Implement pallet view function queries

Cargo.toml

@@ -937,6 +937,7 @@ pallet-example-offchain-worker = { path = "substrate/frame/examples/offchain-wor
 pallet-example-single-block-migrations = { path = "substrate/frame/examples/single-block-migrations", default-features = false }
 pallet-example-split = { path = "substrate/frame/examples/split", default-features = false }
 pallet-example-tasks = { path = "substrate/frame/examples/tasks", default-features = false }
+pallet-example-view-functions = { path = "substrate/frame/examples/view-functions", default-features = false }
 pallet-examples = { path = "substrate/frame/examples" }
 pallet-fast-unstake = { path = "substrate/frame/fast-unstake", default-features = false }
 pallet-glutton = { path = "substrate/frame/glutton", default-features = false }

polkadot/runtime/westend/src/lib.rs

@@ -1595,7 +1595,8 @@ mod runtime {
 		RuntimeHoldReason,
 		RuntimeSlashReason,
 		RuntimeLockId,
-		RuntimeTask
+		RuntimeTask,
+		RuntimeViewFunction
 	)]
 	pub struct Runtime;
 

prdoc/pr_4722.prdoc

@@ -0,0 +1,18 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Implement pallet view functions
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Read-only view functions can now be defined on pallets. These functions provide an interface for querying state,
+      from both outside and inside the runtime. Common queries can be defined on pallets, without users having to
+      access the storage directly.
+  - audience: Runtime User
+    description: |
+      Querying the runtime state is now easier with the introduction of pallet view functions. Clients can call commonly
+      defined view functions rather than accessing the storage directly. These are similar to the Runtime APIs, but
+      are defined within the runtime itself.
+
+crates: [ ]

substrate/bin/node/runtime/src/lib.rs

@@ -101,7 +101,7 @@ use sp_consensus_beefy::{
 	mmr::MmrLeafVersion,
 };
 use sp_consensus_grandpa::AuthorityId as GrandpaId;
-use sp_core::{crypto::KeyTypeId, OpaqueMetadata, H160};
+use sp_core::{crypto::KeyTypeId, OpaqueMetadata, ViewFunctionDispatchError, ViewFunctionId, H160};
 use sp_inherents::{CheckInherentsResult, InherentData};
 use sp_runtime::{
 	curve::PiecewiseLinear,
@@ -2383,7 +2383,8 @@ mod runtime {
 		RuntimeHoldReason,
 		RuntimeSlashReason,
 		RuntimeLockId,
-		RuntimeTask
+		RuntimeTask,
+		RuntimeViewFunction
 	)]
 	pub struct Runtime;
 
@@ -2925,6 +2926,12 @@ impl_runtime_apis! {
 		}
 	}
 
+	impl sp_api::RuntimeViewFunction<Block> for Runtime {
+		fn execute_view_function(id: ViewFunctionId, input: Vec<u8>) -> Result<Vec<u8>, ViewFunctionDispatchError> {
+			Runtime::execute_view_function(id, input)
+		}
+	}
+
 	impl sp_block_builder::BlockBuilder<Block> for Runtime {
 		fn apply_extrinsic(extrinsic: <Block as BlockT>::Extrinsic) -> ApplyExtrinsicResult {
 			Executive::apply_extrinsic(extrinsic)

substrate/frame/examples/Cargo.toml

@@ -25,6 +25,7 @@ pallet-example-offchain-worker = { workspace = true }
 pallet-example-split = { workspace = true }
 pallet-example-single-block-migrations = { workspace = true }
 pallet-example-tasks = { workspace = true }
+pallet-example-view-functions = { workspace = true }
 pallet-example-authorization-tx-extension = { workspace = true }
 
 [features]
@@ -40,6 +41,7 @@ std = [
 	"pallet-example-single-block-migrations/std",
 	"pallet-example-split/std",
 	"pallet-example-tasks/std",
+	"pallet-example-view-functions/std",
 ]
 try-runtime = [
 	"pallet-default-config-example/try-runtime",
@@ -51,4 +53,5 @@ try-runtime = [
 	"pallet-example-single-block-migrations/try-runtime",
 	"pallet-example-split/try-runtime",
 	"pallet-example-tasks/try-runtime",
+	"pallet-example-view-functions/try-runtime",
 ]

substrate/frame/examples/src/lib.rs

@@ -48,6 +48,9 @@
 //!
 //! - [`pallet_example_tasks`]: This pallet demonstrates the use of `Tasks` to execute service work.
 //!
+//! - [`pallet_example_view_functions`]: This pallet demonstrates the use of view functions to query
+//!   pallet state.
+//!
 //! - [`pallet_example_authorization_tx_extension`]: An example `TransactionExtension` that
 //!   authorizes a custom origin through signature validation, along with two support pallets to
 //!   showcase the usage.

