(false);
+    let (_cx, _url, mut visual, _renders) = field_host(false);
     let frames = visual.update(|window, cx| window.simulate_next_frame(cx));
     assert_eq!(
         frames, 0,
@@ -68,12 +84,25 @@ fn an_idle_field_page_schedules_no_frames() {
     );
 }
 
+/// The caret blink is fps-capped: it must never demand a next frame, and its
+/// redraws arrive on the executor timer at the blink cadence.
 #[test]
-fn a_focused_field_page_schedules_frames_for_its_blink() {
-    let (_cx, _url, mut visual) = field_host(true);
+fn a_focused_field_blink_is_fps_capped_and_still_redraws() {
+    let (cx, _url, mut visual, renders) = field_host(true);
+    let renders_after_focus = renders.get();
+
     let frames = visual.update(|window, cx| window.simulate_next_frame(cx));
+    assert_eq!(
+        frames, 0,
+        "the blink must be fps-capped, not per-frame scheduled"
+    );
+
+    // The blink redraws through the executor timer (4 fps cap).
+    cx.executor().advance_clock(Duration::from_millis(251));
+    cx.run_until_parked();
+    visual.update(|window, cx| window.draw(cx).clear(cx));
     assert!(
-        frames > 0,
-        "the focused caret's blink needs frames to redraw at the blink cadence"
+        renders.get() > renders_after_focus,
+        "the capped blink must still redraw its page"
     );
 }