lens/packages/utility-features/event-emitter/src/event-emitter.test.ts

import { EventEmitter } from "./event-emitter";

describe("EventEmitter", () => {
  it("should stop early if a listener returns false", () => {
    let called = false;
    const e = new EventEmitter<[]>();

    e.addListener(() => false, {});
    e.addListener(() => {
      called = true;
    }, {});
    e.emit();

    expect(called).toBe(false);
  });

  it("shouldn't stop early if a listener returns 0", () => {
    let called = false;
    const e = new EventEmitter<[]>();

    e.addListener(() => 0 as never, {});
    e.addListener(() => {
      called = true;
    }, {});
    e.emit();

    expect(called).toBe(true);
  });

  it("prepended listeners should be called before others", () => {
    const callOrder: number[] = [];
    const e = new EventEmitter<[]>();

    e.addListener(() => {
      callOrder.push(1);
    }, {});
    e.addListener(() => {
      callOrder.push(2);
    }, {});
    e.addListener(
      () => {
        callOrder.push(3);
      },
      { prepend: true },
    );
    e.emit();

    expect(callOrder).toStrictEqual([3, 1, 2]);
  });

  it("once listeners should be called only once", () => {
    const callOrder: number[] = [];
    const e = new EventEmitter<[]>();

    e.addListener(() => {
      callOrder.push(1);
    }, {});
    e.addListener(() => {
      callOrder.push(2);
    }, {});
    e.addListener(
      () => {
        callOrder.push(3);
      },
      { once: true },
    );
    e.emit();
    e.emit();

    expect(callOrder).toStrictEqual([1, 2, 3, 1, 2]);
  });

  it("removeListener should stop the listener from being called", () => {
    const callOrder: number[] = [];
    const e = new EventEmitter<[]>();
    const r = () => {
      callOrder.push(3);
    };

    e.addListener(() => {
      callOrder.push(1);
    }, {});
    e.addListener(() => {
      callOrder.push(2);
    }, {});
    e.addListener(r);

    e.emit();
    e.removeListener(r);
    e.emit();

    expect(callOrder).toStrictEqual([1, 2, 3, 1, 2]);
  });

  it("removeAllListeners should stop the all listeners from being called", () => {
    const callOrder: number[] = [];
    const e = new EventEmitter<[]>();

    e.addListener(() => {
      callOrder.push(1);
    });
    e.addListener(() => {
      callOrder.push(2);
    });
    e.addListener(() => {
      callOrder.push(3);
    });

    e.emit();
    e.removeAllListeners();
    e.emit();

    expect(callOrder).toStrictEqual([1, 2, 3]);
  });
});