WIP: Add support in config for transaction creation and begin porting/implementing tx flows

new/src/client.rs

@@ -2,6 +2,7 @@ mod offline_client;
 mod online_client;
 
 use crate::config::{Config, HashFor};
+use crate::transactions::Transactions;
 use core::marker::PhantomData;
 use subxt_metadata::Metadata;
 
@@ -33,11 +34,22 @@ impl<Client, T> ClientAtBlock<Client, T> {
 impl<Client, T> ClientAtBlock<Client, T>
 where
     T: Config,
-    Client: OfflineClientAtBlockT,
+    Client: OfflineClientAtBlockT<T>,
 {
+    /// Construct transactions.
+    pub fn tx(&self) -> Transactions<'_, T, Client> {
+        Transactions::new(&self.client)
+    }
+
+    /// Obtain a reference to the metadata.
     pub fn metadata_ref(&self) -> &Metadata {
         self.client.metadata_ref()
     }
+
+    /// The current block number.
+    pub fn block_number(&self) -> u64 {
+        self.client.block_number()
+    }
 }
 
 impl<Client, T> ClientAtBlock<Client, T>
@@ -45,6 +57,7 @@ where
     T: Config,
     Client: OnlineClientAtBlockT<T>,
 {
+    /// The current block hash.
     pub fn block_hash(&self) -> HashFor<T> {
         self.client.block_hash()
     }

new/src/client/offline_client.rs

@@ -1,5 +1,5 @@
 use crate::client::ClientAtBlock;
-use crate::config::Config;
+use crate::config::{Config, HashFor};
 use crate::error::OfflineClientAtBlockError;
 use std::sync::Arc;
 use subxt_metadata::Metadata;
@@ -21,11 +21,11 @@ impl<T: Config> OfflineClient<T> {
     pub fn at_block(
         &self,
         block_number: impl Into<u64>,
-    ) -> Result<ClientAtBlock<OfflineClientAtBlock, T>, OfflineClientAtBlockError> {
+    ) -> Result<ClientAtBlock<OfflineClientAtBlock<T>, T>, OfflineClientAtBlockError> {
         let block_number = block_number.into();
-        let spec_version = self
+        let (spec_version, transaction_version) = self
             .config
-            .spec_version_for_block_number(block_number)
+            .spec_and_transaction_version_for_block_number(block_number)
             .ok_or(OfflineClientAtBlockError::SpecVersionNotFound { block_number })?;
 
         let metadata = self
@@ -33,29 +33,66 @@ impl<T: Config> OfflineClient<T> {
             .metadata_for_spec_version(spec_version)
             .ok_or(OfflineClientAtBlockError::MetadataNotFound { spec_version })?;
 
-        Ok(ClientAtBlock::new(OfflineClientAtBlock { metadata }))
+        let genesis_hash = self.config.genesis_hash();
+
+        let offline_client_at_block = OfflineClientAtBlock {
+            metadata,
+            block_number,
+            genesis_hash,
+            spec_version,
+            transaction_version,
+        };
+
+        Ok(ClientAtBlock::new(offline_client_at_block))
     }
 }
 
 #[derive(Clone)]
-pub struct OfflineClientAtBlock {
+pub struct OfflineClientAtBlock<T: Config> {
     metadata: Arc<Metadata>,
+    block_number: u64,
+    genesis_hash: Option<HashFor<T>>,
+    spec_version: u32,
+    transaction_version: u32,
 }
 
 /// This represents an offline-only client at a specific block.
 #[doc(hidden)]
-pub trait OfflineClientAtBlockT: Clone {
+pub trait OfflineClientAtBlockT<T: Config>: Clone {
     /// Get a reference to the metadata appropriate for this block.
     fn metadata_ref(&self) -> &Metadata;
     /// Get a clone of the metadata appropriate for this block.
     fn metadata(&self) -> Arc<Metadata>;
+    /// The block number we're operating at.
+    fn block_number(&self) -> u64;
+    /// Return the genesis hash for the chain if it is known.
+    fn genesis_hash(&self) -> Option<HashFor<T>>;
+    /// The spec version at the current block.
+    fn spec_version(&self) -> u32;
+    /// The transaction version at the current block.
+    ///
+    /// Note: This is _not_ the same as the transaction version that
+    /// is encoded at the beginning of transactions (ie 4 or 5).
+    fn transaction_version(&self) -> u32;
 }
 
-impl OfflineClientAtBlockT for OfflineClientAtBlock {
+impl<T: Config> OfflineClientAtBlockT<T> for OfflineClientAtBlock<T> {
     fn metadata_ref(&self) -> &Metadata {
         &self.metadata
     }
     fn metadata(&self) -> Arc<Metadata> {
         self.metadata.clone()
     }
+    fn block_number(&self) -> u64 {
+        self.block_number
+    }
+    fn genesis_hash(&self) -> Option<HashFor<T>> {
+        self.genesis_hash
+    }
+    fn spec_version(&self) -> u32 {
+        self.spec_version
+    }
+    fn transaction_version(&self) -> u32 {
+        self.transaction_version
+    }
 }

new/src/client/online_client.rs

@@ -30,6 +30,9 @@ pub struct OnlineClient<T: Config> {
 struct OnlineClientInner<T: Config> {
     /// The configuration for this client.
     config: T,
+    /// Chain genesis hash. Needed to construct transactions,
+    /// so we obtain it up front on constructing this.
+    genesis_hash: HashFor<T>,
     /// The RPC methods used to communicate with the node.
     backend: Arc<dyn Backend<T>>,
 }
@@ -107,18 +110,31 @@ impl<T: Config> OnlineClient<T> {
             .await
             .map_err(OnlineClientError::CannotBuildCombinedBackend)?;
         let backend: Arc<dyn Backend<T>> = Arc::new(backend);
-        Ok(OnlineClient::from_backend(config, backend))
+        OnlineClient::from_backend(config, backend).await
     }
 
