Consider simplifying attribute conversion

[Reinmar]

Looking at the image feature converters I particularly didn't like this piece:

export function createImageAttributeConverter( dispatchers, attributeName ) {
	for ( let dispatcher of dispatchers ) {
		dispatcher.on( `addAttribute:${ attributeName }:image`, modelToViewAttributeConverter );
		dispatcher.on( `changeAttribute:${ attributeName }:image`, modelToViewAttributeConverter );
		dispatcher.on( `removeAttribute:${ attributeName }:image`, modelToViewAttributeConverter );
	}
}

function modelToViewAttributeConverter( evt, data, consumable, conversionApi ) {
	const parts = evt.name.split( ':' );
	const consumableType = parts[ 0 ] + ':' + parts[ 1 ];

	if ( !consumable.consume( data.item, consumableType ) ) {
		return;
	}

	const figure = conversionApi.mapper.toViewElement( data.item );
	const img = figure.getChild( 0 );

	if ( parts[ 0 ] == 'removeAttribute' ) {
		img.removeAttribute( data.attributeKey );
	} else {
		img.setAttribute( data.attributeKey, data.attributeNewValue );
	}
}

Couldn't we have just one event and the type of change as a data property? Could we simplify that consumable check code too? Maybe we could also have a shorthand method in Element – like updateAttribute() to avoid having to use both remove/setAttribute() methods.

[Reinmar]

@scofalik asked me:

Why converter builder has not been used?

cc @szymonkups – there was some reason AFAIR. But you're right that my ticket should be about improving the builder as well.

[szymonkups]

If I remember correctly, with builder we cannot specify that we want to convert attributes from certain element type only. For example: "convert src attribute only from image elements".

[scofalik]

I can also see that there is figure involved in here, which looks like is mapped to the original data.item.

[szymonkups]

I was mentioning this here: https://github.com/ckeditor/ckeditor5-engine/issues/736.

[Reinmar]

Code sample that we need to improve: https://github.com/ckeditor/ckeditor5-image/pull/32/files#diff-181a641e45476f54a783a7ad41ed6f5bR101

[pjasiun]

Fixed by ckeditor/ckeditor5-engine#1274 and other refactoring tickets.