Add the client-side subscriptions/listen driver (#3047)

docs_src/subscriptions/tutorial004_trio.py

@@ -0,0 +1,32 @@
+import trio
+
+from mcp import Client
+from mcp.client.subscriptions import Subscription
+
+from .tutorial003 import BOARD, read_board
+
+
+async def watch(client: Client, sub: Subscription) -> None:
+    async for _event in sub:
+        board = await read_board(client)
+        print(board)
+        if "[ ]" not in board:
+            return  # sprint finished: the stream closes when run_sprint leaves the block
+
+
+async def run_sprint(client: Client) -> None:
+    async with client.listen(resource_subscriptions=[BOARD]) as sub:
+        print(await read_board(client))  # snapshot: acknowledged, so nothing after this is missed
+        async with trio.open_nursery() as nursery:
+            nursery.start_soon(watch, client, sub)
+            for task in ("design", "build", "ship"):
+                await client.call_tool("complete_task", {"board": "sprint", "task": task})
+
+
+async def main() -> None:
+    async with Client("http://localhost:8000/mcp") as client:
+        await run_sprint(client)
+
+
+if __name__ == "__main__":
+    trio.run(main)