Closes #216.

This PR allows pallets to define a view_functions impl like so:

#[pallet::view_functions]
impl<T: Config> Pallet<T>
where
	T::AccountId: From<SomeType1> + SomeAssociation1,
{
	/// Query value no args.
	pub fn get_value() -> Option<u32> {
		SomeValue::<T>::get()
	}

	/// Query value with args.
	pub fn get_value_with_arg(key: u32) -> Option<u32> {
		SomeMap::<T>::get(key)
	}
}

QueryId

Each view function is uniquely identified by a QueryId, which for this implementation is generated by:

twox_128(pallet_name) ++ twox_128("fn_name(fnarg_types) -> return_ty")

The prefix twox_128(pallet_name) is the same as the storage prefix for pallets and take into account multiple instances of the same pallet.

The suffix is generated from the fn type signature so is guaranteed to be unique for that pallet impl. For one of the view fns in the example above it would be twox_128("get_value_with_arg(u32) -> Option<u32>"). It is a known limitation that only the type names themselves are taken into account: in the case of type aliases the signature may have the same underlying types but a different id; for generics the concrete types may be different but the signatures will remain the same.

The existing Runtime Call dispatchables are addressed by their concatenated indices pallet_index ++ call_index, and the dispatching is handled by the SCALE decoding of the RuntimeCallEnum::PalletVariant(PalletCallEnum::dispatchable_variant(payload)). For view_functions the runtime/pallet generated enum structure is replaced by implementing the DispatchQuery trait on the outer (runtime) scope, dispatching to a pallet based on the id prefix, and the inner (pallet) scope dispatching to the specific function based on the id suffix.

Future implementations could also modify/extend this scheme and routing to pallet agnostic queries.

Executing externally

These view functions can be executed externally via the system runtime api:

pub trait ViewFunctionsApi<QueryId, Query, QueryResult, Error> where
	QueryId: codec::Codec,
	Query: codec::Codec,
	QueryResult: codec::Codec,
	Error: codec::Codec,
{
	/// Execute a view function query.
	fn execute_query(query_id: QueryId, query: Query) -> Result<QueryResult, Error>;
}

XCQ

Currently there is work going on by @xlc to implement XCQ which may eventually supersede this work.

It may be that we still need the fixed function local query dispatching in addition to XCQ, in the same way that we have chain specific runtime dispatchables and XCM.

I have kept this in mind and the high level query API is agnostic to the underlying query dispatch and execution. I am just providing the implementation for the view_function definition.

Metadata

Currently I am utilizing the custom section of the frame metadata, to avoid modifying the official metadata format until this is standardized.

vs runtime_api

There are similarities with runtime_apis, some differences being:

• queries can be defined directly on pallets, so no need for boilerplate declarations and implementations
• no versioning, the QueryId will change if the signature changes.
• possibility for queries to be executed from smart contracts (see below)

Calling from contracts

Future work would be to add weight annotations to the view function queries, and a host function to pallet_contracts to allow executing these queries from contracts.

TODO

• Consistent naming (view functions pallet impl, queries, high level api?)
• End to end tests via runtime_api
• UI tests
• Mertadata tests
• Docs