Macro impl_runtime_apis

impl_runtime_apis!() { /* proc-macro */ }

Tags given trait implementations as runtime apis.

All traits given to this macro, need to be declared with the decl_runtime_apis! macro. The implementation of the trait should follow the declaration given to the decl_runtime_apis! macro, besides the Block type that is required as first generic parameter for each runtime api trait. When implementing a runtime api trait, it is required that the trait is referenced by a path, e.g. impl my_trait::MyTrait for Runtime. The macro will use this path to access the declaration of the trait for the runtime side.

The macro also generates the api implementations for the client side and provides it through the RuntimeApi type. The RuntimeApi is hidden behind a feature called std.

To expose version information about all implemented api traits, the constant RUNTIME_API_VERSIONS is generated. This constant should be used to instantiate the apis field of RuntimeVersion.

Example

extern crate alloc;

/// All runtime api implementations need to be done in one call of the macro!
sp_api::impl_runtime_apis! {

    impl self::Balance<Block> for Runtime {
        fn get_balance() -> u64 {
            1
        }
        fn set_balance(_bal: u64) {
            // Store the balance
        }
    }

    impl self::BlockBuilder<Block> for Runtime {
        fn build_block() -> Block {
             unimplemented!("Please implement me!")
        }
    }
}

/// Runtime version. This needs to be declared for each runtime.
pub const VERSION: sp_version::RuntimeVersion = sp_version::RuntimeVersion {
    spec_name: alloc::borrow::Cow::Borrowed("node"),
    impl_name: alloc::borrow::Cow::Borrowed("test-node"),
    authoring_version: 1,
    spec_version: 1,
    impl_version: 0,
    // Here we are exposing the runtime api versions.
    apis: RUNTIME_API_VERSIONS,
    transaction_version: 1,
    system_version: 1,
};

Implementing specific api version

If decl_runtime_apis! declares multiple versions for an api impl_runtime_apis! should specify which version it implements by adding api_version attribute to the impl block. If omitted - the base/default version is implemented. Here is an example:

sp_api::impl_runtime_apis! {
    #[api_version(3)]
    impl self::Balance<Block> for Runtime {
         // implementation
    }
}

In this case Balance api version 3 is being implemented for Runtime. The impl block must contain all methods declared in version 3 and below.

Conditional version implementation

impl_runtime_apis! supports cfg_attr attribute for conditional compilation. For example let’s say you want to implement a staging version of the runtime api and put it behind a feature flag. You can do it this way:

pub enum Runtime {}
sp_api::decl_runtime_apis! {
    pub trait ApiWithStagingMethod {
        fn stable_one(data: u64);

        #[api_version(99)]
        fn staging_one();
    }
}

sp_api::impl_runtime_apis! {
    #[cfg_attr(feature = "enable-staging-api", api_version(99))]
    impl self::ApiWithStagingMethod<Block> for Runtime {
        fn stable_one(_: u64) {}

        #[cfg(feature = "enable-staging-api")]
        fn staging_one() {}
    }
}

decl_runtime_apis! declares two version of the api - 1 (the default one, which is considered stable in our example) and 99 (which is considered staging). In impl_runtime_apis! a cfg_attr attribute is attached to the ApiWithStagingMethod implementation. If the code is compiled with enable-staging-api feature a version 99 of the runtime api will be built which will include staging_one. Note that staging_one implementation is feature gated by #[cfg(feature = ... )] attribute.

If the code is compiled without enable-staging-api version 1 (the default one) will be built which doesn’t include staging_one.

cfg_attr can also be used together with api_version. For the next snippet will build version 99 if enable-staging-api is enabled and version 2 otherwise because both cfg_attr and api_version are attached to the impl block:

#[cfg_attr(feature = "enable-staging-api", api_version(99))]
#[api_version(2)]
impl self::ApiWithStagingAndVersionedMethods<Block> for Runtime {
 // impl skipped
}