Add example how to wrap async-nats in sync context

Signed-off-by: Tomasz Pietrek <tomasz@nats.io>

async-nats/examples/sync_context.rs

@@ -0,0 +1,49 @@
+// Copyright 2024 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use futures::StreamExt;
+
+fn main() -> Result<(), async_nats::Error> {
+    // Spawn a new runtime that will run on the current thread.
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()?;
+
+    // We are using `block_on` to run the async code in a sync context.
+    let client = rt.block_on(async_nats::connect("nats://localhost:4222"))?;
+
+    // Subscribe a client to the "foo" subject.
+    let subscription = rt.block_on(client.subscribe("foo"))?;
+
+    // Limit the number of messages to 10. This does not have to be done inside the
+    // async context.
+    let mut subscription = subscription.take(10);
+
+    // Publish some messages
+    for _ in 0..10 {
+        rt.block_on(client.publish("foo", "Hello, sync code!".into()))?;
+    }
+
+    // To receive message in a loop, you can use the same pattern as you would in an async context.
+    while let Some(message) = rt.block_on(subscription.next()) {
+        println!("Received message {:?}", message);
+    }
+
+    // You need to drop subscripions in async context, as they do spawn tasks to clean themselves up.
+    rt.block_on(async {
+        drop(subscription);
+        drop(client);
+    });
+
+    Ok(())
+}