Merge pull request #2728 from CapSoftware/perf/cursor-lookup-on-events

perf(recording): read the cursor image only on ticks that record an event

Author: richiemcilroy

crates/recording/src/cursor.rs

@@ -572,7 +572,17 @@ pub fn spawn_cursor_recorder(
                 last_window_position = window_position;
             }
 
-            let cursor_id = if let Some(data) = get_cursor_data() {
+            // The cursor image only labels this tick's move and click events,
+            // and reading it builds a full TIFF of every representation
+            // (~0.5 ms, ~580 KB on a Retina Mac), so skip it on ticks that
+            // record neither.
+            let buttons_changed = buttons_changed(
+                &last_mouse_state.button_pressed,
+                &mouse_state.button_pressed,
+            );
+            let cursor_id = if !position_changed && !buttons_changed && last_cursor_id.is_some() {
+                last_cursor_id.clone()
+            } else if let Some(data) = get_cursor_data() {
                 let cursor_id = cursor_id_for_image(
                     data,
                     &mut response,
@@ -703,6 +713,15 @@ pub fn spawn_cursor_recorder(
     }
 }
 
+/// Whether any button present in both samples changed state: the same pairs
+/// the click events below are generated from.
+fn buttons_changed(previous: &[bool], current: &[bool]) -> bool {
+    previous
+        .iter()
+        .zip(current)
+        .any(|(previous, current)| previous != current)
+}
+
 fn cursor_id_for_image(
     data: CursorData,
     response: &mut CursorActorResponse,
@@ -1634,6 +1653,19 @@ mod cursor_image_tests {
         }
     }
 
+    #[test]
+    fn button_changes_match_the_pairs_clicks_are_built_from() {
+        assert!(!buttons_changed(
+            &[false, false, false],
+            &[false, false, false]
+        ));
+        assert!(buttons_changed(&[false, false], &[false, true]));
+        assert!(buttons_changed(&[true], &[false]));
+        // Clicks only compare buttons present in both samples.
+        assert!(!buttons_changed(&[false], &[false, true]));
+        assert!(!buttons_changed(&[], &[true]));
+    }
+
     #[test]
     fn an_unchanged_cursor_image_reuses_its_id_without_hashing() {
         let dir = tempfile::tempdir().unwrap();