Allow using observables for sensors

This is useful for creating agent sensors on some data coming from a
stream values

Change-type: minor

lib/sensor.spec.ts

@@ -3,6 +3,7 @@ import { Sensor } from './sensor';
 import { Ref } from './ref';
 import { stub } from 'sinon';
 import { setTimeout } from 'timers/promises';
+import { Observable } from './observable';
 
 describe('Sensor', () => {
 	it('only starts execution once subscribers have been added', async () => {
@@ -53,6 +54,25 @@ describe('Sensor', () => {
 		expect(state._.temperature).to.equal(23);
 	});
 
+	it('allows reporting on a value using observable', async () => {
+		type Heater = { temperature: number; on: boolean };
+		const sensor = Sensor.of<Heater>().from({
+			lens: '/temperature',
+			sensor: () => Observable.from([20, 23]),
+		});
+
+		const state = Ref.of({ temperature: 0, on: false });
+
+		const next = stub();
+		sensor(state).subscribe(next);
+
+		await setTimeout(10);
+
+		expect(next).to.have.been.calledWith({ temperature: 20, on: false });
+		expect(next).to.have.been.calledWith({ temperature: 23, on: false });
+		expect(state._.temperature).to.equal(23);
+	});
+
 	it('allows reporting on a value using lenses with args', async () => {
 		type Heater = { temperature: { [room: string]: number }; on: boolean };
 		const sensor = Sensor.of<Heater>().from({

lib/sensor.ts

@@ -17,7 +17,8 @@ export type SensorFn<T, P extends PathType = '/'> = (
 	args: LensArgs<T, P>,
 ) =>
 	| AsyncGenerator<Lens<T, P>, never | void | Lens<T, P>, void>
-	| Generator<Lens<T, P>, never | void, void | undefined>;
+	| Generator<Lens<T, P>, never | void, void | undefined>
+	| Subscribable<Lens<T, P>>;
 
 /**
  * A sensor receives a reference to a global state and