Move ViewCollection.bind() to utils.Collection

[pjasiun]

Since it's a very useful feature to bind two collections, especially if you can do custom mapping, it should be moved to the utils.Collection to let users bind any collections, not only ViewCollection.

Note that you can bind any observable variable, but not any observable collection, what is inconsistent.

The problem is that the binding checks if the parameter is the instance of View to check if it should be used or if the parameter is a creator function (https://github.com/ckeditor/ckeditor5-ui/blob/6b7fd4adfe6ee99fbdf84bfc4940f10ed78493b8/src/viewcollection.js#L227-L246). To solve it we could split the as function to separate function, what would make the code simpler and less magic:

collection1.bindTo( collection2 ).byWrappingInto( MyViewClass );
collection1.bindTo( collection2 ).using( ( object ) => object.propery );

Note that in the future we might introduce another popular mapping helper, like for the last line from the example above:

collection1.bindTo( collection2 ).withProperty( 'property' );

[oleq]

It's probably more a matter of

collection1.bindTo( collection2 ).using( ( object ) => object.propery );

where for UI, it would be

collection1.bindTo( collection2 ).using( viewFactoryInstance );

Current implementation is straightforward. I think it's an easy thing to do.

[pjasiun]

But binding by wrapping might be a common case not only for UI, as @wwalc and @scofalik suggested.

[oleq]

I'm not sure what's the purpose of byWrappingInto. For me there are 2 real use–cases:

1. Binding a property like

collection1.bindTo( collection2 ).using( ( object ) => object.propery );

2. Binging as a factory i.e.

collection1.bindTo( collection2 ).using( ( object ) => new AnotherClass( object ) );

which as good as

collection1.bindTo( collection2 ).as( AnotherClass );

Is there anything more than that?

[pjasiun]

For me that's fine.

[pjasiun]

By the way: it would be nice to have the two-way binding of collections. Will it work out of the box, or we need a separate ticket for it?

[oleq]

@pjasiun I'm not sure what outcome would you expect when the binding is used as a factory.

But anyway, we considered it in the UI https://github.com/ckeditor/ckeditor5-ui/issues/110.