rpc/events: Add events for chainHead methods

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

subxt/src/rpc/rpc.rs

@@ -41,7 +41,10 @@
 
 use super::{
     rpc_params,
-    subscription_events::FollowEvent,
+    subscription_events::{
+        ChainHeadEvent,
+        FollowEvent,
+    },
     RpcClient,
     RpcClientT,
     Subscription,
@@ -629,7 +632,7 @@ impl<T: Config> Rpc<T> {
         &self,
         hash: T::Hash,
         subscription_id: String,
-    ) -> Result<Subscription<FollowEvent<T::Hash>>, Error> {
+    ) -> Result<Subscription<ChainHeadEvent<String>>, Error> {
         let subscription = self
             .client
             .subscribe(

subxt/src/rpc/subscription_events.rs

@@ -125,3 +125,28 @@ pub enum FollowEvent<Hash> {
     /// will be generated.
     Stop,
 }
+
+/// The result of a chain head method.
+#[derive(Debug, Clone, PartialEq, Eq, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ChainHeadResult<T> {
+    /// Result of the method.
+    pub result: T,
+}
+
+/// The event generated by the body / call / storage methods.
+#[derive(Debug, Clone, PartialEq, Eq, Deserialize)]
+#[serde(rename_all = "camelCase")]
+#[serde(tag = "event")]
+pub enum ChainHeadEvent<T> {
+    /// The request completed successfully.
+    Done(ChainHeadResult<T>),
+    /// The resources requested are inaccessible.
+    ///
+    /// Resubmitting the request later might succeed.
+    Inaccessible(ErrorEvent),
+    /// An error occurred. This is definitive.
+    Error(ErrorEvent),
+    /// The provided subscription ID is stale or invalid.
+    Disjoint,
+}