     /// Construct a new [`OnlineClient`] by providing an underlying [`Backend`]
     /// implementation to power it.
-    pub fn from_backend(config: T, backend: impl Into<Arc<dyn Backend<T>>>) -> OnlineClient<T> {
-        OnlineClient {
+    pub async fn from_backend(
+        config: T,
+        backend: impl Into<Arc<dyn Backend<T>>>,
+    ) -> Result<OnlineClient<T>, OnlineClientError> {
+        let backend = backend.into();
+        let genesis_hash = match config.genesis_hash() {
+            Some(hash) => hash,
+            None => backend
+                .genesis_hash()
+                .await
+                .map_err(OnlineClientError::CannotGetGenesisHash)?,
+        };
+
+        Ok(OnlineClient {
             inner: Arc::new(OnlineClientInner {
                 config,
+                genesis_hash,
                 backend: backend.into(),
             }),
-        }
+        })
     }
 
     /// Obtain a stream of all blocks imported by the node.
@@ -212,7 +228,7 @@ impl<T: Config> OnlineClient<T> {
         let number_or_hash = number_or_hash.into();
 
         // We are given either a block hash or number. We need both.
-        let (block_ref, block_num) = match number_or_hash {
+        let (block_ref, block_number) = match number_or_hash {
             BlockNumberOrRef::BlockRef(block_ref) => {
                 let block_hash = block_ref.hash();
                 let block_header = self
@@ -229,26 +245,29 @@ impl<T: Config> OnlineClient<T> {
                     })?;
                 (block_ref, block_header.number())
             }
-            BlockNumberOrRef::Number(block_num) => {
+            BlockNumberOrRef::Number(block_number) => {
                 let block_ref = self
                     .inner
                     .backend
-                    .block_number_to_hash(block_num)
+                    .block_number_to_hash(block_number)
                     .await
                     .map_err(|e| OnlineClientAtBlockError::CannotGetBlockHash {
-                        block_number: block_num,
+                        block_number,
                         reason: e,
                     })?
-                    .ok_or(OnlineClientAtBlockError::BlockNotFound {
-                        block_number: block_num,
-                    })?;
-                (block_ref, block_num)
+                    .ok_or(OnlineClientAtBlockError::BlockNotFound { block_number })?;
+                (block_ref, block_number)
             }
         };
         let block_hash = block_ref.hash();
 
         // Obtain the spec version so that we know which metadata to use at this block.
-        let spec_version = match self.inner.config.spec_version_for_block_number(block_num) {
+        // Obtain the transaction version because it's required for constructing extrinsics.
+        let (spec_version, transaction_version) = match self
+            .inner
+            .config
+            .spec_and_transaction_version_for_block_number(block_number)
+        {
             Some(version) => version,
             None => {
                 let spec_version_bytes = self
@@ -267,13 +286,18 @@ impl<T: Config> OnlineClient<T> {
                     _impl_name: String,
                     _authoring_version: u32,
                     spec_version: u32,
+                    _impl_version: u32,
+                    _apis: Vec<([u8; 8], u32)>,
+                    transaction_version: u32,
                 }
-                SpecVersionHeader::decode(&mut &spec_version_bytes[..])
-                    .map_err(|e| OnlineClientAtBlockError::CannotDecodeSpecVersion {
-                        block_hash: block_hash.into(),
-                        reason: e,
-                    })?
-                    .spec_version
+                let version =
+                    SpecVersionHeader::decode(&mut &spec_version_bytes[..]).map_err(|e| {
+                        OnlineClientAtBlockError::CannotDecodeSpecVersion {
+                            block_hash: block_hash.into(),
+                            reason: e,
+                        }
+                    })?;
+                (version.spec_version, version.transaction_version)
             }
         };
 
@@ -391,6 +415,10 @@ impl<T: Config> OnlineClient<T> {
             metadata,
             backend: self.inner.backend.clone(),
             block_ref,
+            block_number,
+            spec_version,
+            genesis_hash: self.inner.genesis_hash,
+            transaction_version,
         };
 
