1 files changed, 47 insertions, 23 deletions

diff --git a/lib/jotai.dart b/lib/jotai.dart
index a415e13..16de4e7 100644
--- a/lib/jotai.dart
+++ b/lib/jotai.dart
@@ -1,3 +1,19 @@
+// 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';
@@ -9,9 +25,10 @@ String _defaultToString(dynamic value) => value.toString();
 void Function(T) storageObserver<T>(
   String key, [
   String Function(T) toString = _defaultToString,
-]) {
-  return (T value) => SharedPreferencesAsync().setString(key, toString(value));
-}
+]) =>
+    // 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,
@@ -19,8 +36,11 @@ Future<T> fromString<T>(
   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) => options.firstWhere((option) => value == toString(option)),
       );
 }
@@ -30,9 +50,9 @@ Observable<T> observablePreference<T>(
   T defaultValue,
   List<T> options, [
   String Function(T) toString = _defaultToString,
-]) => Observable(
-  defaultValue,
-  loadValue: fromString(key, options, toString),
+]) => Observable.fromFuture(
+  initialValue: defaultValue,
+  future: fromString(key, options, toString),
   observers: [storageObserver(key, toString)],
 );
 
@@ -64,20 +84,26 @@ final locationAccuracyObservable = observablePreference(
 
 class Observable<T> {
   T _value;
-  final List<void Function(T)?> _observers = [];
+  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, {
-    Future<T>? loadValue,
-    List<void Function(T)?>? observers,
-  }) {
+  Observable(this._value, {List<void Function(T)>? observers}) {
     if (observers != null) {
-      _observers.addAll(observers);
+      _observers.addAll(observers.asMap());
     }
+  }
 
-    if (loadValue != null) {
-      loadValue.then((value) => this.value = value);
-    }
+  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;
@@ -88,21 +114,19 @@ class Observable<T> {
 
     _value = value;
 
-    for (var observer in _observers) {
-      if (observer != null) {
-        observer(_value);
-      }
+    for (var observer in _observers.values) {
+      observer(_value);
     }
   }
 
   int subscribe(void Function(T) onChange) {
-    final id = _observers.length;
-    _observers.add(onChange);
+    final id = _nextId++;
+    _observers[id] = onChange;
     return id;
   }
 
   void unsubscribe(int subscriberId) {
-    _observers[subscriberId] = null;
+    _observers.remove(subscriberId);
   }
 }