.

src/module/State.ts

@@ -0,0 +1,32 @@
+export class State<T> {
+    subscribers = new Map<Symbol, SubscribeCallback<T>>();
+    value: T
+
+    constructor(value: T) {
+        this.value = value;
+    }
+
+    set(value: T) {
+        this.value = value;
+        for (const [_, subscriber] of this.subscribers) {
+            subscriber(value)
+        }
+    }
+
+    update(callback: (currentValue: T) => T) {
+        this.set(callback(this.value))
+    }
+
+    subscribe(subscriber: SubscribeCallback<T>) {
+        const subscriberSymbol = Symbol(subscriber.name)
+        this.subscribers.set(subscriberSymbol, subscriber);
+        subscriber(this.value);
+        return subscriberSymbol;
+    }
+
+    unsubscribe(subscriberSymbol: Symbol) {
+        this.subscribers.delete(subscriberSymbol);
+    }
+}
+
+type SubscribeCallback<T> = (value: T) => any