Filter one or multiple events by type from an EventSubscription

examples/examples/submit_and_watch.rs

@@ -67,7 +67,7 @@ async fn simple_transfer() -> Result<(), Box<dyn std::error::Error>> {
         .await?;
 
     let transfer_event =
-        balance_transfer.find_first_event::<polkadot::balances::events::Transfer>()?;
+        balance_transfer.find_first::<polkadot::balances::events::Transfer>()?;
 
     if let Some(event) = transfer_event {
         println!("Balance transfer success: {event:?}");
@@ -108,7 +108,7 @@ async fn simple_transfer_separate_events() -> Result<(), Box<dyn std::error::Err
     let events = balance_transfer.fetch_events().await?;
 
     let failed_event =
-        events.find_first_event::<polkadot::system::events::ExtrinsicFailed>()?;
+        events.find_first::<polkadot::system::events::ExtrinsicFailed>()?;
 
     if let Some(_ev) = failed_event {
         // We found a failed event; the transfer didn't succeed.
@@ -117,7 +117,7 @@ async fn simple_transfer_separate_events() -> Result<(), Box<dyn std::error::Err
         // We didn't find a failed event; the transfer succeeded. Find
         // more details about it to report..
         let transfer_event =
-            events.find_first_event::<polkadot::balances::events::Transfer>()?;
+            events.find_first::<polkadot::balances::events::Transfer>()?;
         if let Some(event) = transfer_event {
             println!("Balance transfer success: {event:?}");
         } else {
@@ -161,7 +161,7 @@ async fn handle_transfer_events() -> Result<(), Box<dyn std::error::Error>> {
 
             let events = details.wait_for_success().await?;
             let transfer_event =
-                events.find_first_event::<polkadot::balances::events::Transfer>()?;
+                events.find_first::<polkadot::balances::events::Transfer>()?;
 
             if let Some(event) = transfer_event {
                 println!(
@@ -181,7 +181,7 @@ async fn handle_transfer_events() -> Result<(), Box<dyn std::error::Error>> {
 
             let events = details.wait_for_success().await?;
             let transfer_event =
-                events.find_first_event::<polkadot::balances::events::Transfer>()?;
+                events.find_first::<polkadot::balances::events::Transfer>()?;
 
             if let Some(event) = transfer_event {
                 println!("Balance transfer success: {event:?}");

examples/examples/subscribe_all_events.rs

@@ -103,7 +103,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
         // Or we can dynamically find the first transfer event, ignoring any others:
         let transfer_event =
-            events.find_first_event::<polkadot::balances::events::Transfer>()?;
+            events.find_first::<polkadot::balances::events::Transfer>()?;
 
         if let Some(ev) = transfer_event {
             println!("  - Balance transfer success: value: {:?}", ev.amount);

examples/examples/subscribe_one_event.rs

@@ -22,11 +22,7 @@
 //! polkadot --dev --tmp
 //! ```
 
-use futures::{
-    future,
-    stream,
-    StreamExt,
-};
+use futures::StreamExt;
 use sp_keyring::AccountKeyring;
 use std::time::Duration;
 use subxt::{
@@ -51,21 +47,13 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         .await?
         .to_runtime_api::<polkadot::RuntimeApi<DefaultConfig, DefaultExtra<DefaultConfig>>>();
 
-    // Subscribe to just balance transfer events, making use of `flat_map` and
-    // `filter_map` from the StreamExt trait to filter everything else out.
+    // Subscribe to just balance transfer events, making use of `filter_events`
+    // to select a single event type (note the 1-tuple) to filter out and return.
     let mut transfer_events = api
         .events()
         .subscribe()
         .await?
-        // Ignore errors returning events:
-        .filter_map(|events| future::ready(events.ok()))
-        // Map events to just the one we care about:
-        .flat_map(|events| {
-            let transfer_events = events
-                .find::<polkadot::balances::events::Transfer>()
-                .collect::<Vec<_>>();
-            stream::iter(transfer_events)
-        });
+        .filter_events::<(polkadot::balances::events::Transfer,)>();
 
     // While this subscription is active, we imagine some balance transfers are made somewhere else:
     async_std::task::spawn(async {

examples/examples/subscribe_some_events.rs

@@ -0,0 +1,100 @@
+// 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/>.
+
+//! To run this example, a local polkadot node should be running. Example verified against polkadot 0.9.13-82616422d0-aarch64-macos.
+//!
+//! E.g.
+//! ```bash
+//! curl "https://github.com/paritytech/polkadot/releases/download/v0.9.13/polkadot" --output /usr/local/bin/polkadot --location
+//! polkadot --dev --tmp
+//! ```
+
+use futures::StreamExt;
+use sp_keyring::AccountKeyring;
+use std::time::Duration;
+use subxt::{
+    ClientBuilder,
+    DefaultConfig,
+    DefaultExtra,
+    PairSigner,
+};
+
+#[subxt::subxt(runtime_metadata_path = "examples/polkadot_metadata.scale")]
+pub mod polkadot {}
+
+/// Subscribe to all events, and then manually look through them and
+/// pluck out the events that we care about.
+#[async_std::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    env_logger::init();
+
+    // Subscribe to any events that occur:
+    let api = ClientBuilder::new()
+        .build()
+        .await?
+        .to_runtime_api::<polkadot::RuntimeApi<DefaultConfig, DefaultExtra<DefaultConfig>>>();
+
+    // Subscribe to several balance related events. If we ask for more than one event,
+    // we'll be given a correpsonding tuple of `Option`'s, with exactly one
+    // variant populated each time.
+    let mut balance_events = api.events().subscribe().await?.filter_events::<(
+        polkadot::balances::events::Withdraw,
+        polkadot::balances::events::Transfer,
+        polkadot::balances::events::Deposit,
+    )>();
+
+    // While this subscription is active, we imagine some balance transfers are made somewhere else:
+    async_std::task::spawn(async {
+        let signer = PairSigner::new(AccountKeyring::Alice.pair());
+        let api = ClientBuilder::new()
+            .build()
+            .await
+            .unwrap()
+            .to_runtime_api::<polkadot::RuntimeApi<DefaultConfig, DefaultExtra<DefaultConfig>>>();
+
+        // Make small balance transfers from Alice to Bob in a loop:
+        loop {
+            api.tx()
+                .balances()
+                .transfer(AccountKeyring::Bob.to_account_id().into(), 1_000_000_000)
+                .sign_and_submit(&signer)
+                .await
+                .unwrap();
+            async_std::task::sleep(Duration::from_secs(10)).await;
+        }
+    });
+
+    // Our subscription will see all of the balance events we're filtering on:
+    while let Some(ev) = balance_events.next().await {
+        let event_details = ev?;
+
+        let block_hash = event_details.block_hash;
+        let event = event_details.event;
+        println!("Event at {:?}:", block_hash);
+
+        if let (Some(withdraw), _, _) = &event {
+            println!("  Withdraw event: {withdraw:?}");
+        }
+        if let (_, Some(transfer), _) = &event {
+            println!("  Transfer event: {transfer:?}");
+        }
+        if let (_, _, Some(deposit)) = &event {
+            println!("  Deposit event: {deposit:?}");
+        }
+    }
+
+    Ok(())
+}