Allowed to easily set, change, save and load root attributes

[scofalik]

Suggested merge commit message (convention)

Other (engine): RootAttributeOperation is now correctly handled by Differ. Root attribute changes will be returned in Differ#getChangedRoots().

Internal (editor-multi-root): Introduced API to easily save and load root attributes. Introduced EditorConfig#rootsAttributes and MultiRootEditor#getRootsAttributes(). Closes #13395.

Internal (editor-multi-root): Introduced MultiRootEditor#getFullData(). The method returns document data for all roots in multi-root editor.

Internal (editor-multi-root): MultiRootEditor#addRoot() has a new option attributes.

Internal (engine): model.Writer#detachRoot() will now remove attributes from the detached root.

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Additional information

Most things as internal because the changes will be in the changelog as "introduced multi-root editor type".

See #13395 (comment) for summary of proposed solution.

[Inviz]

oh that's great that you are addressing this too!

[Inviz]

Will this sync attributes set via dom using mutation observer?

[scofalik]

This solution is for attributes understood as, hm, roots metadata (additional model root data). You can write converters to convert these roots attributes from model to view and then DOM. We will probably write such guide at some point. But in general, the flow should be:

• initialize root attributes when initing editor (you can set DOM attributes in DOM/HTML too)
• custom features allow to change these attributes through a custom UI
• converters (in custom features) handle these attributes (e.g. change something in DOM in the process)
• when you save the data, you save the roots attributes as well

Automatic sync through mutation observer in a way that you described (DOM -> model) is against the philosophy of how CKE5 works. Instead, the changes should be done on model and be propagated downwards to DOM. You can introduce a custom UI or any other method to change model roots attributes, but the propagation should always be model -> view -> DOM.

[Inviz]

You can write converters to convert these roots attributes from model to view and then DOM.

Is it true now? I remember it was not possible before, for one reason or another it didn't really work. I remember filed issue on that, and i stumbled upon that myself too.

Automatic sync through mutation observer in a way that you described (DOM -> model) is against the philosophy of how CKE5 works. Instead, the changes should be done on model and be propagated downwards to DOM. You can introduce a custom UI or any other method to change model roots attributes, but the propagation should always be model -> view -> DOM.

You are right. I think i was confused about some other part that relied on syncing changes from the dom, but perhaps it was selection or something like that. Either way it doesn't matter.

[scofalik]

Is it true now? I remember it was not possible before, for one reason or another it didn't really work. I remember filed issue on that, and i stumbled upon that myself too.

I think it is difficult currently, as root attributes changes were not buffered in Differ. After this PR, it will be a bit easier. But in general, even now, you can set attributes on roots. But it is not convenient to use them afterwards. One official plugin that uses it is HTML comments.

[niegowski]

LGTM. Only minor comments.

[huzedong2015]

addRoot cannot add a new HTML element