Use preloading for initial requests in importer and setup wizard

[yscik]

This uses the @wordpress/api-fetch library's functionality to pre-resolve some REST-API requests on the server and send the results along in the initial HTML response. With this, we don't have to show a spinner when opening the importer/exporter/setup wizard while fetching the current state (active step/ongoing job), but can display the correct screen immediately.

Changes proposed in this Pull Request

• Add an apiFetch middleware that makes sure that preloaded responses are used only once
  • Used by importing shared/data/api-fetch-preloaded-once
• Add preloading for getting the active job in importer/exporter
  • Adjust the response of this endpoint to return an empty job instead of 404, so that it works with preloading
• Add preloading for getting the setup wizard state

Testing instructions

• Open the Setup Wizard again after finishing it
  • It should display the correct state (all steps completed), without an additional network request and waiting
• Open the Importer, start a new import. Reload the page while it's still in progress.
  • It should show the progress screen immediately

[renatho]

Good idea! It improved a lot the experience \o/

Just a minor comment and there are some conflicts in the branch.

[renatho]

🚀

The base branch was changed.

[yscik]

Rebased and removed the exporter bit so it can be merged to master directly.

[jsnajdr]

Hi @yscik 👋 I found the preloadedDataUsedOnceMiddleware utility by complete coincidence, when grepping for api-fetch in the wpcom codebase (it ships the Sensei plugin).

Just wanted to point out that "preloading just once" is what apiFetch preloading middleware now does natively, after a bugfix was merged two months ago in WordPress/gutenberg#25550.

In future versions of Sensei, when the underlying WordPress Core ships the fixed api-fetch library, you might not need the workaround any more.

[yscik]

Hey @jsnajdr! Thanks for the heads up, that is good news :) We support a few older versions of Wordpress in Sensei, so this might remain for a while, but we'll flag this as something that can be removed in a future release.