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diff --git a/lib/jotai.dart b/lib/jotai.dart
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+// When I started this project, I knew I would eventually need either a library
+// similar to streaming_shared_preferences, or a global state management system
+// to cache the shared preferences. But I could not get that library to work,
+// and I don't like any of the state management solutions that exist for
+// Flutter.
+//
+// What I really wanted was something like jotai, and eventually I decided to
+// just write my own. I'm very happy with how simple it is. It's just an
+// Observable class, and a widget that listens for changes to the observable.
+// It was very easy to extend it to save the preferences, and it's blazingly
+// fast.
+//
+// There is a library called fl_observable that does essentially this but
+// better. I didn't find it until after I finished the settings page, and it
+// hasn't been updated for over a year. It also has very few users.
+
+import 'package:flutter/material.dart';
+import 'package:geolocator/geolocator.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+import 'package:speedometer/main.dart';
+
+String _defaultToString(dynamic value) => value.toString();
+
+void Function(T) storageObserver<T>(
+  String key, [
+  String Function(T) toString = _defaultToString,
+]) =>
+    // Loading the shared preferences for asll of these might not be
+    // particularly efficient, but it seems fine on my Moto G Stylus.
+    (T value) => SharedPreferencesAsync().setString(key, toString(value));
+
+Future<T> fromString<T>(
+  String key,
+  List<T> options, [
+  String Function(T) toString = _defaultToString,
+]) {
+  return SharedPreferencesAsync()
+      // This takes time to load in, but I've never noticed the delay
+      .getString(key)
+      .then(
+        // Going through all of these sounds like it's slower than necessary,
+        // but it's very simple, and most of the options lists are very small.
+        (value) {
+          try {
+            return options.firstWhere((option) => value == toString(option));
+          } catch (error, stack) {
+            logObservable.value = logObservable.value
+              ..add('While loading $key:\n$error\n$stack');
+            rethrow;
+          }
+        },
+      );
+}
+
+Observable<T> observablePreference<T>(
+  String key,
+  T defaultValue,
+  List<T> options, [
+  String Function(T) toString = _defaultToString,
+]) => Observable.fromFuture(
+  initialValue: defaultValue,
+  future: fromString(key, options, toString),
+  observers: [storageObserver(key, toString)],
+);
+
+final speedUnitsObservable = observablePreference('speedUnits', null, [
+  ...SpeedUnit.values,
+  null,
+]);
+final themeModeObservable = observablePreference(
+  'themeMode',
+  ThemeMode.system,
+  ThemeMode.values,
+);
+final primaryColorObservable = observablePreference(
+  "primaryColor",
+  Colors.red,
+  [...Colors.primaries, Colors.grey],
+  (color) => color.toARGB32().toString(),
+);
+final showMarginOfErrorObservable = observablePreference(
+  "showMarginOfError",
+  true,
+  [true, false],
+);
+final locationAccuracyObservable = observablePreference(
+  "locationAccuracy",
+  LocationAccuracy.best,
+  LocationAccuracy.values,
+);
+
+final logObservable = Observable(<String>[]);
+void log(String message) {
+  logObservable.value = logObservable.value..add(message);
+}
+
+class Observable<T> {
+  T _value;
+  int _nextId = 0;
+
+  // A map is used instead of a list for fast deletions and to avoid memory
+  // leaks. Storing IDs in a list would make deletion slow, and setting entries
+  // in the list to null would cause memory leaks.
+  final Map<int, void Function(T)> _observers = {};
+
+  Observable(this._value, {List<void Function(T)>? observers}) {
+    if (observers != null) {
+      for (var observer in observers) {
+        subscribe(observer);
+      }
+    }
+  }
+
+  factory Observable.fromFuture({
+    required T initialValue,
+    required Future<T> future,
+    List<void Function(T)>? observers,
+  }) {
+    final self = Observable(initialValue, observers: observers);
+    future.then((value) => self.value = value);
+    return self;
+  }
+
+  T get value => _value;
+  set value(T value) {
+    if (value == _value) {
+      return;
+    }
+
+    _value = value;
+
+    for (var observer in _observers.values) {
+      observer(_value);
+    }
+  }
+
+  int subscribe(void Function(T) onChange) {
+    final id = _nextId++;
+    _observers[id] = onChange;
+    return id;
+  }
+
+  void unsubscribe(int subscriberId) {
+    _observers.remove(subscriberId);
+  }
+}
+
+class ObserverBuilder<T> extends StatefulWidget {
+  final Observable<T> observable;
+  final Widget Function(BuildContext, T, void Function(T)) builder;
+
+  const ObserverBuilder({
+    required this.observable,
+    required this.builder,
+    super.key,
+  });
+
+  @override
+  State<ObserverBuilder<T>> createState() => _ObserverState<T>();
+}
+
+class _ObserverState<T> extends State<ObserverBuilder<T>> {
+  late T _value;
+  late int _subscriberId;
+
+  @override
+  void initState() {
+    super.initState();
+    _subscriberId = widget.observable.subscribe(
+      (value) => setState(() => _value = value),
+    );
+    _value = widget.observable.value;
+  }
+
+  @override
+  void dispose() {
+    super.dispose();
+    widget.observable.unsubscribe(_subscriberId);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return widget.builder(
+      context,
+      _value,
+      (value) => widget.observable.value = value,
+    );
+  }
+}