potentially resolve #4

src/select.ts

@@ -0,0 +1,12 @@
+import { Source } from './source'
+import { SourceLike } from './types'
+
+
+export function select<T>(source: SourceLike<T>) {
+  return new Source<T>(emit => {
+    const listener = (val: T) => emit(val)
+    source.get(listener)
+
+    return () => source.remove(listener)
+  })
+}

src/state.ts

@@ -0,0 +1,9 @@
+import { Subject } from './subject'
+
+
+export class State<T> extends Subject<T> {
+  constructor(initial: T) {
+    super()
+    this.set(initial)
+  }
+}

src/test/select.test.ts

@@ -0,0 +1,73 @@
+import { State } from '../state'
+import { observe } from '../observe'
+import { select } from '../select'
+
+
+describe(select, () => {
+  test('without select, higher order sources are stopped after switch.', () => {
+    const flag = new State(false)
+    const a = new State('a')
+    const b = new State('b')
+
+    const cb = jest.fn()
+    const switched = $ => $(flag) ? a : b
+
+    observe($ => cb($($(switched)!)))
+
+    expect(a.stopped).toBe(false)
+    expect(b.stopped).toBe(false)
+
+    flag.set(true)
+
+    expect(a.stopped).toBe(false)
+    expect(b.stopped).toBe(true)
+
+    flag.set(false)
+
+    expect(a.stopped).toBe(true)
+    expect(b.stopped).toBe(true)
+  })
+
+  test('with select, higher order sources are not stopped after switch.', () => {
+    const flag = new State(false)
+    const a = new State('a')
+    const b = new State('b')
+
+    const cb = jest.fn()
+    const switched = $ => $(flag) ? select(a) : select(b)
+
+    observe($ => cb($($(switched)!)))
+
+    expect(a.stopped).toBe(false)
+    expect(b.stopped).toBe(false)
+
+    flag.set(true)
+
+    expect(a.stopped).toBe(false)
+    expect(b.stopped).toBe(false)
+
+    flag.set(false)
+
+    expect(a.stopped).toBe(false)
+    expect(b.stopped).toBe(false)
+  })
+
+  test('removes its subscription when stopped.', () => {
+    const get = jest.fn()
+    const remove = jest.fn()
+    const stop = jest.fn()
+    const src = select({
+      get,
+      remove,
+      stop,
+      stops: jest.fn(),
+      stopped: false
+    })
+
+    expect(get).toHaveBeenCalled()
+
+    src.stop()
+    expect(remove).toHaveBeenCalled()
+    expect(stop).not.toHaveBeenCalled()
+  })
+})