Extract startable-stoppable to NPM package

Signed-off-by: Janne Savolainen <janne.savolainen@live.fi>

packages/utility-features/startable-stoppable/src/get-startable-stoppable.test.ts

@@ -0,0 +1,62 @@
+import type { StartableStoppable } from "./get-startable-stoppable";
+import { getStartableStoppable } from "./get-startable-stoppable";
+
+describe("getStartableStoppable", () => {
+  let stopMock: jest.MockedFunction<() => void>;
+  let startMock: jest.MockedFunction<() => () => void>;
+  let actual: StartableStoppable;
+
+  beforeEach(() => {
+    stopMock = jest.fn();
+    startMock = jest.fn().mockImplementation(() => stopMock);
+    actual = getStartableStoppable("some-id", startMock);
+  });
+
+  it("does not start yet", () => {
+    expect(startMock).not.toHaveBeenCalled();
+  });
+
+  it("does not stop yet", () => {
+    expect(stopMock).not.toHaveBeenCalled();
+  });
+
+  it("when stopping before ever starting, throws", () => {
+    expect(() => actual.stop()).toThrow('Tried to stop "some-id", but it is already stopped.');
+  });
+
+  it("is not started", () => {
+    expect(actual.started).toBe(false);
+  });
+
+  describe("when started", () => {
+    beforeEach(() => {
+      actual.start();
+    });
+
+    it("calls start function", () => {
+      expect(startMock).toHaveBeenCalled();
+    });
+
+    it("is started", () => {
+      expect(actual.started).toBe(true);
+    });
+
+    it("when started again, throws", () => {
+      expect(() => actual.start()).toThrow('Tried to start "some-id", but it is already started.');
+    });
+
+    describe("when stopped", () => {
+      beforeEach(() => {
+        actual.stop();
+      });
+
+      it("calls stop function", () => {
+        expect(stopMock).toBeCalled();
+      });
+
+      it("is stopped", () => {
+        expect(actual.started).toBe(false);
+      });
+    });
+  });
+});

packages/utility-features/startable-stoppable/src/get-startable-stoppable.ts

@@ -0,0 +1,40 @@
+export type Stopper = () => void;
+export type Starter = () => Stopper;
+
+export interface StartableStoppable {
+  readonly started: boolean;
+  start: () => void;
+  stop: () => void;
+}
+
+type StartableStoppableState = "stopped" | "started" | "starting";
+
+export function getStartableStoppable(id: string, startAndGetStopper: Starter): StartableStoppable {
+  let stop: Stopper;
+  let state: StartableStoppableState = "stopped";
+
+  return {
+    get started() {
+      return state === "started";
+    },
+
+    start: () => {
+      if (state !== "stopped") {
+        throw new Error(`Tried to start "${id}", but it is already ${state}.`);
+      }
+
+      state = "starting";
+      stop = startAndGetStopper();
+      state = "started";
+    },
+
+    stop: () => {
+      if (state !== "started") {
+        throw new Error(`Tried to stop "${id}", but it is already ${state}.`);
+      }
+
+      stop();
+      state = "stopped";
+    },
+  };
+}