An Interface represents a standardized way to translate all the information pertaining to a compiled contract, as well as its states and operation, in a human-readable and wallet-readable format.
An RGB Interface can be compared to the very same "Interface" construct of various Object Oriented Programming (OOP) Languages such as Java's Interfaces. In Rust, the equivalent concept is expressed by the trait statement.
The use of an interface in RGB is made necessary by the design choices of the contract construction. Indeed, as described in the previous chapters, Schema is expressed in a purely declarative form which is then eventually compiled in a binary file which is then imported into the wallet. Throughout the Schema, the definition of states, the related variable, and contract operations are basically not accessible without additional pieces of code that actually can parse this information once the Schema is compiled.
Definition and purposes of an RGB Interface.
With Interfaces we can have some general semantic definitions that can be connected to the relevant types encoded in the strict type system, in particular, an Interface:
- Allows to parse the available states, read the state's name and state data, and access their value, for example: the ticker name, the maximum amount token, etc.
- Access the operations' name and access the data contained in a specific contract operation, for example: the amount of tokens being transferred.
- Allows for the construction of contract operation permitted in the Schema, by using the name data mapping.
In general, we can state that contracts having different purposes possess different interfaces that are built to parse them. For instance, an Interface for a Non-Inflatable Asset would be different from that parsing a DAO with identities. However, a single interface may provide a standard way to parse a large number of contracts of the same kind. This approach provides several benefits:
- Standardization of contract interface, fostering code re-usability for common wallet tasks. In particular, this is the purpose of the Interfaces provided by the LNP/BP association described in the next section.
- A clear separation between the development of schema and the development of interfaces that can happen in different development streams. For instance, the Schema developers can focus on the contract features paying attention only to the connections of those features in an already coded interface (a task which is accomplished by interface implementation we will be discussing just after).
- A contract may even be parsable by more than one interface in order to provide separated functionality for different class of wallet and users. For example the features of an issuer would be more advanced to those of a normal user, which can lack all the complex part related to the contract issuance.
Wallet that interacts with multiple contracts through various Interfaces.
From this important points, it's helpful to stress, once more, the major difference of RGB in respect to Ethereum smart contract system. In the case of Ethereum, as well as other blockchain-based smart contract systems, all the interface related part is encoded directly inside the contract and committed into the blockchain at issuance time. This approach leads, inevitably, to an high degree of rigidity in the possible evolution of the contract, since it results hard to make modification to some specific part of the interface without modify the contract altogether. With RGB the clear separation of contract Schema and Interface allows to easily modify the interface (and also the Interface Implementation) even after assets issuance. This way the contract can contain some advance functionality whose Interface code imported in the wallet can be written later and independently from the development of the contract.ù
In the same way as schema, interface are compiled from source code in a .rgb binary file or in a .rgba armored binary file which can then be imported into the RGB wallet.
In the next section we will be going through a description of some Interface which are shipped by default with RGB Standard Library and which were provided by LNP/BP association.