Introduced util for checking whether the editor wasn't initialized on the same element more than once

[pomek]

Suggested merge commit message (convention)

Feature: Introduced a secureSourceElement() utility that makes sure source DOM elements are not shared between editor instances. See ckeditor/ckeditor5#746.

[coveralls]

Coverage remained the same at 100.0% when pulling b8656e4 on t/ckeditor5/746 into cba2fd6 on master.

[jodator]

Some updates to docs are needed - the PR is OK :) I have only one question to @Reinmar about naming the error.

[jodator]

Suggested change

	* An element passed to the editor creator has been passed more than once. The element can be used only once.
	* Another editor instance was already initialized on this element. You cannot create more then one editor on the same element.

Or something similar - I get the feeling that the error description should reflect how someone could use our API. So it is rather creating two or more instances of the ediotr on some element then passing something.

[oleq]

Why not a part of ElementApiMixin?

[jodator]

Why not a part of ElementApiMixin?

I think that probably for the same reason as #173 (comment)

I think that not every editor build is initialized over an element.

Edit: I was wrong - @oleq this looks like a place for this util.

[oleq]

I moved the method and updated other PRs.

But don't merge it please, because I figured we can go further and have ElementDataMixin#setSourceElement(), that will handle these

if ( isElement( sourceElementOrData ) ) {
	this.sourceElement = sourceElementOrData;
	this.secureSourceElement();
}

in editor constructors as

if ( isElement( sourceElementOrData ) ) {
	this.setSourceElement( sourceElementOrData );
}

with backwards compatibility (setting this.sourceElement directly will warn about this way being obsolete like we did with EditorUI.editableElements).

Gonna create it ASAP.

[oleq]

I reverted some of the changes I made because it turned out the ElementApi interface does not make sense the way we use it (RFC). Securing the editor element should remain a helper. The code is ready for review.

[oleq]

Self–review: maybe instead of using data- attributes we should simply use DOM element properties? Less clutter in the view/DOM.

[jodator]

Self–review: maybe instead of using data- attributes we should simply use DOM element properties? Less clutter in the view/DOM.

Sounds cool :)

[Reinmar]

Self–review: maybe instead of using data- attributes we should simply use DOM element properties? Less clutter in the view/DOM.

Just like editableElement.ckeditorInstance? In fact, we could assign ckeditorInstance there for the same reasons why we marked editable elements with it :)

[oleq]

In fact... we could use #ckeditorInstance that we recently implemented.

[oleq]

Perfect timing.

[oleq]

Ready for another round of review.

[jodator]

👍 I've fixed only one docs as I couldn't parse it :)