         Ok(ClientAtBlock {
@@ -402,7 +430,7 @@ impl<T: Config> OnlineClient<T> {
 
 /// This represents an online client at a specific block.
 #[doc(hidden)]
-pub trait OnlineClientAtBlockT<T: Config>: OfflineClientAtBlockT {
+pub trait OnlineClientAtBlockT<T: Config>: OfflineClientAtBlockT<T> {
     /// Return the RPC methods we'll use to interact with the node.
     fn backend(&self) -> &dyn Backend<T>;
     /// Return the block hash for the current block.
@@ -418,6 +446,10 @@ pub struct OnlineClientAtBlock<T: Config> {
     backend: Arc<dyn Backend<T>>,
     hasher: T::Hasher,
     block_ref: BlockRef<HashFor<T>>,
+    block_number: u64,
+    spec_version: u32,
+    genesis_hash: HashFor<T>,
+    transaction_version: u32,
 }
 
 impl<T: Config> OnlineClientAtBlockT<T> for OnlineClientAtBlock<T> {
@@ -432,13 +464,25 @@ impl<T: Config> OnlineClientAtBlockT<T> for OnlineClientAtBlock<T> {
     }
 }
 
-impl<T: Config> OfflineClientAtBlockT for OnlineClientAtBlock<T> {
+impl<T: Config> OfflineClientAtBlockT<T> for OnlineClientAtBlock<T> {
     fn metadata_ref(&self) -> &Metadata {
         &self.metadata
     }
     fn metadata(&self) -> Arc<Metadata> {
         self.metadata.clone()
     }
+    fn block_number(&self) -> u64 {
+        self.block_number
+    }
+    fn genesis_hash(&self) -> Option<HashFor<T>> {
+        Some(self.genesis_hash)
+    }
+    fn spec_version(&self) -> u32 {
+        self.spec_version
+    }
+    fn transaction_version(&self) -> u32 {
+        self.transaction_version
+    }
 }
 
 fn get_legacy_types<T: Config>(

new/src/config.rs

@@ -26,7 +26,7 @@ use subxt_metadata::Metadata;
 use subxt_rpcs::RpcConfig;
 
 pub use default_extrinsic_params::{DefaultExtrinsicParams, DefaultExtrinsicParamsBuilder};
-pub use extrinsic_params::{ExtrinsicParams, ExtrinsicParamsEncoder};
+pub use extrinsic_params::{ClientState, ExtrinsicParams, ExtrinsicParamsEncoder};
 pub use polkadot::{PolkadotConfig, PolkadotExtrinsicParams, PolkadotExtrinsicParamsBuilder};
 pub use substrate::{SubstrateConfig, SubstrateExtrinsicParams, SubstrateExtrinsicParamsBuilder};
 pub use transaction_extensions::TransactionExtension;
@@ -59,11 +59,23 @@ pub trait Config: Clone + Debug + Sized + Send + Sync + 'static {
     /// can then be used to hash things at that block.
     type Hasher: Hasher;
 
-    /// Return the spec version for a given block number, if available.
+    /// The starting hash for the chain we're connecting to. This is required for constructing transactions.
+    ///
+    /// If not provided by the config implementation, it will be obtained from the chain in the case of the
+    /// [`crate::client::OnlineClient`]. It must be provided to construct transactions via the
+    /// [`crate::client::OfflineClient`], else an error will be returned.
+    fn genesis_hash(&self) -> Option<HashFor<Self>> {
+        None
+    }
+
+    /// Return a tuple of the spec version and then transaction version for a given block number, if available.
     ///
     /// The [`crate::client::OnlineClient`] will look this up on chain if it's not available here,
     /// but the [`crate::client::OfflineClient`] will error if this is not available for the required block number.
-    fn spec_version_for_block_number(&self, _block_number: u64) -> Option<u32> {
+    fn spec_and_transaction_version_for_block_number(
+        &self,
+        _block_number: u64,
+    ) -> Option<(u32, u32)> {
         None
     }
 

new/src/config/extrinsic_params.rs

@@ -21,31 +21,14 @@ use subxt_metadata::Metadata;
 pub struct ClientState<T: Config> {
     /// Genesis hash.
     pub genesis_hash: HashFor<T>,
-    /// Runtime version.
-    pub runtime_version: RuntimeVersion,
+    /// Spec version.
+    pub spec_version: u32,
+    /// Transaction version.
+    pub transaction_version: u32,
     /// Metadata.
     pub metadata: Arc<Metadata>,
 }
 
-/// Runtime version information needed to submit transactions.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub struct RuntimeVersion {
-    /// Version of the runtime specification. A full-node will not attempt to use its native
-    /// runtime in substitute for the on-chain Wasm runtime unless all of `spec_name`,
-    /// `spec_version` and `authoring_version` are the same between Wasm and native.
-    pub spec_version: u32,
-    /// All existing dispatches are fully compatible when this number doesn't change. If this
-    /// number changes, then `spec_version` must change, also.
-    ///
-    /// This number must change when an existing dispatchable (module ID, dispatch ID) is changed,
-    /// either through an alteration in its user-level semantics, a parameter
-    /// added/removed/changed, a dispatchable being removed, a module being removed, or a
-    /// dispatchable/module changing its index.
-    ///
-    /// It need *not* change when a new module is added or when a dispatchable is added.
-    pub transaction_version: u32,
-}
-
 /// This trait allows you to configure the "signed extra" and
 /// "additional" parameters that are a part of the transaction payload
 /// or the signer payload respectively.

