In moving towards fully-automated NFT contract processing, the overrides mechanism on
NFTFactory can be configured to specify which strategy to use to dynamically extract
a value from the many non-standard metadata sources on music NFTs.
This is done by specifying a strategy in the typeMetadata.overrides.extractor property
on an NFTFactory and will only work when combined with the
multi-track-multiprint-contract platform type.
A minimal library of common extractor functions is available in fieldExtractor.ts and should
be extended with new strategies as they're encountered in newer contracts.
The goal is to be able to simply add a seed NFTFactory using the CRDT messages system, which
then automatically processes any dynamic fields on a collection of music NFTs. In the meantime
though, migrations are being used to add new contracts to the Spindexer.
There are three new concepts introduced in this system.
An StrategyExtractor is responsible for understanding how the NFTFactory has been
configured, and which strategy has been chosen for a particular field. It usually
fetches a property from typeMetadata.overrides.extractor which is in turn used to
fetch the appropriate function from an ExtractorMapping.
ExtractorMappings are a mapping of a Strategy to their corresponding Extractors.
Extractors operate directly on an NFT to return a string by fetching a specific value
from an NFT's metadata.
The Extractor functions can be arbitrarily complex, composing different metadata fields or applying data sanitization as necessary.
When processing a multi-track-multiprint-contract, the Resolvers are given both the
NFTFactory and an associated NFT.
The Resolver is responsible for using the selected Strategy to find the correct Extractor function from the ExtractorMappings, and subsequently applies the Extractor to the NFT resolve the value.