Create a dotcom-ui-client-embed package for sharing data between the server and the client

[magsallen]

Currently we have no defined method for apps to share data between the client and the server. Some apps have resorted to updating the appContext object as a way around this but we want to discourage this practice and v0.7.0 of Page Kit introduced errors when modifying appContext to prevent this behaviour.

Proposal:

• Create a new dotcom-ui-client-embed package containing all of the logic required to pass data from the server to the client without modifying appContext.
• Refactor dotcom-middleware-app-context and dotcom-middleware-flags to use this new package rather than duplicating the same functionality in multiple places.
• Document a process for other apps to make use of the new package to pass custom properties from the client to the server.
• Incorporate this functionality into an example app.

[andygout]

Case study: next-video-page, which currently adds this to appContext:

{
	permutiveContext: {
		publicId: process.env.PERMUTIVE_PUBLIC_ID,
		publicApikey: process.env.PERMUTIVE_PUBLIC_API_KEY
	}
}

[NickColley]

One thought... If someone fails the appContext schema validation we could consider pointing them to the documentation for this...

[magsallen]

Yes, I think so 👍 At the moment I believe we serve a Typescript error but we could definitely do better than this!

We actually have a design decisions doc about appContext which has become out-of-date; if we update that as part of this work then it might be a good resource to point to in the event of a validation error https://github.com/Financial-Times/dotcom-page-kit/blob/master/docs/design-decisions/app-context.md