Editor: Refactor currentPost state to use core-data package

[aduth]

Previously: #16402 (comment)

The editor maintains a copy of the original post being edited in its state, as currentPost. This value is always representative of the last saved value of the post; edits are maintained separately. There should be effectively no difference between this value and what would be produced by...

wp.data.select( 'core' ).getEntityRecord( 'postType', 'post', wp.data.select( 'core/editor' ).getCurrentPostId() );

By using the entity provided from the core-data data store, we gain some benefit:

• There is only at most one copy of the post held in memory at a given time
• Updates to the canonical post are reflected in the editor, and vice-versa
• The implementation will likely push us to implement saving and setting behaviors for core-data entities (and a subsequent refactor of savePost to use core-data as well)
• Reduced implementation complexity and maintenance overhead of the editor reducer file

Implementation Specifics:

For the most part, the crux of the implementation requires changing...

gutenberg/packages/editor/src/store/selectors.js

Lines 156 to 158 in 8995efd

	export function getCurrentPost( state ) {
	return state.currentPost;
	}

...to:

export const getCurrentPost = createRegistrySelector( ( registry ) => ( state ) => {
	return registry.select( 'core' ).getEntityRecord( 'postType', 'post' /* !!! */, getCurrentPostId( state ) );
} );

Some details:

• How do we know the post type? We may still need to track this in editor state, or as an edit. This is unfortunately required because an entity cannot be retrieved from the REST API without providing its post type (related discussion).
• We should see about providing a simpler interface to retrieving entities (I thought we already had getPost etc. auto-generated for entities?)

[gziolo]

gutenberg/packages/editor/src/store/selectors.js

Line 162 in ba4793b

const post = select( 'core' ).getRawEntityRecord( 'postType', postType, postId );

It looks like it was addressed in #16932 by @epiqueras.

[Mamaduka]

@gziolo, should we close this issue?

[gziolo]

I don't know and that's why I didn't close this issue 😄

[gziolo]

I did another check. Everything is implemented.