nep-393: add class metadata interface

neps/nep-0393.md

@@ -269,8 +269,9 @@ The Soulbound Token follows the NFT [NEP-171](https://github.com/near/NEPs/blob/
 All time related attributes are defined in milliseconds (as per NEP-171).
 
 ```rust
-/// ContractMetadata defines contract wide attributes, which describes the whole contract.
-pub struct ContractMetadata {
+/// IssuerMetadata defines contract wide attributes, which describes the whole contract.
+/// Must be provided by the Issuer contract. See the `SBTIssuer` trait.
+pub struct IssuerMetadata {
     /// Version with namespace, example: "sbt-1.0.0". Required.
     pub spec: String,
     /// Issuer Name, required, ex. "Mosaics"
@@ -290,6 +291,25 @@ pub struct ContractMetadata {
     pub reference_hash: Option<Base64VecU8>,
 }
 
+/// ClassMetadata defines SBT class wide attributes, which are shared and default to all SBTs of
+/// the given class. Must be provided by the Issuer contract. See the `SBTIssuer` trait.
+pub struct ClassMetadata {
+    /// Issuer class name. Required to be not empty.
+    pub name: String,
+    /// If defined, should be used instead of `IssuerMetadata::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 `IssuerMetadata::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 +468,18 @@ 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: The issuer can set a default icon for all tokens (SBT) using `IssuerMetadata::icon` and additionally it can customize an icon of a particular token via `TokenMetadata::icon`.
 
 ```rust
-pub trait SBTContract {
-    /// returns contract metadata
-    fn sbt_metadata(&self) -> ContractMetadata;
+pub trait SBTIssuer {
+    /// Returns contract metadata.
+    fn sbt_metadata(&self) -> IssuerMetadata;
+    /// Returns SBT class metadata, or `None` if the class is not found.
+    fn sbt_class_metadata(&self, class: ClassId) -> Option<ClassMetadata>;
 }
 ```
 
@@ -594,7 +620,7 @@ trait SBT {
 
 ## Reference Implementation
 
-- Common [type definitions](https://github.com/near-ndc/i-am-human/tree/main/contracts/sbt) (events, traits).
+- Common [type definitions](https://github.com/near-ndc/i-am-human/tree/master/contracts/sbt) (events, traits).
 - [I Am Human](https://github.com/near-ndc/i-am-human) registry and issuers.
 
 ## Consequences