Add observable utilities

src/observable.ts

···
+import { Source, Sink, SignalKind, TalkbackKind, Observer, Subject, TeardownFn } from './types'
+import { push, start, talkbackPlaceholder } from './helpers'
+
+interface ObservableSubscription {
+  closed?: boolean;
+  unsubscribe(): void;
+}
+
+export interface ObservableObserver<T> {
+  next(value: T): void;
+  error(error: any): void;
+  complete(): void;
+}
+
+interface Observable<T> {
+  subscribe(observer: ObservableObserver<T>): ObservableSubscription;
+}
+
+const observableSymbol: unique symbol = typeof Symbol === 'function'
+  ? (Symbol as any).observable || ((Symbol as any).observable = Symbol('observable'))
+  : '@@observable'
+
+export function fromObservable<T>(input: Observable<T>): Source<T> {
+  const observable: Observable<T> = input[observableSymbol]
+    ? (input as any)[observableSymbol]()
+    : input;
+  return (sink) => {
+    const subscription = observable.subscribe({
+      next(value: T) {
+        sink(push(value));
+      },
+      complete() {
+        sink(SignalKind.End);
+      },
+      error() {/*noop*/},
+    });
+    sink(start((signal) => {
+      if (signal === TalkbackKind.Close) subscription.unsubscribe();
+    }));
+  };
+}
+
+export function toObservable<T>(source: Source<T>): Observable<T> {
+  const observable: Observable<T> = {
+    subscribe(observer: ObservableObserver<T>) {
+      let talkback = talkbackPlaceholder;
+      let ended = false;
+      source((signal) => {
+        if (ended) {
+          /*noop*/
+        } else if (signal === SignalKind.End) {
+          ended = true;
+          observer.complete();
+        } else if (signal.tag === SignalKind.Start) {
+          (talkback = signal[0])(TalkbackKind.Pull);
+        } else {
+          observer.next(signal[0]);
+          talkback(TalkbackKind.Pull);
+        }
+      });
+      const subscription = {
+        closed: false,
+        unsubscribe() {
+          subscription.closed = true;
+          ended = true;
+          talkback(TalkbackKind.Close);
+        },
+      };
+      return subscription;
+    },
+  };
+  observable[observableSymbol] = () => observable;
+  return observable;
+}