blocks: Fetch the block's header

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

subxt/src/blocks/block_types.rs

@@ -69,6 +69,8 @@ pub enum ChainHeadError {
     Error(String),
     /// The provided subscription ID is stale or invalid.
     Disjoint,
+    /// The RPC target node does not contain the given resource.
+    ResourceNonExistent,
     /// An error occurred internally. This is definitive.
     Other(String),
 }
@@ -103,6 +105,18 @@ where
     T: Config,
     C: OnlineClientT<T>,
 {
+    /// Fetch the body (vector of extrinsics) of this block.
+    pub async fn body(&self) -> Result<Vec<Vec<u8>>, ChainHeadError> {
+        self.fetch_body(self.subscription_id.clone(), self.hash)
+            .await
+    }
+
+    /// Fetch the header of this block.
+    pub async fn header(&self) -> Result<T::Header, ChainHeadError> {
+        self.fetch_header(self.subscription_id.clone(), self.hash)
+            .await
+    }
+
     /// Wrapper to fetch the block's body from the `chainHead_body` subscription.
     async fn fetch_body(
         &self,
@@ -128,10 +142,29 @@ where
         Err(Error::Other("Failed to fetch the block body".into()).into())
     }
 
-    /// Fetch the body (vector of extrinsics) of this block.
-    pub async fn body(&self) -> Result<Vec<Vec<u8>>, ChainHeadError> {
-        self.fetch_body(self.subscription_id.clone(), self.hash)
-            .await
+    /// Wrapper to fetch the block's header from the `chainHead_header` method.
+    async fn fetch_header(
+        &self,
+        subscription_id: String,
+        hash: T::Hash,
+    ) -> Result<T::Header, ChainHeadError> {
+        let header = self
+            .client
+            .rpc()
+            .chainhead_header(subscription_id, hash)
+            .await?;
+
+        let header = match header {
+            Some(header) => header,
+            None => return Err(ChainHeadError::ResourceNonExistent),
+        };
+
+        let bytes = hex::decode(header.trim_start_matches("0x"))
+            .map_err(|err| Error::Other(err.to_string()))?;
+
+        let header: T::Header =
+            Decode::decode(&mut &bytes[..]).map_err(Into::<Error>::into)?;
+        Ok(header)
     }
 }