Attachments

[pgebheim]

https://github.com/onflow/flips/blob/main/cadence/20220921-attachments.md

[sisyphusSmiling]

I've gotten a chance to play around with Attachments as implemented and have found two primary features that would be very helpful.

One is issue is that I'd like to define attachments for composite restricted types. So something like the following won't work, which I can see being very useful and even expected given how we define and use composite types in resources.

pub attachment Hat for INFT, MetadataViews.Resolver

I could design an attachment for use on any resource that implements a set of interfaces. Any one interface might not be sufficient for the functionality I'm after in my attachment, and defining a single implementation might be more limiting than I wish. My thought is this could greatly improve composability of attachments as it allows them to define more generalized base resources.

It would also be very helpful for a base resource to be able to access its attachments. So I could do something like

pub resource NFT {
    ...
    pub fun resolveAttachmentView(attachmentType: Type, view: Type): &AnyStruct? {
        pre {
            attachmentType.isSubtype(of: &{MetadataViews.Resolver}): "Attachment is not a Resolver!"
        }
        // Get a reference to the attachment from the resource its attached to
        if let attachmentRef = self[attachmentType] {
            let attachmentView: &AnyStruct? = attachmentRef.invokeFunction<((Type): AnyStruct?)>("resolveView", args: view)
            return attachmentViews
        }
        return nil
    }
    ...
}

Lastly, removal by the naming the Attachment explicitly prevents me from doing things like dynamically removing Attachments. If possible, being able to remove by runtime Type would be much easier and would allow for something like:

pub resource Collection {
    ...
    pub fun removeAttachment(attachmentType: Type, nftID: UInt64) {
        let baseNFT <-self.withdraw(withdrawID: nftID)
        remove attachmentType from baseNFT
        self.deposit(token: <-baseNFT)
    }
    ...
}

I didn't see anything in the Flip that explicitly eliminates the possibility of these two feature. I think devs would find them helpful, myself included!

[jasamba]

01/19 still in discussion

[franklywatson]

@franklywatson to follow up with Cadence team on this

[j1010001]

• Release preview to get more feedback @turbolent

[j1010001]

Flip is accepted and closed