new/src/config/polkadot.rs

@@ -71,12 +71,20 @@ impl Config for PolkadotConfig {
     // because we need to pass the PolkadotConfig trait as a param.
     type ExtrinsicParams = PolkadotExtrinsicParams<Self>;
 
+    fn genesis_hash(&self) -> Option<super::HashFor<Self>> {
+        self.0.genesis_hash()
+    }
+
     fn legacy_types_for_spec_version(&'_ self, spec_version: u32) -> Option<TypeRegistrySet<'_>> {
         self.0.legacy_types_for_spec_version(spec_version)
     }
 
-    fn spec_version_for_block_number(&self, block_number: u64) -> Option<u32> {
-        self.0.spec_version_for_block_number(block_number)
+    fn spec_and_transaction_version_for_block_number(
+        &self,
+        block_number: u64,
+    ) -> Option<(u32, u32)> {
+        self.0
+            .spec_and_transaction_version_for_block_number(block_number)
     }
 
     fn metadata_for_spec_version(&self, spec_version: u32) -> Option<Arc<Metadata>> {

new/src/config/substrate.rs

@@ -20,7 +20,8 @@ use subxt_metadata::Metadata;
 /// Construct a [`SubstrateConfig`] using this.
 pub struct SubstrateConfigBuilder {
     legacy_types: Option<ChainTypeRegistry>,
-    spec_version_for_block_number: RangeMap<u64, u32>,
+    spec_and_transaction_version_for_block_number: RangeMap<u64, (u32, u32)>,
+    genesis_hash: Option<H256>,
     metadata_for_spec_version: Mutex<HashMap<u32, Arc<Metadata>>>,
     use_old_v9_hashers_before_spec_version: u32,
 }
@@ -36,12 +37,19 @@ impl SubstrateConfigBuilder {
     pub fn new() -> Self {
         SubstrateConfigBuilder {
             legacy_types: None,
-            spec_version_for_block_number: RangeMap::empty(),
+            genesis_hash: None,
+            spec_and_transaction_version_for_block_number: RangeMap::empty(),
             metadata_for_spec_version: Mutex::new(HashMap::new()),
             use_old_v9_hashers_before_spec_version: 0,
         }
     }
 
+    /// Set the genesis hash for this chain.
+    pub fn set_genesis_hash(mut self, genesis_hash: H256) -> Self {
+        self.genesis_hash = Some(genesis_hash);
+        self
+    }
+
     /// Set the legacy types to use for this configuration. This enables support for
     /// blocks produced by Runtimes that emit metadata older than V14.
     pub fn set_legacy_types(mut self, legacy_types: ChainTypeRegistry) -> Self {
@@ -73,9 +81,10 @@ impl SubstrateConfigBuilder {
             let start = version_for_range.block_range.start;
             let end = version_for_range.block_range.end;
             let spec_version = version_for_range.spec_version;
-            m = m.add_range(start, end, spec_version);
+            let transaction_version = version_for_range.transaction_version;
+            m = m.add_range(start, end, (spec_version, transaction_version));
         }
-        self.spec_version_for_block_number = m.build();
+        self.spec_and_transaction_version_for_block_number = m.build();
         self
     }
 
@@ -91,7 +100,8 @@ impl SubstrateConfigBuilder {
         SubstrateConfig {
             inner: Arc::new(SubstrateConfigInner {
                 legacy_types: self.legacy_types,
-                spec_version_for_block_number: self.spec_version_for_block_number,
+                spec_and_transaction_version_for_block_number: self
+                    .spec_and_transaction_version_for_block_number,
                 metadata_for_spec_version: self.metadata_for_spec_version,
             }),
         }
@@ -107,6 +117,8 @@ pub struct SpecVersionForRange {
     pub block_range: std::ops::Range<u64>,
     /// The spec version at this block range.
     pub spec_version: u32,
+    /// The transaction version at this block range.
+    pub transaction_version: u32,
 }
 
 /// Configuration that's suitable for standard Substrate chains (ie those
@@ -119,7 +131,7 @@ pub struct SubstrateConfig {
 #[derive(Debug)]
 struct SubstrateConfigInner {
     legacy_types: Option<ChainTypeRegistry>,
-    spec_version_for_block_number: RangeMap<u64, u32>,
+    spec_and_transaction_version_for_block_number: RangeMap<u64, (u32, u32)>,
     metadata_for_spec_version: Mutex<HashMap<u32, Arc<Metadata>>>,
 }
 
@@ -146,9 +158,12 @@ impl Config for SubstrateConfig {
             .map(|types| types.for_spec_version(spec_version as u64))
     }
 
-    fn spec_version_for_block_number(&self, block_number: u64) -> Option<u32> {
+    fn spec_and_transaction_version_for_block_number(
+        &self,
+        block_number: u64,
+    ) -> Option<(u32, u32)> {
         self.inner
-            .spec_version_for_block_number
+            .spec_and_transaction_version_for_block_number
             .get(block_number)
             .copied()
     }

new/src/config/transaction_extensions.rs

@@ -169,7 +169,7 @@ impl<T: Config> ExtrinsicParams<T> for CheckSpecVersion {
     type Params = ();
 
     fn new(client: &ClientState<T>, _params: Self::Params) -> Result<Self, ExtrinsicParamsError> {
-        Ok(CheckSpecVersion(client.runtime_version.spec_version))
+        Ok(CheckSpecVersion(client.spec_version))
     }
 }
 
