Adds AB Testing support (ExPlat)

[leandroalonso]

This PR adds AB Testing support.

Configuration

(WPiOS PR here: wordpress-mobile/WordPress-iOS#15236)

Before having fun with AB testing, the app needs to configure the ExPlat instance. To do so, the switchToAuthenticatedUserWithUsername method was changed and now it requires a token parameter.

An example from WPiOS:

[self.tracksService switchToAuthenticatedUserWithUsername:username userID:@"" token:self.token skipAliasEventCreation:NO];

Then, you need to call the method refreshIfNeeded() in didFinishLaunchingWithOptions.

ExPlat.shared.refreshIfNeeded()

And that's all. :)

Usage

Once it's configured you can use like this:

switch ExPlat.shared.experiment("experiment-name") {
    case .treatment:
        // Show treatment option
    case .control:
        // Show control option
    case .unknown:
        // Experiment doesn't exist or wasn't retrieved
    case .other(let value) where value == "my-value":
        // any other value 
}

Or, more simpler:

switch ExPlat.shared.experiment("experiment-name") {
    case .treatment:
        // Show treatment option
    default:
        // Show control option
}

[leandroalonso]

@mokagio I removed your private specs because the build was failing on the CI machine. Just want to double-check if this will not cause any unintended issue.

[astralbodies]

It probably makes sense to tie the ExPlat service into the Tracks service somehow so it can respond to auth/de-auth scenarios already set up in the host app.

Methods used by the host app to track auth: https://github.com/Automattic/Automattic-Tracks-iOS/blob/trunk/Automattic-Tracks-iOS/Services/TracksService.h#L29

Those two methods are already hooked in somewhere in the host app based on the auth lifecycle. The only thing this doesn't give you is the OAuth bearer token, of course. Just figured since we already deal with going from anon -> logged in -> anon with Tracks.

[astralbodies]

Also, consider adding the OAuth Bearer token to the methods tracking authentication state. Since that token changes at the same time (usually), the TracksService could be the owner of that data, and then later inject that into the ExPlat service for use.

Does ExPlat work without a Bearer token? Wonder how that affects anonymous users / users not yet logged in.

[mokagio]

Hey @leandroalonso, sorry for the long time it took me to get to this 😞 🙏

You did well. I'm pretty sure I added them in there only because the CDN didn't seem to find the .podspec for my Sodium-Fork for some reason (update propagation maybe?). The error we got in CI clearly shows that the spec is now there, so good to clean it 🧹

[leandroalonso]

@astralbodies ready for another review!

ExPlat is now coupled with Tracks so it doesn't need to be configured, but it needs to be started in didFinishLaunchingWithOptions.

I have one question: I kept both the old method and the new one with a token param. My intention is to not break the apps that use this library. So, if the app wants to use the AB testing feature it needs to switch to the switchToAuthenticatedUserWithUsername with the token param. Thoughts about that?

[astralbodies]

@leandroalonso I'm terribly sorry for missing this code review. I will be reviewing it today!

[astralbodies]

I was thinking about maybe having you add a deprecation warning here... But then I think Simplenote uses this client as well, and they don't use WordPress.com logins. Now that has me wondering if we should name the token in the new method to be a wpcom bearer token for clarity.

[leandroalonso]

Do you mean something like that?

- (void)switchToAuthenticatedUserWithUsername:(NSString *)username userID:(NSString *)userID wpComToken:(NSString *)token skipAliasEventCreation:(BOOL)skipEvent;

[astralbodies]

Yup, like that!

[leandroalonso]

Done in 7f8c484

[astralbodies]

This looks good! I didn't test out the functionality with the backend system. I did suggest maybe changing the parameter name a bit for clarity's sake.

[astralbodies]

@leandroalonso Ah crap, I didn't notice this PR was targeting trunk.

@jkmassel can we fix that?

[leandroalonso]

@astralbodies I already fixed. 😅

[withinboredom]

It may be better to consider returning .control instead of .unknown so that an old client can properly fall back to the control experience if the experiment is no longer available.

[withinboredom]

Explat will never return the name of the control variation, For control, it always returns null and/or it won't exist in the response. It only returns the name of treatment variations. Right now, we only support a singular treatment, but multi-variate experiments are in the works.