nep-393: add class metadata interface

Author: robert-zaremba

neps/nep-0393.md

@@ -290,6 +290,23 @@ pub struct ContractMetadata {
     pub reference_hash: Option<Base64VecU8>,
 }
 
+pub struct ClassMetadata {
+    /// Issuer class name. Required.
+    pub name: String,
+    /// If defined, should be used instead of `contract_metadata.symbol`.
+    pub symbol: Option<String>,
+    /// An URL to an Icon. To protect fellow developers from unintentionally triggering any
+    /// SSRF vulnerabilities with URL parsers, we don't allow to set an image bytes here.
+    /// If it doesn't start with a scheme (eg: https://)
+    /// then `contract_metadata.base_uri` should be prepended.
+    pub icon: Option<String>,
+    /// JSON or an URL to a JSON file with more info. If it doesn't start with a scheme
+    /// (eg: https://) then base_uri should be prepended.
+    pub reference: Option<String>,
+    /// Base64-encoded sha256 hash of JSON from reference field. Required if `reference` is included.
+    pub reference_hash: Option<Base64VecU8>,
+}
+
 /// TokenMetadata defines attributes for each SBT token.
 pub struct TokenMetadata {
     pub class: ClassId, // token class. Required. Must be non zero.
@@ -448,12 +465,18 @@ Example **Soul Transfer** interface:
 
 ### SBT Issuer interface
 
-SBTContract is the minimum required interface to be implemented by issuer. Other methods, such as a mint function, which requests the registry to proceed with token minting, is specific to an Issuer implementation (similarly, mint is not part of the FT standard).
+SBTIssuer is the minimum required interface to be implemented by issuer. Other methods, such as a mint function, which requests the registry to proceed with token minting, is specific to an Issuer implementation (similarly, mint is not part of the FT standard).
+
+The issuer must provide metadata object of the Issuer. Optionally, Issuer can also provide metadata object for each token class.
+Issuer level (contract) metadata, must provide information common to all tokens and all classes defined by the issuer. Class level metadata, must provide information common to all tokens of a given class. Information should be deduplicated and denormalized whenever possible.
+Example: if all tokens of a give class share the same symbol and icon, then both icon and symbol must be defined in the `ClassMetadata` and left empty (`None`) in the token metadata level. If a particular SBT (token) has to have unique icon, but we would like to keep a default icon for all tokens of a given issuer, then we can have default icon defined in the `ContractMetadata.icon` and each SBT, that has to have different icon, can have it specified in `TokenMetadata.icon`.
 
 ```rust
-pub trait SBTContract {
+pub trait SBTIssuer {
     /// returns contract metadata
     fn sbt_metadata(&self) -> ContractMetadata;
+    /// returns sbt class metadata, or None if class is not found.
+    fn sbt_class_metadata(&self, class: ClassId) -> Option<ClassMetadata>;
 }
 ```