Modify SchemaContext

[pjasiun]

At this moment, the schema is not mutable and there is no easy way to create one schema based on another. The simplest way right now is to create a new definition and the new schema based on it. Even the current docs do not look very simple:

https://github.com/ckeditor/ckeditor5-engine/blob/d00c24c0fefad47f6a48abe8bd43c71ee8fddf58/src/model/schema.js#L1018

You also, need to remember about all these fixes done in the SchemaContext creator if you want to get items from the position manually:

https://github.com/ckeditor/ckeditor5-engine/blob/d00c24c0fefad47f6a48abe8bd43c71ee8fddf58/src/model/schema.js#L877-L890

This is why I think that schema SchemaContext should have methods like add/delete or push/pop to create one context based one another. These methods could modify the context object, but if we want to keep a context immutable (as @Reinmar suggested) these methods could return the new context object (do not change the current one).

Also, checkChild and checkAttribute should accept SchemaContext instance. To do so, we could make SchemaContext constructor accepts SchemaContext instance as a parameter. In such case, the same (or even the same instance) should be returned from the constructor.

[Reinmar]

Off topic:

	constructor( context ) {
		if ( Array.isArray( context ) ) {
			if ( context[ 0 ] && typeof context[ 0 ] != 'string' && context[ 0 ].is( 'documentFragment' ) ) {
				context.shift();
			}
		}
		else {
			// `context` is item or position.
			// Position#getAncestors() doesn't accept any parameters but it works just fine here.
			context = context.getAncestors( { includeSelf: true } );

			if ( context[ 0 ].is( 'documentFragment' ) ) {
				context.shift();
			}
		}

		this._items = context.map( mapContextItem );
	}

Why is the documentFragment check implemented twice? It can be done once, after the if-else part, once context is an array.

[Reinmar]

Also, checkChild and checkAttribute should accept SchemaContext instance. To do so, we could make SchemaContext constructor accepts SchemaContext instance as a parameter. In such case, the same (or even the same instance) should be returned from the constructor.

I'd go with the same instance. Since it's immutable anyway, that doesn't matter.

[pjasiun]

We decided to call it concatand it will accept any form of the schema definition, which can be string now too (it is useful also in other cases). Such string will be treated as a name of a single element.

[Reinmar]

I'm afraid that I don't like the new method and other changes. I don't understand why we needed it.

Plus, I'd ask for clarification about this:

Also, checkChild and checkAttribute should accept SchemaContext instance. To do so, we could make SchemaContext constructor accepts SchemaContext instance as a parameter. In such case, the same (or even the same instance) should be returned from the constructor.

Because once it landed it just looks very... unnecessary.

EDIT: OK, that paragraph explains the "why". Sorry – I focused so much on concat() that I thought that it was added because of it.

Let's talk further under ckeditor/ckeditor5-engine#1263.