pybricksdev.connections.pybricks: add stdout_observable property

This adds a new property for subscribing to each received stdout
notification.

Fixes: https://github.com/orgs/pybricks/discussions/1038

CHANGELOG.md

@@ -6,9 +6,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Added `PybricksHub.stdout_observable` property ([support#1038]).
+
 ### Fixed
 - Fixed endline in `PybricksHub.write_line()`.
 
+[support#1038]: https://github.com/orgs/pybricks/discussions/1038
+
 ## [1.0.0-alpha.44] - 2023-04-20
 
 ### Fixed

pybricksdev/connections/pybricks.py

@@ -13,6 +13,7 @@
 from bleak import BleakClient
 from bleak.backends.device import BLEDevice
 from packaging.version import Version
+from reactivex import Observable
 from reactivex.subject import BehaviorSubject, Subject
 from tqdm.auto import tqdm
 from tqdm.contrib.logging import logging_redirect_tqdm
@@ -80,6 +81,7 @@ class PybricksHub:
     def __init__(self):
         self.connection_state_observable = BehaviorSubject(ConnectionState.DISCONNECTED)
         self.status_observable = BehaviorSubject(StatusFlag(0))
+        self._stdout_subject = Subject()
         self.nus_observable = Subject()
         self.print_output = True
         self.fw_version = None
@@ -118,6 +120,13 @@ def __init__(self):
         # File handle for logging
         self.log_file = None
 
+    @property
+    def stdout_observable(self) -> Observable[bytes]:
+        """
+        Observable used to subscribe to stdout of the hub.
+        """
+        return self._stdout_subject
+
     def _line_handler(self, line: bytes) -> None:
         """
         Handles new incoming lines. Handle special actions if needed,
@@ -201,6 +210,8 @@ def _nus_handler(self, sender, data: bytearray) -> None:
         # support legacy firmware where the Nordic UART service
         # was used for stdio
         if self._legacy_stdio:
+            self._stdout_subject.on_next(data)
+
             if self._enable_line_handler:
                 self._handle_line_data(data)
 
@@ -211,6 +222,7 @@ def _pybricks_service_handler(self, _: int, data: bytes) -> None:
             self.status_observable.on_next(StatusFlag(flags))
         elif data[0] == Event.WRITE_STDOUT:
             payload = data[1:]
+            self._stdout_subject.on_next(payload)
 
             if self._enable_line_handler:
                 self._handle_line_data(payload)
@@ -403,7 +415,17 @@ async def read_line(self) -> str:
 
         Returns:
             The next line read from stdout (without the newline).
+
+        Raises:
+            RuntimeError:
+                if line handler is disabled (e.g. :meth:`run` is
+                called with ``line_handler=False``)
+            RuntimeError:
+                if hub becomes disconnected
         """
+        if not self._enable_line_handler:
+            raise RuntimeError("line handler is disabled, method would block forever")
+
         return await self.race_disconnect(self._stdout_line_queue.get())
 
     async def start_user_program(self) -> None: