add constants api to codegen (#402)

* add constants api to codegen

Signed-off-by: Gregory Hill <gregorydhill@outlook.com>

* handle constant decoding error

Signed-off-by: Gregory Hill <gregorydhill@outlook.com>

* fix clippy and remove extra constants allocation

Signed-off-by: Gregory Hill <gregorydhill@outlook.com>

codegen/src/api/constants.rs

@@ -0,0 +1,56 @@
+// Copyright 2019-2022 Parity Technologies (UK) Ltd.
+// This file is part of subxt.
+//
+// subxt 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.
+//
+// subxt 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 subxt.  If not, see <http://www.gnu.org/licenses/>.
+
+use crate::types::TypeGenerator;
+use frame_metadata::PalletConstantMetadata;
+use heck::SnakeCase as _;
+use proc_macro2::TokenStream as TokenStream2;
+use quote::{
+    format_ident,
+    quote,
+};
+use scale_info::form::PortableForm;
+
+pub fn generate_constants(
+    type_gen: &TypeGenerator,
+    constants: &[PalletConstantMetadata<PortableForm>],
+    types_mod_ident: &syn::Ident,
+) -> TokenStream2 {
+    let constant_fns = constants.iter().map(|constant| {
+        let fn_name = format_ident!("{}", constant.name.to_snake_case());
+        let return_ty = type_gen.resolve_type_path(constant.ty.id(), &[]);
+
+        let ref_slice = constant.value.as_slice();
+
+        quote! {
+            pub fn #fn_name(&self) -> ::core::result::Result<#return_ty, ::subxt::BasicError> {
+                Ok(::subxt::codec::Decode::decode(&mut &[#(#ref_slice,)*][..])?)
+            }
+        }
+    });
+
+    quote! {
+        pub mod constants {
+            use super::#types_mod_ident;
+
+            pub struct ConstantsApi;
+
+            impl ConstantsApi {
+                #(#constant_fns)*
+            }
+        }
+    }
+}

codegen/src/api/mod.rs

@@ -15,6 +15,7 @@
 // along with subxt.  If not, see <http://www.gnu.org/licenses/>.
 
 mod calls;
+mod constants;
 mod errors;
 mod events;
 mod storage;
@@ -176,12 +177,23 @@ impl RuntimeGenerator {
                 quote!()
             };
 
+            let constants_mod = if !pallet.constants.is_empty() {
+                constants::generate_constants(
+                    &type_gen,
+                    &pallet.constants,
+                    types_mod_ident,
+                )
+            } else {
+                quote!()
+            };
+
             quote! {
                 pub mod #mod_name {
                     use super::#types_mod_ident;
                     #calls
                     #event
                     #storage_mod
+                    #constants_mod
                 }
             }
         });
@@ -207,6 +219,12 @@ impl RuntimeGenerator {
         };
 
         let mod_ident = item_mod_ir.ident;
+        let pallets_with_constants =
+            pallets_with_mod_names
+                .iter()
+                .filter_map(|(pallet, pallet_mod_name)| {
+                    (!pallet.constants.is_empty()).then(|| pallet_mod_name)
+                });
         let pallets_with_storage =
             pallets_with_mod_names
                 .iter()
@@ -273,6 +291,10 @@ impl RuntimeGenerator {
                     T: ::subxt::Config,
                     X: ::subxt::SignedExtra<T>,
                 {
+                    pub fn constants(&'a self) -> ConstantsApi {
+                        ConstantsApi
+                    }
+
                     pub fn storage(&'a self) -> StorageApi<'a, T> {
                         StorageApi { client: &self.client }
                     }
@@ -282,6 +304,17 @@ impl RuntimeGenerator {
                     }
                 }
 
+                pub struct ConstantsApi;
+
+                impl ConstantsApi
+                {
+                    #(
+                        pub fn #pallets_with_constants(&self) -> #pallets_with_constants::constants::ConstantsApi {
+                            #pallets_with_constants::constants::ConstantsApi
+                        }
+                    )*
+                }
+
                 pub struct StorageApi<'a, T: ::subxt::Config> {
                     client: &'a ::subxt::Client<T>,
                 }