afterDrop is now bound to context can optionally be set through `bind:`

[mblarsen]

This PR implements a default binding of the afterDrop method to the elements binding context. This is to give the developer access to mainly the $parent object, since a common use case is to have a list model with a number of item models in it.

class ListViewModel () {
  constructor () {
    this.items = ko.observableArray([]);
  }

  afterDrop () {
    // this would otherwise be `undefined`
    for ([idx, item] of this.$parent.items().entries()) {
      item.sortOrder(idx);
    }
  }
}

In the example above this would normally be undefined. A more natural bind value would be to bind it to $parent since that is the object in which the afterDrop method is defined and you'll be able to use this as you would in other class methods / object functions. But since this is not always the scenario I've left it as an option (see README.md):

<div data-bind="dragula: { data: items, afterDrop: afterDrop, bind: '$parent' }"></div>

Doing so will set this to be the "real" this of afterDrop.

[mblarsen]

This is an option as well, but a default would be nice:

<div data-bind="dragula: { data: categories, afterDrop: afterDrop.bind($data) }">

[laurentlbm-rc]

Thanks for the pull request. I like the change to bind afterDrop by default, but I'm not sure the bind option is necessary. I personally find that afterDrop.bind($data) is enough. I still want to merge the whole pull request, though, since it adds flexibility. But, before I do, could you make the same changes for the afterDelete callback? Should the bind option apply to both or should they each have an option to set their bind value?

[mblarsen]

Yes afterDrop.bind($data) with a default binding would do fine. I do see the idea of binding to both, however I think it could lead to confusing as well if it is the same "class" of view model. What about just binding to the context then? Then you could get this.$data, this.$parent or this.$root even? @laurentlbm-rc

[mblarsen]

@laurentlbm-rc I've updated the code so that the bind-option is removed and it defaults to binding-context. Updated the README.md as well.

[laurentlbm-rc]

Looks good to me, thank you very much @mblarsen!