[Bug]: Multiple inheritance with custom shapes not working (as expected?)

[Alexander Nestorov]

What happened?

I'm playing with the examples at https://resources.jointjs.com/tutorial/ts-shape and I figured out I'd try to create a Base shape that extends from dia.Link, which then I'd use to create MyLink. It turns out this doesn't work.

I'm not really sure why (maybe it has something to do with the way defaults is being called?).

Demo: https://codesandbox.io/s/joint-custom-link-extend-playground-ngs9zn

Note that

class CLink extends CBase {                       // <--- doesn't render the link

class CLink extends joint.shapes.standard.Link {  // <--- renders the link

Version

3.7.5

What browsers are you seeing the problem on?

Firefox, Chrome, Safari

What operating system are you seeing the problem on?

Mac

[Roman Bruckner]

It is the defaults :)

• wrong order of merge arguments
• super.default is a function in CBase

class CLink extends CBase {
  defaults() {
    return joint.util.merge(
      {},
      super.defaults(),
      {
        type: "CLink",
        attrs: {
          line: {
            stroke: "#ccc",
            strokeWidth: 2
          }
        }
      },
    );
  }
}

[Alexander Nestorov]

Ooohh! 🤔

Ok, I have another questions. Why CBase is merging super.defaults, but CLink is merging super.defaults()? What is the difference between using super.defaults as a value and calling super.defaults() as a function?

[Roman Bruckner]

The shapes.standard.Link class is defined via the define method which using the extend method of Backbone. If you create a Backbone.Model this way (ES5 way), the defaults is a property, not a method. Therefore CBase needs to access it as a property.

The ES Class CBase defines the defaults as a method. Therefore CLink has to call it as a function.

I recommend reading this article which should explain this in detail. Also reading this Backbone issue might be helpful.

[Alexander Nestorov]

Oohh, now I get it. Good thing to know :)
Thank you!!

[Alexander Nestorov]

I have an extra question 🙏

Would it be possible to extend from 2 classes using this method? Lets say that I have CBaseLinkA which defines the stroke color (using the defaults()) and CBaseLinkB which defines the strokeWidth (also using the defaults()) and the markup attribute. Would it be possible to do something like

class MyLink extends merge(CBaseLinkA, CBaseLinkB) {
  defaults() {
    return joint.util.merge(
      {
        attrs: { line: { fill: "red" } }
      },
      super.defaults() // ???? 
    )
  }
}

[Roman Bruckner]

Perhaps you can check this discussion.

[Alexander Nestorov]

Hmmm, I think I saw similar stuff in SO, but I'm not really sure how to handle the defaults(), since that is threated in a very special way. I'll play around and report back if I find a way of achieving this!