@@ -240,7 +240,7 @@ impl<T: Config> ExtrinsicParams<T> for CheckTxVersion {
     type Params = ();
 
     fn new(client: &ClientState<T>, _params: Self::Params) -> Result<Self, ExtrinsicParamsError> {
-        Ok(CheckTxVersion(client.runtime_version.transaction_version))
+        Ok(CheckTxVersion(client.transaction_version))
     }
 }
 

new/src/lib.rs

@@ -36,6 +36,7 @@ pub mod backend;
 pub mod client;
 pub mod config;
 pub mod error;
+pub mod transactions;
 pub mod utils;
 // pub mod book;
 // pub mod blocks;

new/src/transactions.rs

@@ -0,0 +1,260 @@
+mod payload;
+mod signer;
+
+use crate::client::{OfflineClientAtBlockT, OnlineClientAtBlockT};
+use crate::config::{ClientState, Config};
+use crate::error::ExtrinsicError;
+use codec::Compact;
+use core::marker::PhantomData;
+
+pub use payload::Payload;
+pub use signer::Signer;
+
+/// A client for working with transactions.
+#[derive(Clone)]
+pub struct Transactions<'atblock, T, Client> {
+    client: &'atblock Client,
+    marker: PhantomData<T>,
+}
+
+impl<'atblock, T, Client> Transactions<'atblock, T, Client> {
+    pub(crate) fn new(client: &'atblock Client) -> Self {
+        Transactions {
+            client,
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<'atblock, T: Config, C: OfflineClientAtBlockT<T>> Transactions<'atblock, T, C> {
+    /// Run the validation logic against some transaction you'd like to submit. Returns `Ok(())`
+    /// if the call is valid (or if it's not possible to check since the call has no validation hash).
+    /// Return an error if the call was not valid or something went wrong trying to validate it (ie
+    /// the pallet or call in question do not exist at all).
+    pub fn validate<Call>(&self, call: &Call) -> Result<(), ExtrinsicError>
+    where
+        Call: Payload,
+    {
+        let Some(details) = call.validation_details() else {
+            return Ok(());
+        };
+
+        let pallet_name = details.pallet_name;
+        let call_name = details.call_name;
+
+        let expected_hash = self
+            .client
+            .metadata_ref()
+            .pallet_by_name(pallet_name)
+            .ok_or_else(|| ExtrinsicError::PalletNameNotFound(pallet_name.to_string()))?
+            .call_hash(call_name)
+            .ok_or_else(|| ExtrinsicError::CallNameNotFound {
+                pallet_name: pallet_name.to_string(),
+                call_name: call_name.to_string(),
+            })?;
+
+        if details.hash != expected_hash {
+            Err(ExtrinsicError::IncompatibleCodegen)
+        } else {
+            Ok(())
+        }
+    }
+
+    /// Return the SCALE encoded bytes representing the call data of the transaction.
+    pub fn call_data<Call>(&self, call: &Call) -> Result<Vec<u8>, ExtrinsicError>
+    where
+        Call: Payload,
+    {
+        let mut bytes = Vec::new();
+        let metadata = self.client.metadata_ref();
+        call.encode_call_data_to(metadata, &mut bytes)?;
+        Ok(bytes)
+    }
+
+    /// Creates an unsigned transaction without submitting it. Depending on the metadata, we might end
+    /// up constructing either a v4 or v5 transaction. See [`Self::create_v4_unsigned`] or
+    /// [`Self::create_v5_bare`] if you'd like to explicitly create an unsigned transaction of a certain version.
+    pub fn create_unsigned<Call>(
+        &self,
+        call: &Call,
+    ) -> Result<SubmittableTransaction<T, C>, ExtrinsicError>
+    where
+        Call: Payload,
+    {
+        let tx = match self.default_transaction_version()? {
+            TransactionVersion::V4 => self.create_v4_unsigned(call),
+            TransactionVersion::V5 => self.create_v5_bare(call),
+        }?;
+
+        Ok(SubmittableTransaction {
+            client: self.client.clone(),
+            inner: tx,
+        })
+    }
+
+    /// Creates a V4 "unsigned" transaction without submitting it.
+    pub fn create_v4_unsigned<Call>(&self, call: &Call) -> Result<Transaction<T>, ExtrinsicError>
+    where
+        Call: Payload,
+    {
+        self.create_unsigned_at_version(call, 4)
+    }
+
+    /// Creates a V5 "bare" transaction without submitting it.
+    pub fn create_v5_bare<Call>(&self, call: &Call) -> Result<Transaction<T>, ExtrinsicError>
+    where
+        Call: Payload,
+    {
+        self.create_unsigned_at_version(call, 5)
+    }
+
+    /// Create a partial transaction. Depending on the metadata, we might end up constructing either a v4 or
+    /// v5 transaction. See [`subxt_core::tx`] if you'd like to manually pick the version to construct
+    ///
+    /// Note: if not provided, the default account nonce will be set to 0 and the default mortality will be _immortal_.
+    /// This is because this method runs offline, and so is unable to fetch the data needed for more appropriate values.
+    pub fn create_partial_offline<Call>(
+        &self,
+        call: &Call,
+        params: <T::ExtrinsicParams as ExtrinsicParams<T>>::Params,
+    ) -> Result<PartialTransaction<T, C>, ExtrinsicError>
+    where
+        Call: Payload,
+    {
+        let metadata = self.client.metadata();
+        let client_state = ClientState {
