Add observable pattern implementation with Subject and ReadOnlySubject classes

- Introduced Subject class for managing state and notifying listeners
- Added ReadOnlySubject class to provide read-only access to Subject values
- Created assertUnreachable utility function for unreachable code handling
- Updated observables index file to export Subject and ReadOnlySubject

src/lang/assertUnreachable.ts

@@ -0,0 +1,3 @@
+export default function assertUnreachable(x: never): never {
+  throw new Error(`Unreachable code [${x}]`);
+}

src/lang/observables/ReadOnlySubject.ts

@@ -0,0 +1,18 @@
+import Disposable from '../disposables/Disposable';
+import Subject from './Subject';
+
+export default class ReadOnlySubject<T> {
+  private readonly _subject: Subject<T>;
+
+  constructor(subject: Subject<T>) {
+    this._subject = subject;
+  }
+
+  get value(): T {
+    return this._subject.value;
+  }
+
+  public subscribe(listener: (value: T) => void): Disposable {
+    return this._subject.subscribe(listener);
+  }
+}

src/lang/observables/Subject.ts

@@ -0,0 +1,39 @@
+import Disposable from '../disposables/Disposable';
+
+export default class Subject<T> {
+  private readonly _listeners: Array<(value: T) => void> = [];
+  private _value: T;
+
+  constructor(value: T) {
+    this._value = value;
+  }
+
+  set value(value: T) {
+    if (this._value === value) {
+      return;
+    }
+
+    this._value = value;
+    this.notifyListeners(value);
+  }
+
+  get value(): T {
+    return this._value;
+  }
+
+  public subscribe(listener: (value: T) => void): Disposable {
+    const listenerIndex = this._listeners.push(listener);
+
+    listener(this.value);
+
+    return new Disposable(() => {
+      this._listeners.splice(listenerIndex, 1);
+    });
+  }
+
+  private notifyListeners(value: T): void {
+    for (const listener of this._listeners) {
+      listener(value);
+    }
+  }
+}

src/lang/observables/index.ts

@@ -0,0 +1,4 @@
+import Subject from './Subject';
+import ReadOnlySubject from './ReadOnlySubject';
+
+export {Subject, ReadOnlySubject};