substrate/frame/examples/view-functions/Cargo.toml

@@ -0,0 +1,60 @@
+[package]
+name = "pallet-example-view-functions"
+version = "1.0.0"
+authors.workspace = true
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+description = "Pallet to demonstrate the usage of view functions to query pallet state"
+
+[lints]
+workspace = true
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]
+
+[dependencies]
+codec = { package = "parity-scale-codec", version = "3.6.12", default-features = false }
+log = { workspace = true }
+scale-info = { version = "2.11.1", default-features = false, features = ["derive"] }
+frame-metadata = { features = ["current"], workspace = true }
+
+frame-support = { path = "../../support", default-features = false }
+frame-system = { path = "../../system", default-features = false }
+
+sp-io = { path = "../../../primitives/io", default-features = false }
+sp-metadata-ir = { path = "../../../primitives/metadata-ir", default-features = false }
+sp-runtime = { path = "../../../primitives/runtime", default-features = false }
+sp-std = { path = "../../../primitives/std", default-features = false }
+sp-core = { default-features = false, path = "../../../primitives/core" }
+
+frame-benchmarking = { path = "../../benchmarking", default-features = false, optional = true }
+
+[dev-dependencies]
+pretty_assertions = { version = "1.3.0" }
+
+[features]
+default = ["std"]
+std = [
+	"codec/std",
+	"frame-benchmarking?/std",
+	"frame-support/std",
+	"frame-system/std",
+	"log/std",
+	"scale-info/std",
+	"sp-core/std",
+	"sp-io/std",
+	"sp-runtime/std",
+	"sp-std/std",
+]
+runtime-benchmarks = [
+	"frame-benchmarking/runtime-benchmarks",
+	"frame-support/runtime-benchmarks",
+	"frame-system/runtime-benchmarks",
+	"sp-runtime/runtime-benchmarks",
+]
+try-runtime = [
+	"frame-support/try-runtime",
+	"frame-system/try-runtime",
+	"sp-runtime/try-runtime",
+]

substrate/frame/examples/view-functions/src/lib.rs

@@ -0,0 +1,114 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! This pallet demonstrates the use of the `pallet::view_functions_experimental` api for service
+//! work.
+#![cfg_attr(not(feature = "std"), no_std)]
+
+pub mod tests;
+
+use frame_support::Parameter;
+use scale_info::TypeInfo;
+
+pub struct SomeType1;
+impl From<SomeType1> for u64 {
+	fn from(_t: SomeType1) -> Self {
+		0u64
+	}
+}
+
+pub trait SomeAssociation1 {
+	type _1: Parameter + codec::MaxEncodedLen + TypeInfo;
+}
+impl SomeAssociation1 for u64 {
+	type _1 = u64;
+}
+
+#[frame_support::pallet]
+pub mod pallet {
+	use super::*;
+	use frame_support::pallet_prelude::*;
+
+	#[pallet::error]
+	pub enum Error<T> {}
+
+	#[pallet::config]
+	pub trait Config: frame_system::Config {}
+
+	#[pallet::pallet]
+	pub struct Pallet<T>(_);
+
+	#[pallet::storage]
+	pub type SomeValue<T: Config> = StorageValue<_, u32>;
+
+	#[pallet::storage]
+	pub type SomeMap<T: Config> = StorageMap<_, Twox64Concat, u32, u32, OptionQuery>;
+
+	#[pallet::view_functions_experimental]
+	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)
+		}
+	}
+}
+
+#[frame_support::pallet]
+pub mod pallet2 {
+	use super::*;
+	use frame_support::pallet_prelude::*;
+
+	#[pallet::error]
+	pub enum Error<T, I = ()> {}
+
+	#[pallet::config]
+	pub trait Config<I: 'static = ()>: frame_system::Config {}
+
+	#[pallet::pallet]
+	pub struct Pallet<T, I = ()>(PhantomData<(T, I)>);
+
+	#[pallet::storage]
+	pub type SomeValue<T: Config<I>, I: 'static = ()> = StorageValue<_, u32>;
+
+	#[pallet::storage]
+	pub type SomeMap<T: Config<I>, I: 'static = ()> =
+		StorageMap<_, Twox64Concat, u32, u32, OptionQuery>;
+
+	#[pallet::view_functions_experimental]
+	impl<T: Config<I>, I: 'static> Pallet<T, I>
+	where
+		T::AccountId: From<SomeType1> + SomeAssociation1,
+	{
+		/// Query value no args.
+		pub fn get_value() -> Option<u32> {
+			SomeValue::<T, I>::get()
+		}
+
+		/// Query value with args.
+		pub fn get_value_with_arg(key: u32) -> Option<u32> {
+			SomeMap::<T, I>::get(key)
+		}
+	}
+}