WIP: Add naive support for action hooks

[davejm]

Summary

Related to #1348. This is just an outline of how hooks might be implemented. I don't anticipate working too much on this as I haven't the time, but maybe this is useful to the discussion.

Heavily inspired by WordPress' 'action hooks'. You register a function to run on a particular hook and can optionally assign it a priority (not implemented). This could be complemented or merged with WordPress' idea of filter hooks. I think the functionality provided by action/filter hooks could provide a lot of possibilities for netlify-cms.

Test plan

WIP

A picture of a cute animal (not mandatory but encouraged)

[netlify]

Preview proposed changes to the CMS demo site in the link below:

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[netlify]

Preview proposed changes to netlifycms.org in the link below:

Built with commit 73e355b

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[erquhart]

This is compelling - I have some lower level thoughts about the API (registerActionHook should accept an object, for example, as that's we're taking most of our reg methods), but I'm more interested in broad strokes discussion of this approach.

@Benaiah @talves @barthc @tomrutgers thoughts?

[talves]

I like the idea of having some type of action Hook in the CMS, but need to think on whether it needs to be a registered Hook or just a listener type event handler.

function hookHandler(event, args) {
  If (event.action === "SOME_ACTION") {
    // Do something
  }
}

const options = { action: 'SOME_ACTION' }
const cmsHook = CMS.addHookListener(hookHandler, options)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[cpjolicoeur]

Please, what is the status of this?

[erquhart]

@cpjolicoeur the issue is still something we want to fulfill. This PR is almost a duplicate of your issue as the code is pretty simplistic, almost a POC. Going to close this until we're ready to implement (as it feels mostly like a duplicate issue with a code example), but I would consider the issue not far from top of pile.