+            genesis_hash: self.client.genesis_hash(),
+            spec_version: self.client.spec_version(),
+            transaction_version: self.client.transaction_version(),
+            metadata: self.client.metadata(),
+        };
+
+        let tx = match self.default_transaction_version(&metadata)? {
+            TransactionVersion::V4 => {
+                PartialTransactionInner::V4(self.create_v4_signed(call, &client_state, params)?)
+            }
+            TransactionVersion::V5 => {
+                PartialTransactionInner::V5(self.create_v5_general(call, &client_state, params)?)
+            }
+        };
+
+        Ok(PartialTransaction {
+            client: self.client.clone(),
+            inner: tx,
+        })
+    }
+
+    /// Create a v4 partial transaction, ready to sign.
+    ///
+    /// Note: if not provided, the default account nonce will be set to 0 and the default mortality will be _immortal_.
+    /// This is because this method runs offline, and so is unable to fetch the data needed for more appropriate values.
+    ///
+    /// Prefer [`Self::create_partial_offline()`] if you don't know which version to create; this will pick the
+    /// most suitable one for the given chain.
+    pub fn create_v4_partial_offline<Call>(
+        &self,
+        call: &Call,
+        params: <T::ExtrinsicParams as ExtrinsicParams<T>>::Params,
+    ) -> Result<PartialTransaction<T, C>, ExtrinsicError>
+    where
+        Call: Payload,
+    {
+        let tx = PartialTransactionInner::V4(subxt_core::tx::create_v4_signed(
+            call,
+            &self.client.client_state(),
+            params,
+        )?);
+
+        Ok(PartialTransaction {
+            client: self.client.clone(),
+            inner: tx,
+        })
+    }
+
+    /// Create a v5 partial transaction, ready to sign.
+    ///
+    /// Note: if not provided, the default account nonce will be set to 0 and the default mortality will be _immortal_.
+    /// This is because this method runs offline, and so is unable to fetch the data needed for more appropriate values.
+    ///
+    /// Prefer [`Self::create_partial_offline()`] if you don't know which version to create; this will pick the
+    /// most suitable one for the given chain.
+    pub fn create_v5_partial_offline<Call>(
+        &self,
+        call: &Call,
+        params: <T::ExtrinsicParams as ExtrinsicParams<T>>::Params,
+    ) -> Result<PartialTransaction<T, C>, ExtrinsicError>
+    where
+        Call: Payload,
+    {
+        let tx = PartialTransactionInner::V5(subxt_core::tx::create_v5_general(
+            call,
+            &self.client.client_state(),
+            params,
+        )?);
+
+        Ok(PartialTransaction {
+            client: self.client.clone(),
+            inner: tx,
+        })
+    }
+
+    // Create a V4 "unsigned" transaction or V5 "bare" transaction.
+    fn create_unsigned_at_version<Call: Payload>(
+        &self,
+        call: &Call,
+        tx_version: u8,
+    ) -> Result<Transaction<T>, ExtrinsicError> {
+        let metadata = self.client.metadata_ref();
+
+        // 1. Validate this call against the current node metadata if the call comes
+        // with a hash allowing us to do so.
+        self.validate(call)?;
+
+        // 2. Encode extrinsic
+        let extrinsic = {
+            let mut encoded_inner = Vec::new();
+            // encode the transaction version first.
+            tx_version.encode_to(&mut encoded_inner);
+            // encode call data after this byte.
+            call.encode_call_data_to(metadata, &mut encoded_inner)?;
+            // now, prefix byte length:
+            let len = Compact(
+                u32::try_from(encoded_inner.len()).expect("extrinsic size expected to be <4GB"),
+            );
+            let mut encoded = Vec::new();
+            len.encode_to(&mut encoded);
+            encoded.extend(encoded_inner);
+            encoded
+        };
+
+        // Wrap in Encoded to ensure that any more "encode" calls leave it in the right state.
+        Ok(Transaction::from_bytes(extrinsic))
+    }
+
+    /// Returns the suggested transaction versions to build for a given chain, or an error
+    /// if Subxt doesn't support any version expected by the chain.
+    ///
+    /// If the result is [`TransactionVersion::V4`], use the `v4` methods in this module. If it's
+    /// [`TransactionVersion::V5`], use the `v5` ones.
+    pub fn default_transaction_version(&self) -> Result<TransactionVersion, ExtrinsicError> {
+        let metadata = self.client.metadata_ref();
+        let versions = metadata.extrinsic().supported_versions();
+
+        if versions.contains(&4) {
+            Ok(TransactionVersion::V4)
+        } else if versions.contains(&5) {
+            Ok(TransactionVersion::V5)
+        } else {
+            Err(ExtrinsicError::UnsupportedVersion)
+        }
+    }
+}
+
+/// The transaction versions supported by Subxt.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
+pub enum TransactionVersion {
+    /// v4 transactions (signed and unsigned transactions)
+    V4,
+    /// v5 transactions (bare and general transactions)
+    V5,
+}

