safeTransferFrom()
function) - which is the normal way that those are transferred between owners - does actually test if the new owner is a simple account or a contract, and if it actually is a contract, it "asks" if that contract can receive tokens via a contract function call. The collection contract does use that function call to register any such transfer into the collection and puts such received assets into a list. As for transferring away an asset, you need to make a function call on the collection contract anyhow, removing from that list can be done there. So, this list can be made available for querying and will always be accurate - as long as "safe" transfers are used. Unfortunately, ERC-721 allows "unsafe" transfers via transferFrom()
even though it warns that NFTs "MAY BE PERMANENTLY LOST
" when that function is used. This was probably added into the standard mostly for compatibility with CryptoKitties, which predate this standard and only supported "unsafe" transfers. To deal with that, the collections contract has a function to "sync" ownership, which is given a contract address and token ID, and it adjusts it assets list accordingly by either adding or removing it from there. Note that there is a theoretical possibility to also lose an assets without being able to track it there, that's why both directions are supported there. (Note: OpenSea has used "unsafe" transfers in their "gift" functionality at least in the past, but that hopefully has been fixed by now.)externalCall()
function, to which the caller needs to hand over a contract address to call and an encoded payload (which can relatively easily be generated e.g. via the web3.js
library). The result is that the Collection can e.g. call the function for Crypto stamps sold via the OnChain shop to have their physical versions sent to a postage address, which is a function that only the owner of a Crypto stamp can call - as the Collection is that owner and its own owner can call this "external" function, things like this can still be achieved.