Make sure channel is always set, be able to raise UI errors with WebRTCError (#45)

* Code

* test

* code

* user guide

backend/gradio_webrtc/webrtc.py

@@ -44,6 +44,7 @@ from gradio_client import handle_file
 from gradio_webrtc.utils import (
     AdditionalOutputs,
     DataChannel,
+    current_channel,
     player_worker_decode,
     split_output,
 )
@@ -84,9 +85,12 @@ class VideoCallback(VideoStreamTrack):
         self.set_additional_outputs = set_additional_outputs
         self.thread_quit = asyncio.Event()
         self.mode = mode
+        self.channel_set = asyncio.Event()
 
     def set_channel(self, channel: DataChannel):
         self.channel = channel
+        current_channel.set(channel)
+        self.channel_set.set()
 
     def set_args(self, args: list[Any]):
         self.latest_args = ["__webrtc_value__"] + list(args)
@@ -122,6 +126,12 @@ class VideoCallback(VideoStreamTrack):
         logger.debug("video callback stop")
         self.thread_quit.set()
 
+    async def wait_for_channel(self):
+        if not self.channel_set.is_set():
+            await self.channel_set.wait()
+        if current_channel.get() != self.channel:
+            current_channel.set(self.channel)
+
     async def recv(self):
         try:
             try:
@@ -129,6 +139,8 @@ class VideoCallback(VideoStreamTrack):
             except MediaStreamError:
                 self.stop()
                 return
+
+            await self.wait_for_channel()
             frame_array = frame.to_ndarray(format="bgr24")
 
             if self.latest_args == "not_set":
@@ -180,6 +192,7 @@ class StreamHandlerBase(ABC):
         self._channel: DataChannel | None = None
         self._loop: asyncio.AbstractEventLoop
         self.args_set = asyncio.Event()
+        self.channel_set = asyncio.Event()
 
     @property
     def loop(self) -> asyncio.AbstractEventLoop:
@@ -191,6 +204,7 @@ class StreamHandlerBase(ABC):
 
     def set_channel(self, channel: DataChannel):
         self._channel = channel
+        self.channel_set.set()
 
     async def fetch_args(
         self,
@@ -203,6 +217,9 @@ class StreamHandlerBase(ABC):
         await self.fetch_args()
         await self.args_set.wait()
 
+    def wait_for_args_sync(self):
+        asyncio.run_coroutine_threadsafe(self.wait_for_args(), self.loop).result()
+
     def set_args(self, args: list[Any]):
         logger.debug("setting args in audio callback %s", args)
         self.latest_args = ["__webrtc_value__"] + list(args)
@@ -275,6 +292,7 @@ class AudioCallback(AudioStreamTrack):
         channel: DataChannel | None = None,
         set_additional_outputs: Callable | None = None,
     ) -> None:
+        super().__init__()
         self.track = track
         self.event_handler = event_handler
         self.current_timestamp = 0
@@ -286,7 +304,6 @@ class AudioCallback(AudioStreamTrack):
         self.last_timestamp = 0
         self.channel = channel
         self.set_additional_outputs = set_additional_outputs
-        super().__init__()
 
     def set_channel(self, channel: DataChannel):
         self.channel = channel
@@ -295,6 +312,10 @@ class AudioCallback(AudioStreamTrack):
     def set_args(self, args: list[Any]):
         self.event_handler.set_args(args)
 
+    def event_handler_receive(self, frame: tuple[int, np.ndarray]) -> None:
+        current_channel.set(self.event_handler.channel)
+        return cast(Callable, self.event_handler.receive)(frame)
+
     async def process_input_frames(self) -> None:
         while not self.thread_quit.is_set():
             try:
@@ -307,7 +328,7 @@ class AudioCallback(AudioStreamTrack):
                         )
                     else:
                         await anyio.to_thread.run_sync(
-                            self.event_handler.receive, (frame.sample_rate, numpy_array)
+                            self.event_handler_receive, (frame.sample_rate, numpy_array)
                         )
             except MediaStreamError:
                 logger.debug("MediaStreamError in process_input_frames")
@@ -342,7 +363,13 @@ class AudioCallback(AudioStreamTrack):
             if self.readyState != "live":
                 raise MediaStreamError
 
+            if not self.event_handler.channel_set.is_set():
+                await self.event_handler.channel_set.wait()
+            if current_channel.get() != self.event_handler.channel:
+                current_channel.set(self.event_handler.channel)
+
             self.start()
+
             frame = await self.queue.get()
             logger.debug("frame %s", frame)
 
@@ -415,7 +442,7 @@ class ServerToClientVideo(VideoStreamTrack):
                 self.generator = cast(
                     Generator[Any, None, Any], self.event_handler(*self.latest_args)
                 )
-
+                current_channel.set(self.channel)
             try:
                 next_array, outputs = split_output(next(self.generator))
                 if (
@@ -470,6 +497,7 @@ class ServerToClientAudio(AudioStreamTrack):
 
     def next(self) -> tuple[int, np.ndarray] | None:
         self.args_set.wait()
+        current_channel.set(self.channel)
         if self.generator is None:
             self.generator = self.event_handler(*self.latest_args)
         if self.generator is not None:
@@ -946,6 +974,7 @@ class WebRTC(Component):
         answer = await pc.createAnswer()
         await pc.setLocalDescription(answer)  # type: ignore
         logger.debug("done handling offer about to return")
+        await asyncio.sleep(0.1)
 
         return {
             "sdp": pc.localDescription.sdp,