FIX Don't try to preview unlocalised objects

[GuySartorelli]

I don't think there's ever a scenario where you'd want to consider previews as not frontend so I don't think this is a breaking change.
I'm on the fence about whether updatePreviewLink() belongs here or in FluentExtension directly. Probably it belongs on FluentExtension so it can affect anything else which have a preview (e.g. elemental blocks which aren't inline-editable) - but I'll leave it like this for now (it is still an improvement and solves the specific exact scenario mentioned in the issue) until someone opines.

Parent issue

• CMSPreview should be considered "frontend" #775

[GuySartorelli]

Also happy to change the commit name - when I wrote it I was thinking of the last piece of the chain that I was solving, not the first part as described in the issue itself.

[tractorcow]

The only risk this introduced, as we identified in a prior conversation, is that some pages visible in the admin won't appear at all (i.e. 404) in preview. I still this this fix is a more consistent and better functionality, rather than showing a false view of content that then breaks after publish. :)

[GuySartorelli]

@tractorcow I'm not sure if your comment is in response to something I've said, or to the changes I've introduced with this PR.... if the latter, then there is no 404 in the preview (for pages specifically) with this PR. That's what setting the preview url to null ensures. Instead, the preview panel is collapsed by default for those pages and a "there is no preview available" message is displayed if the user explicitly swaps to split or preview mode.

[GuySartorelli]

@tractorcow @chrispenny do either of you have an opinion as to whether this change belongs on the FluentSiteTreeExtension or directly on FluentExtension?

[chrispenny]

@GuySartorelli Good question... Can any DataObject implement a Preview window? I don't think I've ever seen it outside of SiteTree, but I vaguely remember reading that it was possible.

If any DO can use Preview, then I guess it makes sense to be on FluentExtension.

CC: @mfendeksilverstripe

[mfendeksilverstripe]

I think the ability to have a preview is not limited to only SiteTree. I think this belongs more to the FluentExtension. Alternatively, this can be a separate extension which is enabled by default but if needed can be disabled / overwritten via yaml config. This could give projects more flexibility over how to use this feature.

[GuySartorelli]

Can any DataObject implement a Preview window?

Yup (I even did a whole talk on it ;p). Any DataObject that implements the CMSPreviewable interface and returns some valid link path from PreviewLink() will have a preview available in gridfield edit forms and modeladmins - or in any LeftAndMain for that matter. You can even slap a preview in the siteconfig if you really want.

If any DO can use Preview, then I guess it makes sense to be on FluentExtension.
...
I think this belongs more to the FluentExtension.

I'll make the change 👍

Alternatively, this can be a separate extension which is enabled by default but if needed can be disabled / overwritten via yaml config. This could give projects more flexibility over how to use this feature.

I don't think it makes sense to configure this separately - I can't think of any use case where you'd want anything other than the correct preview being displayed in the preview panel.

[GuySartorelli]

Oops I broke some tests - will look at that now

[mfendeksilverstripe]

Nice work 👍