Allow templates to change list block placeholder by adding it as an attribute

[annestk]

Description

Heading blocks and paragraph blocks allows templates (for CPT/InnerBlocks) to set a custom placeholder via attributes - like so:

const TEMPLATE =[ ['core/heading', { placeholder: 'Enter recipe title...' }] ]

This is not possible with the list block, as its placeholder is only defined on the <RichText />-component.

Types of changes

This PR updates the list block to insert placeholders the same way the paragraph and heading blocks do. They do this by adding the placeholder as an empty attribute - so the standard placeholder defined on the RichText-component is used when it's not defined.

Screenshots

List block with default placeholder

List block as part of a custom block that uses an <InnerBlocks /> component with a template that sets the list block's placeholder to be Enter steps...

I tried to search the GitHub repo to see if there has been any discussion around what blocks can set placeholder text in a template and which can't - but I didn't find anything.

It seems like it's possible to set the placeholder attributes for:

• Heading block
• Paragraph block

But not for (I didn't check all blocks, but are willing to):

• List block
• Button block
• Code block
• Quote (neither quote field or citation field)
• Pullquote (same)

Is this a conscious decision/is there a reason for this?
Allowing templates to have custom placeholders really increase their usefulness (in my humble opinion).

How has this been tested?

Tested defining the placeholder attribute for a list block as part of a template for and a custom post type.
Tested on a local install with WordPress 5.0.1

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

(I'm quite new at this is, so if you have to tell I've done something wrong/it's terrible/it can't be used there will be no hard feelings.)

[jorgefilipecosta]

Hi @annestk thank you for your contribution 👍

I agree we should be able to change placeholder of other blocks besides the paragraph.
But I'm not sure if adding attributes that are not used to generate the markup and are just used as part of the edit function is the correct approach.
There are many situations where we may want to customize the block besides the placeholder (disable some alignments, change the block description for users with vision problems and using screen readers etc...), we should work on an API that addresses all these cases.
We have an open issue #7931 to track this. As soon as we have this general mechanism we should be able to change the placeholder of other blocks.

[jorgefilipecosta]

I'm closing this PR as we can track this problem in issue #7931.
Thank you @annestk for the effort in submitting this PR, I added a comment in #7931 describing this need, and the description/conceptualization of the use case was beneficial.