Support subsets of block types in block templates

[mboynes]

As a theme and plugin developer, I want to be able to define positions in block templates that are restricted to a subset of block types so that I can maintain structure in page templates while still supporting flexibility for similar media.

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When defining block templates, I've found myself wanting to support multiple block types in a template position, which as best I can tell is not yet possible. A straightforward example of this is defining a region in a post that can be either an image or a video.

Examples

This is an example of one option for how we might define a region that could be either an image or a video, using a new block type block-set:

$post_type_object->template = [
    [ 'core/block-set', [
        [ 'core/image', [] ],
        [ 'core/video', [] ],
    ] ],
];

[ZebulanStanphill]

Currently, the way you would do something like this is by using a block that only allows certain blocks as children, like the Media Area child block in #7414. Not sure if that is the best approach, though.

[designsimply]

@mboynes are you able to do what you're after using child blocks?

[mboynes]

@designsimply that's slightly different from what I'm suggesting -- or at least considerably more work. Going that route, and please correct me if I'm mistaken, requires one to create a custom block, in that custom block define the allowed child blocks, then use the new custom block in the template. I think this would be a useful enough feature to warrant core support, and not make developers go through that level of effort.

Possibly related: #7763, if InnerBlocks would eventually be used in the solution for this feature.

[nerrad]

I think you can now do something like what is requested here within template registration via InnerBlocks and also nested templates.

If that is not the case please feel free to comment with more details about your use-case. There's been a number of changes in the api since this issue was created and it's possible there may be an existing solution. (We can re-open this issue if necessary).