Return account's asset balances (#13352)

* Runtime method to get user's assets balances

* Fix test (typo)

* Update frame/assets/src/functions.rs

* Remove instance param

* Update frame/assets/src/functions.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Remove instance param

* Refactor

* Chore

* Update doc

---------

Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: parity-processbot <>

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

@@ -0,0 +1,34 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2018-2022 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+//! Runtime API definition for assets.
+
+use codec::Codec;
+use sp_std::vec::Vec;
+
+sp_api::decl_runtime_apis! {
+	pub trait AssetsApi<AccountId, AssetBalance, AssetId>
+	where
+		AccountId: Codec,
+		AssetBalance: Codec,
+		AssetId: Codec,
+	{
+		/// Returns the list of `AssetId`s and corresponding balance that an `AccountId` has.
+		fn account_balances(account: AccountId) -> Vec<(AssetId, AssetBalance)>;
+	}
+}