ENH Pass form validation result to client

[emteknetnz]

Issue silverstripe/silverstripe-elemental#764

Allows the ValidationResult to be passed to the client as part of a pjax response

Wrapping the JSON in a <script type="application/json"> is something of a workaround due to the expectation that pjax returns HTML. If it's not wrapped in a <script type="application/json"> tag it will fail here https://github.com/silverstripe/silverstripe-admin/blob/1/client/src/legacy/LeftAndMain.js#L775

[michalkleiner]

That's an interesting mechanism with the header, didn't know there was something like that. Always learning :)

Looks ok to me, let's wait for the tests.

[brynwhyman]

Just came across this PR which I thought was worth noting here incase yo uhadn't seen it: silverstripe/silverstripe-elemental#921

[emteknetnz]

I'd tick "Approve" if I wasn't the original author on this PR

[michalkleiner]

Approved and merged, but now I wonder whether it should've targeted 2 as an enhancement? But seeing the target branch was updated last week by @GuySartorelli I'm not sure. LMK.

[GuySartorelli]

On its own, yes this is just an enhancement and belongs in 2 - but it was done specifically to work with silverstripe/silverstripe-elemental#930 in order to solve silverstripe/silverstripe-elemental#764 - so it's part of a wider work to fix a bug that's pretty egregious, and I think in this case warrants being released in CMS 4 as a patch.