new/src/transactions/payload.rs

@@ -0,0 +1,272 @@
+// Copyright 2019-2024 Parity Technologies (UK) Ltd.
+// This file is dual-licensed as Apache-2.0 or GPL-3.0.
+// see LICENSE for license details.
+
+//! This module contains the trait and types used to represent
+//! transactions that can be submitted.
+
+use crate::error::ExtrinsicError;
+use codec::Encode;
+use scale_encode::EncodeAsFields;
+use scale_value::{Composite, Value, ValueDef, Variant};
+use std::borrow::Cow;
+use subxt_metadata::Metadata;
+
+/// This represents a transaction payload that can be submitted
+/// to a node.
+pub trait Payload {
+    /// Encode call data to the provided output.
+    fn encode_call_data_to(
+        &self,
+        metadata: &Metadata,
+        out: &mut Vec<u8>,
+    ) -> Result<(), ExtrinsicError>;
+
+    /// Encode call data and return the output. This is a convenience
+    /// wrapper around [`Payload::encode_call_data_to`].
+    fn encode_call_data(&self, metadata: &Metadata) -> Result<Vec<u8>, ExtrinsicError> {
+        let mut v = Vec::new();
+        self.encode_call_data_to(metadata, &mut v)?;
+        Ok(v)
+    }
+
+    /// Returns the details needed to validate the call, which
+    /// include a statically generated hash, the pallet name,
+    /// and the call name.
+    fn validation_details(&self) -> Option<ValidationDetails<'_>> {
+        None
+    }
+}
+
+macro_rules! boxed_payload {
+    ($ty:path) => {
+        impl<T: Payload + ?Sized> Payload for $ty {
+            fn encode_call_data_to(
+                &self,
+                metadata: &Metadata,
+                out: &mut Vec<u8>,
+            ) -> Result<(), ExtrinsicError> {
+                self.as_ref().encode_call_data_to(metadata, out)
+            }
+            fn encode_call_data(&self, metadata: &Metadata) -> Result<Vec<u8>, ExtrinsicError> {
+                self.as_ref().encode_call_data(metadata)
+            }
+            fn validation_details(&self) -> Option<ValidationDetails<'_>> {
+                self.as_ref().validation_details()
+            }
+        }
+    };
+}
+
+boxed_payload!(Box<T>);
+boxed_payload!(std::sync::Arc<T>);
+boxed_payload!(std::rc::Rc<T>);
+
+/// Details required to validate the shape of a transaction payload against some metadata.
+pub struct ValidationDetails<'a> {
+    /// The pallet name.
+    pub pallet_name: &'a str,
+    /// The call name.
+    pub call_name: &'a str,
+    /// A hash (this is generated at compile time in our codegen)
+    /// to compare against the runtime code.
+    pub hash: [u8; 32],
+}
+
+/// A transaction payload containing some generic `CallData`.
+#[derive(Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
+pub struct DefaultPayload<CallData> {
+    pallet_name: Cow<'static, str>,
+    call_name: Cow<'static, str>,
+    call_data: CallData,
+    validation_hash: Option<[u8; 32]>,
+}
+
+/// The payload type used by static codegen.
+pub type StaticPayload<Calldata> = DefaultPayload<Calldata>;
+/// The type of a payload typically used for dynamic transaction payloads.
+pub type DynamicPayload = DefaultPayload<Composite<()>>;
+
+impl<CallData> DefaultPayload<CallData> {
+    /// Create a new [`DefaultPayload`].
+    pub fn new(
+        pallet_name: impl Into<String>,
+        call_name: impl Into<String>,
+        call_data: CallData,
+    ) -> Self {
+        DefaultPayload {
+            pallet_name: Cow::Owned(pallet_name.into()),
+            call_name: Cow::Owned(call_name.into()),
+            call_data,
+            validation_hash: None,
+        }
+    }
+
+    /// Create a new [`DefaultPayload`] using static strings for the pallet and call name.
+    /// This is only expected to be used from codegen.
+    #[doc(hidden)]
+    pub fn new_static(
+        pallet_name: &'static str,
+        call_name: &'static str,
+        call_data: CallData,
+        validation_hash: [u8; 32],
+    ) -> Self {
+        DefaultPayload {
+            pallet_name: Cow::Borrowed(pallet_name),
+            call_name: Cow::Borrowed(call_name),
+            call_data,
+            validation_hash: Some(validation_hash),
+        }
+    }
+
+    /// Do not validate this call prior to submitting it.
+    pub fn unvalidated(self) -> Self {
+        Self {
+            validation_hash: None,
+            ..self
+        }
+    }
+
+    /// Returns the call data.
+    pub fn call_data(&self) -> &CallData {
+        &self.call_data
+    }
+
+    /// Returns the pallet name.
+    pub fn pallet_name(&self) -> &str {
+        &self.pallet_name
+    }
+
+    /// Returns the call name.
+    pub fn call_name(&self) -> &str {
+        &self.call_name
+    }
+}
+
+impl DefaultPayload<Composite<()>> {
+    /// Convert the dynamic `Composite` payload into a [`Value`].
+    /// This is useful if you want to use this as an argument for a
+    /// larger dynamic call that wants to use this as a nested call.
