[Docs] A snippet in Preserving custom content does not work

[AnnaTomanek]

The following snippet does not work on current master:
https://ckeditor5.github.io/docs/nightly/ckeditor5/latest/framework/guides/deep-dive/ui/conversion-preserving-custom-content.html#adding-extra-attributes-to-elements-contained-in-a-figure

{@snippet framework/extending-content-custom-figure-attributes}

The following errors can be found in the console:

plugincollection.js:235
plugincollection-load: It was not possible to load the plugin. Read more: https://ckeditor.com/docs/ckeditor5/latest/framework/guides/support/error-codes.html#error-plugincollection-load
 {plugin: ƒ}
(anonymous) @ plugincollection.js:235
Promise.catch (async)
l @ plugincollection.js:214
init @ plugincollection.js:198
initPlugins @ editor.js:229
(anonymous) @ classiceditor.js:193
create @ classiceditor.js:189
389 @ extending-content-custom-figure-attributes.js:157
r @ bootstrap:19
(anonymous) @ bootstrap:83
(anonymous) @ bootstrap:83

extending-content-custom-figure-attributes.js:168
CKEditorError: schema-cannot-extend-missing-item: Cannot extend an item which was not registered yet. Read more: https://ckeditor.com/docs/ckeditor5/latest/framework/guides/support/error-codes.html#error-schema-cannot-extend-missing-item
 {"itemName":"image"}
    at ae.extend (https://ckeditor5.github.io/docs/nightly/ckeditor5/latest/snippets/framework/build-extending-content-source/snippet.js:4:326616)
    at o (https://ckeditor5.github.io/docs/nightly/ckeditor5/latest/snippets/framework/extending-content-custom-figure-attributes/snippet.js:4:1242)
    at new ClassicEditor.create.extraPlugins (https://ckeditor5.github.io/docs/nightly/ckeditor5/latest/snippets/framework/extending-content-custom-figure-attributes/snippet.js:4:2493)
    at https://ckeditor5.github.io/docs/nightly/ckeditor5/latest/snippets/framework/build-extending-content-source/snippet.js:4:320828
    at new Promise (<anonymous>)
    at https://ckeditor5.github.io/docs/nightly/ckeditor5/latest/snippets/framework/build-extending-content-source/snippet.js:4:320560
    at l (https://ckeditor5.github.io/docs/nightly/ckeditor5/latest/snippets/framework/build-extending-content-source/snippet.js:4:320865)
    at Array.map (<anonymous>)
    at Yt.init (https://ckeditor5.github.io/docs/nightly/ckeditor5/latest/snippets/framework/build-extending-content-source/snippet.js:4:320402)
    at P.initPlugins (https://ckeditor5.github.io/docs/nightly/ckeditor5/latest/snippets/framework/build-extending-content-source/snippet.js:4:385183)
(anonymous) @ extending-content-custom-figure-attributes.js:168
Promise.catch (async)
389 @ extending-content-custom-figure-attributes.js:167
r @ bootstrap:19
(anonymous) @ bootstrap:83
(anonymous) @ bootstrap:83

[Reinmar]

Ee... it's loaded via extraPlugins. I'd say, a plugin loaded with it should be loaded at the very end of the queue. Does it happen this way, if we get this error?

[jodator]

@Reinmar it doesn't matter if it is loaded at the end or at the beginning. The function() plugin code is being executed on loadPlugin phase while "image" schema item is being defined later during the "init" phase. It will not work because when the function() plugin is called the "image" schema item is not yet defined.

The order of plugins initialization is:

1. Call each plugin constructor (class or funciton()).
2. Call each plugin instance init() method.
3. Call each plugin instance afterInit() method.

It is this issue again: #2905 (comment).

You cannot use function() as a plugin if you wish to extend schema or alter conversion which might be defined in Plugin.init() method.

[Reinmar]

IMO, we need to give more power to simple function plugins. I explained the two options that I see in ckeditor/ckeditor5-engine#1801 (comment).