Add onSessionChangeCallback to react-native tracker

[dnedo]

This PR resolves #1432 by adding the ability to pass an onSessionUpdateCallback callback to the react-native tracker.

This has worked for our team when used as a patch. However, please let me know if there are any further changes required to get this merged into the library. For example, it looks like the session plugin exposes the startNewSession method, which as far as I can tell is not called anywhere right now. Perhaps the team has additional context or future plans that would require adjusting the implementation to account for sessions updated this way?

[snowplowcla]

Thanks for your pull request. Is this your first contribution to a Snowplow open source project? Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://docs.snowplowanalytics.com/docs/contributing/contributor-license-agreement/ to learn more and sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

[matus-tomlein]

This looks great, thank you for the contribution!

Can I please ask you to add a unit test in the session.test.ts file in the RN tracker project. Something like this would do:

  it('calls onSessionUpdateCallback when a new session starts', async () => {
    jest.setSystemTime(new Date('2022-04-17T00:00:00.000Z'));

    const callback = jest.fn();

    const sessionPlugin = await newSessionPlugin({
      asyncStorage: AsyncStorage,
      namespace: 'test-callback',
      foregroundSessionTimeout: 5,
      backgroundSessionTimeout: 5,
      onSessionUpdateCallback: callback,
    });

    const tracker = trackerCore({ corePlugins: [sessionPlugin.plugin] });

    tracker.track(buildPageView({ pageUrl: 'http://localhost' }));
    expect(callback).toHaveBeenCalledTimes(1);
    const initialState = await sessionPlugin.getSessionState();
    expect(callback).toHaveBeenCalledWith(expect.objectContaining({
      sessionId: initialState.sessionId,
    }));

    // Advance time to force session timeout
    jest.setSystemTime(new Date('2022-04-17T00:00:10.000Z'));

    tracker.track(buildPageView({ pageUrl: 'http://localhost' }));
    expect(callback).toHaveBeenCalledTimes(2);
    const updatedState = await sessionPlugin.getSessionState();
    expect(callback).toHaveBeenCalledWith(expect.objectContaining({
      sessionId: updatedState.sessionId,
    }));
    expect(initialState.sessionId).not.toEqual(updatedState.sessionId);
  });

About the startNewSession method – it's true that it is not used/exposed anywhere, but that is an oversight. The thinking with that function was to expose it from the tracker so that users can force the start of a new session.

[matus-tomlein]

Also please sign the CLA in order for us to be able to integrate this, thanks!