+    pub fn into_value(self) -> Value<()> {
+        let call = Value {
+            context: (),
+            value: ValueDef::Variant(Variant {
+                name: self.call_name.into_owned(),
+                values: self.call_data,
+            }),
+        };
+
+        Value::unnamed_variant(self.pallet_name, [call])
+    }
+}
+
+impl<CallData: EncodeAsFields> Payload for DefaultPayload<CallData> {
+    fn encode_call_data_to(
+        &self,
+        metadata: &Metadata,
+        out: &mut Vec<u8>,
+    ) -> Result<(), ExtrinsicError> {
+        let pallet = metadata
+            .pallet_by_name(&self.pallet_name)
+            .ok_or_else(|| ExtrinsicError::PalletNameNotFound(self.pallet_name.to_string()))?;
+        let call = pallet
+            .call_variant_by_name(&self.call_name)
+            .ok_or_else(|| ExtrinsicError::CallNameNotFound {
+                pallet_name: pallet.name().to_string(),
+                call_name: self.call_name.to_string(),
+            })?;
+
+        let pallet_index = pallet.call_index();
+        let call_index = call.index;
+
+        pallet_index.encode_to(out);
+        call_index.encode_to(out);
+
+        let mut fields = call
+            .fields
+            .iter()
+            .map(|f| scale_encode::Field::new(f.ty.id, f.name.as_deref()));
+
+        self.call_data
+            .encode_as_fields_to(&mut fields, metadata.types(), out)
+            .map_err(ExtrinsicError::CannotEncodeCallData)?;
+        Ok(())
+    }
+
+    fn validation_details(&self) -> Option<ValidationDetails<'_>> {
+        self.validation_hash.map(|hash| ValidationDetails {
+            pallet_name: &self.pallet_name,
+            call_name: &self.call_name,
+            hash,
+        })
+    }
+}
+
+/// Construct a transaction at runtime; essentially an alias to [`DefaultPayload::new()`]
+/// which provides a [`Composite`] value for the call data.
+pub fn dynamic(
+    pallet_name: impl Into<String>,
+    call_name: impl Into<String>,
+    call_data: impl Into<Composite<()>>,
+) -> DynamicPayload {
+    DefaultPayload::new(pallet_name, call_name, call_data.into())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use codec::Decode;
+    use scale_value::Composite;
+
+    fn test_metadata() -> Metadata {
+        let metadata_bytes = include_bytes!("../../../artifacts/polkadot_metadata_small.scale");
+        Metadata::decode(&mut &metadata_bytes[..]).expect("Valid metadata")
+    }
+
+    #[test]
+    fn encode_call_with_incompatible_types_returns_error() {
+        let metadata = test_metadata();
+
+        let incompatible_data = Composite::named([
+            ("dest", scale_value::Value::bool(true)), // Boolean instead of MultiAddress
+            ("value", scale_value::Value::string("not_a_number")), // String instead of u128
+        ]);
+
+        let payload = DefaultPayload::new("Balances", "transfer_allow_death", incompatible_data);
+
+        let mut out = Vec::new();
+        let result = payload.encode_call_data_to(&metadata, &mut out);
+
+        assert!(
+            result.is_err(),
+            "Expected error when encoding with incompatible types"
+        );
+    }
+
+    #[test]
+    fn encode_call_with_valid_data_succeeds() {
+        let metadata = test_metadata();
+
+        // Create a valid payload to ensure our error handling doesn't break valid cases
+        // For MultiAddress, we'll use the Id variant with a 32-byte account
+        let valid_address =
+            scale_value::Value::unnamed_variant("Id", [scale_value::Value::from_bytes([0u8; 32])]);
+
+        let valid_data = Composite::named([
+            ("dest", valid_address),
+            ("value", scale_value::Value::u128(1000)),
+        ]);
+
+        let payload = DefaultPayload::new("Balances", "transfer_allow_death", valid_data);
+
+        // This should succeed
+        let mut out = Vec::new();
+        let result = payload.encode_call_data_to(&metadata, &mut out);
+
+        assert!(
+            result.is_ok(),
+            "Expected success when encoding with valid data"
+        );
+        assert!(!out.is_empty(), "Expected encoded output to be non-empty");
+    }
+}

new/src/transactions/signer.rs

@@ -0,0 +1,15 @@
+use crate::config::Config;
+
+/// Signing transactions requires a [`Signer`]. This is responsible for
+/// providing the "from" account that the transaction is being signed by,
+/// as well as actually signing a SCALE encoded payload.
+pub trait Signer<T: Config> {
+    /// Return the "from" account ID.
+    fn account_id(&self) -> T::AccountId;
+
+    /// Takes a signer payload for an extrinsic, and returns a signature based on it.
+    ///
+    /// Some signers may fail, for instance because the hardware on which the keys are located has
+    /// refused the operation.
+    fn sign(&self, signer_payload: &[u8]) -> T::Signature;
+}