feat: add observable()

src/context/__tests__/observable.test.ts

@@ -0,0 +1,63 @@
+import {observable} from '../observable';
+
+describe('observable()', () => {
+  it('creates and object', () => {
+    const obs = observable(123);
+    expect(typeof obs).toBe('object');
+});
+  it('if no initial value provided state equals undefined', () => {
+    const obs = observable();
+    expect(obs.get()).toBe(void 0);
+  });
+  it('sets state to the default specified value, .get() returns it', () => {
+    const state = {};
+    const obs = observable(state);
+    expect(obs.get()).toBe(state);
+  });
+  it('returns mutated state', () => {
+    const obs = observable();
+    const state = {};
+    obs.set(state);
+    expect(obs.get()).toBe(state);
+  });
+  it('.sub() returns an unsub function', () => {
+    const obs = observable();
+    const unsub = obs.sub(() => {});
+    expect(typeof unsub).toBe('function');
+  });
+  it('calls listener on on state change', () => {
+    const listener = jest.fn();
+    const obs = observable();
+    obs.sub(listener);
+    obs.set(123);
+    expect(listener).toHaveBeenCalledTimes(1);
+  });
+  it('calls correct number of times', () => {
+    const listener = jest.fn();
+    const obs = observable();
+    obs.sub(listener);
+    obs.set(1);
+    obs.set(2);
+    obs.set(3);
+    expect(listener).toHaveBeenCalledTimes(3);
+  });
+  it('calls listener with the correct data', () => {
+    const listener = jest.fn();
+    const obs = observable();
+    obs.sub(listener);
+    obs.set(1);
+    obs.set(2);
+    obs.set(3);
+    expect(listener.mock.calls).toEqual([[1], [2], [3]]);
+  });
+  it('stops calling listener when unsubscribed', () => {
+    const listener = jest.fn();
+    const obs = observable();
+    const unsub = obs.sub(listener);
+    obs.set(1);
+    obs.set(2);
+    unsub();
+    obs.set(3);
+    expect(listener.mock.calls).toEqual([[1], [2]]);
+  });
+});

src/context/observable.ts

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+export type TObservalbeUnsub = (() => void);
+export type TObservableSet<T> = (state: T) => void;
+export interface IObservable<T> {
+  get: () => T;
+  set: TObservableSet<T>;
+  sub: (listener: TObservableSet<T>) => TObservalbeUnsub;
+}
+
+export const observable: <T>(state?: T) => IObservable<T> = state => {
+  let listeners = [];
+  let currentState = state;
+
+  const get = () => currentState;
+
+  const set = state => {
+    currentState = state;
+    for (const listener of listeners) listener(currentState);
+  };
+
+  const sub = listener => {
+    listeners.push(listener);
+
+    return () => (listeners = listeners.filter(item => item !== listener));
+  };
+
+  return {
+    get,
+    set,
+    sub,
+  };
+};