Nested Blocks: Inner block arrangement options

[aduth]

Previously: #2995 (#2995 (comment)), #3745 (#3745 (comment))

The InnerBlocks component, used for block nesting, should allow a block implementer to assign a predefined block arrangement, optionally locking or otherwise restricting the types of blocks which are available for insertion in the block.

This could potentially align with existing block templating

Pseudo-code examples:

<InnerBlocks
	allowedBlockNames={ [ 'core/image', 'core/paragraph' ] }
	template={ [
		createBlock( 'core/image' ),
		createBlock( 'core/paragraph' ),
	] }
/>

For the inverse relationship (blocks restricted to certain parent contexts), we could either...

• Rely on a combination of block's isPrivate API (already used to omit block from inserter) and treat allowedBlockNames as overriding this preference
• Add a property to the block API, allowedRootBlockNames: [ ... ]

Related Work:

Discussion by @jorgefilipecosta about defining inner blocks supported:

https://wordpress.slack.com/archives/C02QB2JS7/p1520252046000229

cc @jorgefilipecosta

[robbisy]

The InnerBlocks component link are broken :

[aduth]

@robbisy The component has been moved to the editor directory. Here's the updated link:

https://github.com/WordPress/gutenberg/tree/master/editor/components/inner-blocks

(I've revised the original comment as well)

[ajitbohra]

Closing this as Nested / Inner blocks have been updated https://github.com/WordPress/gutenberg/tree/master/editor/components/inner-blocks

cc: @aduth

[jessicaplant]

Link is broken again :(

[aduth]

@jessicaplant Here's the updated link. I expect this one should remain relatively stable (famous last words):

https://github.com/WordPress/gutenberg/tree/master/packages/editor/src/components/inner-blocks