Upcasting a $text element over an already upcasted property should work

[idiazroncero]

📝 Provide detailed reproduction steps (if any)

This is related to Upcasting an element that has multiple attributes may not work #4306 and it might be considered a feature request instead of a bug.

Summary: Issue #4306 solved a bug where multiple upcasts couldn't address the same $text element because the first one to act would consume the tagname.

The solution was to consume the item when view only specifies name and, if not, consume the property/properties (i.e class, ID, data-attribute, etc)

The problem is that this also prevents upcasting the same property several times, which might make sense for plugins wanting to act independently of each other on the same properties.

Scenario we use a plugin that stores on linkClass any existing link classes on an attribute to expose an UI for handling them. This upcast acts on a elements with classes: true.

    conversion.for('upcast')
      .elementToAttribute( {
        model: {
          key: 'linkClass',
          value: viewElement => {
            console.log('Upcasted classes');
            return viewElement.getAttribute('class');
          }
        },
        view: {
          name: 'a',
          classes: true,
        },
      });

We have another plugin that looks for an specific class to populate a linkSize attribute. This upcast acts on a classes that have an specific class:

    conversion.for('upcast')
      .elementToAttribute( {
        model: {
          key: 'linkSize',
          value: viewElement => {
            console.log('Upcasted size');
            return 'lg';
          },
        },
        view: {
          name: 'a',
          classes: 'btn-lg',
        }
      });

✔️ Expected result

Data upcasting should populate both attributes:

❌ Actual result

Data upcasting will only happen for the first occurence (or higher priority) upcast. It will consume the a.classes and further upcasts will fail to act.

❓ Possible solution

1. Don't consume $text properties. Of course I'm aware that this might have tons of architectural implications but at the same time I find counterintuitive the idea that each element property can be consumed only once.

2. Provide developers with an API or a way to easily "bypass" property consumption.

3. ...or maybe Iim just missing something.

📃 Other details

CKEDITOR_VERSION: 36.0.0
Installed plugins: Essentials, Paragraph, Heading, List, Bold, Italic, Link and a custom testing plugin that pretty much includes the aforementioned code.

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[idiazroncero]

I tried to solve this with a custom dispatcher that should consume and then revert the classes: ['btn-lg'], but it didn't work.

    conversion.for('upcast')
      .add(dispatcher => {
        dispatcher.on('element:a', (evt, data, conversionApi) => {
          if ( conversionApi.consumable.consume(data.viewItem, { classes: ['btn-lg'] })) {
            const { modelRange } = conversionApi.convertChildren( data.viewItem, data.modelCursor );

            for ( let item of modelRange.getItems() ) {
              if ( conversionApi.schema.checkAttribute( item, 'linkSize' ) ) {
                conversionApi.writer.setAttribute( 'linkSize', 'custom-lg', item );
              }
            }
            conversionApi.consumable.revert(data.viewItem, { classes: ['btn-lg'] });
          }
        });
      });

This upcast actually prevents other upcasts from acting on links (see the lacking linkHref on links 2 and 3, which have the btn-lg class)

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