Some comments and cleanup

src/analytics/PassiveAnalytics.tsx

@@ -0,0 +1,32 @@
+import {useEffect, useRef} from 'react'
+
+import {getCurrentState, onAppStateChange} from '#/lib/appState'
+import {useAnalytics} from '#/analytics'
+
+/**
+ * Tracks passive analytics like app foreground/background time.
+ */
+export function PassiveAnalytics() {
+  const ax = useAnalytics()
+  const lastActive = useRef(
+    getCurrentState() === 'active' ? performance.now() : null,
+  )
+
+  useEffect(() => {
+    const sub = onAppStateChange(state => {
+      if (state === 'active') {
+        lastActive.current = performance.now()
+        ax.metric('state:foreground', {})
+      } else if (lastActive.current !== null) {
+        ax.metric('state:background', {
+          secondsActive: Math.round(
+            (performance.now() - lastActive.current) / 1e3,
+          ),
+        })
+      }
+    })
+    return () => sub.remove()
+  }, [ax])
+
+  return null
+}