Radar loop: tick the frame outside MapView state (#237)

The 1 h loop set MapView state every 600 ms, re-rendering the whole map
component and rebuilding (setIcon) every marker icon on each tick. The
frame now lives in a small external store that only the radar frames and
the valid-time badge subscribe to.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-frontend/components/MapView.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { createContext, useCallback, useContext, useEffect, useMemo, useRef, useState } from "react";
+import { createContext, useCallback, useContext, useEffect, useMemo, useRef, useState, useSyncExternalStore } from "react";
 import { createPortal } from "react-dom";
 import {
   FeatureGroup,
@@ -346,6 +346,38 @@ const LOOP_HOLD_FRAMES = 3;   // the newest frame is held this many ticks before
 const RADAR_LEGEND_DBZ = [5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75];
 const RADAR_LEGEND_TICKS = [5, 40, 75];   // evenly spaced over RADAR_LEGEND_DBZ, so justify-between places them right
 
+/**
+ * The loop's current frame, kept OUTSIDE MapView's state: it ticks every
+ * LOOP_FRAME_MS, and a MapView re-render rebuilds every marker icon on the map
+ * (setIcon on each), so only the radar frames and the badge subscribe to it.
+ */
+function createLoopClock() {
+  let frame = 0;
+  let id: ReturnType<typeof setInterval> | undefined;
+  const subs = new Set<() => void>();
+  const set = (f: number) => { if (f !== frame) { frame = f; subs.forEach((s) => s()); } };
+  return {
+    subscribe: (cb: () => void) => { subs.add(cb); return () => { subs.delete(cb); }; },
+    get: () => frame,
+    run(on: boolean) {
+      clearInterval(id);
+      id = undefined;
+      set(0);
+      if (!on) return;
+      const last = RADAR_LOOP_OFFSETS_MIN.length - 1;
+      const cycle = last + LOOP_HOLD_FRAMES;
+      let tick = 0;
+      id = setInterval(() => {
+        if (document.hidden) return;
+        tick = (tick + 1) % cycle;
+        set(Math.min(tick, last));
+      }, LOOP_FRAME_MS);
+    },
+  };
+}
+type LoopClock = ReturnType<typeof createLoopClock>;
+const useLoopFrame = (c: LoopClock) => useSyncExternalStore(c.subscribe, c.get, c.get);
+
 function radarBuster(valid: Date | null, clock: number): string {
   return `?_=${valid ? valid.getTime() : clock}`;
 }
@@ -363,7 +395,8 @@ function RadarFrame({ offsetMin, buster, opacity }: { offsetMin: number; buster:
  * All frames stay loaded; only the shown one is opaque, so the loop plays
  * without re-fetching tiles every cycle.
  */
-function RadarLayer({ valid, loop, frame }: { valid: Date | null; loop: boolean; frame: number }) {
+function RadarLayer({ valid, loop, loopClock }: { valid: Date | null; loop: boolean; loopClock: LoopClock }) {
+  const frame = useLoopFrame(loopClock);
   const [clock, setClock] = useState(() => Math.floor(Date.now() / RADAR_REFRESH_MS));
   useEffect(() => {
     if (valid) return;   // the valid time drives refreshes while it is known
@@ -387,9 +420,10 @@ function hhmmLocal(d: Date): string {
 }
 
 /** Radar valid-time badge + loop toggle: what time the map's radar shows, and whether that is stale. */
-function RadarBadge({ meta, loop, frame, onLoop }: {
-  meta: RadarMeta; loop: boolean; frame: number; onLoop: (on: boolean) => void;
+function RadarBadge({ meta, loop, loopClock, onLoop }: {
+  meta: RadarMeta; loop: boolean; loopClock: LoopClock; onLoop: (on: boolean) => void;
 }) {
+  const frame = useLoopFrame(loopClock);
   const [, tick] = useState(0);
   useEffect(() => {
     const id = setInterval(() => tick((t) => t + 1), 30_000);
@@ -1530,7 +1564,7 @@ export default function MapView({
   const [radarShown, setRadarShown] = useState(() => startLayers.has("radar"));
   const [wxShown, setWxShown] = useState(() => startLayers.has("wx"));
   const [radarLoop, setRadarLoop] = useState(false);   // off by default (#237); not persisted
-  const [loopFrame, setLoopFrame] = useState(0);
+  const loopClock = useMemo(createLoopClock, []);
 
   // The altitude key only belongs in the legend while the opt-in Aircraft
   // overlay is actually on (server-26#185); same for the outage key (#184).
@@ -1566,18 +1600,9 @@ export default function MapView({
   const wxAlerts = useMapWeatherAlerts(mapInstance, wxShown);
   const loopOn = radarShown && radarLoop;
   useEffect(() => {
-    if (!loopOn) { setLoopFrame(0); return; }
-    const last = RADAR_LOOP_OFFSETS_MIN.length - 1;
-    let tick = 0;
-    const cycle = last + LOOP_HOLD_FRAMES;
-    setLoopFrame(0);
-    const id = setInterval(() => {
-      if (document.hidden) return;
-      tick = (tick + 1) % cycle;
-      setLoopFrame(Math.min(tick, last));
-    }, LOOP_FRAME_MS);
-    return () => clearInterval(id);
-  }, [loopOn]);
+    loopClock.run(loopOn);
+    return () => loopClock.run(false);
+  }, [loopClock, loopOn]);
 
   // eslint-disable-next-line react-hooks/exhaustive-deps
   const ago = useMemo(() => (lastUpdated ? timeAgo(lastUpdated) : null), [lastUpdated, agoClock]);
@@ -1791,7 +1816,7 @@ export default function MapView({
           {/* Overlay: Weather Radar — NEXRAD n0q via Iowa Env Mesonet (server-26#237) */}
           <LayersControl.Overlay checked={layerOn("radar")} name={LAYER_NAMES.radar}>
             <FeatureGroup>
-              <RadarLayer valid={radarMeta.valid} loop={loopOn} frame={loopFrame} />
+              <RadarLayer valid={radarMeta.valid} loop={loopOn} loopClock={loopClock} />
             </FeatureGroup>
           </LayersControl.Overlay>
 
@@ -1823,7 +1848,7 @@ export default function MapView({
       {(radarShown || wxShown) && (
         <div className="absolute bottom-10 md:bottom-8 left-1/2 -translate-x-1/2 z-[1001] flex flex-col items-center gap-1 w-max max-w-[min(30rem,calc(100%-1.5rem))] pointer-events-none">
           {wxShown && <WeatherWarningsStatus state={wxAlerts} />}
-          {radarShown && <RadarBadge meta={radarMeta} loop={radarLoop} frame={loopFrame} onLoop={setRadarLoop} />}
+          {radarShown && <RadarBadge meta={radarMeta} loop={radarLoop} loopClock={loopClock} onLoop={setRadarLoop} />}
         </div>
       )}