impl_stubs! - Wiki Parity Tech Documentation

impl_stubs! is a macro that defines entry points to the runtime. It is just one way to define external WASM entry points, which can also be done manually.

Runtime may be seen as an independent library with quite a thin API. Almost all interaction between Substrate and runtime is done by calling one of the functions defined in the impl_stubs!.

Typically it is defined as follows:

pub mod api {
		version => |()| super::Version::version(),
		authorities => |()| super::Consensus::authorities(),
		initialise_block => |header| super::Executive::initialise_block(&header),
		apply_extrinsic => |extrinsic| super::Executive::apply_extrinsic(extrinsic),
		execute_block => |block| super::Executive::execute_block(block),
		finalise_block => |()| super::Executive::finalise_block(),
		inherent_extrinsics => |(timestamp, heads)| super::inherent_extrinsics(timestamp, heads),
		validator_count => |()| super::Session::validator_count(),
		validators => |()| super::Session::validators()

Most important functions are:

If the block was already created elsewhere it may be imported just by calling the execute_block function which is essentially a combination of calls to initialise_block, apply_extrinsic, and finalize_block.