Bus#
Superclasses: Object, InitiallyUnowned, Object
The Bus is an object responsible for delivering Message packets in
a first-in first-out way from the streaming threads (see Task) to the
application.
Since the application typically only wants to deal with delivery of these messages from one thread, the GstBus will marshall the messages between different threads. This is important since the actual streaming of media is done in another thread than the application.
The GstBus provides support for Source based notifications. This makes it
possible to handle the delivery in the glib MainLoop.
The Source callback function async_signal_func() can be used to
convert all bus messages into signal emissions.
A message is posted on the bus with the post() method. With the
peek() and pop() methods one can look at or retrieve a
previously posted message.
The bus can be polled with the poll() method. This methods blocks
up to the specified timeout value until one of the specified messages types
is posted on the bus. The application can then pop() the messages
from the bus to handle them.
Alternatively the application can register an asynchronous bus function
using add_watch_full() or add_watch(). This function will
install a Source in the default glib main loop and will deliver messages
a short while after they have been posted. Note that the main loop should
be running for the asynchronous callbacks.
It is also possible to get messages from the bus without any thread
marshalling with the set_sync_handler() method. This makes it
possible to react to a message in the same thread that posted the
message on the bus. This should only be used if the application is able
to deal with messages from different threads.
Every Pipeline has one bus.
Note that a Pipeline will set its bus into flushing state when changing
from READY to NULL state.
Constructors#
Methods#
- class Bus
- add_signal_watch() None#
Adds a bus signal watch to the default main context with the default priority ( %G_PRIORITY_DEFAULT ). It is also possible to use a non-default main context set up using
push_thread_default()(before one had to create a bus watch source and attach it to the desired main context ‘manually’).After calling this statement, the bus will emit the “message” signal for each message posted on the bus.
This function may be called multiple times. To clean up, the caller is responsible for calling
remove_signal_watch()as many times as this function is called.
- add_signal_watch_full(priority: int) None#
Adds a bus signal watch to the default main context with the given
priority(e.g. %G_PRIORITY_DEFAULT). It is also possible to use a non-default main context set up usingpush_thread_default()(before one had to create a bus watch source and attach it to the desired main context ‘manually’).After calling this statement, the bus will emit the “message” signal for each message posted on the bus when the
MainLoopis running.This function may be called multiple times. To clean up, the caller is responsible for calling
remove_signal_watch()as many times as this function is called.There can only be a single bus watch per bus, you must remove any signal watch before you can set another type of watch.
- Parameters:
priority – The priority of the watch.
- add_watch(priority: int, func: Callable[[Bus, Message, Any], bool], user_data: Any = None) int#
Adds a bus watch to the default main context with the default priority ( %G_PRIORITY_DEFAULT ). It is also possible to use a non-default main context set up using
push_thread_default()(before one had to create a bus watch source and attach it to the desired main context ‘manually’).This function is used to receive asynchronous messages in the main loop. There can only be a single bus watch per bus, you must remove it before you can set a new one.
The bus watch will only work if a
MainLoopis being run.The watch can be removed using
remove_watch()or by returningFalsefromfunc. If the watch was added to the default main context it is also possible to remove the watch usingremove().The bus watch will take its own reference to the
bus, so it is safe to unrefbususingunref()after setting the bus watch.- Parameters:
priority
func – A function to call when a message is received.
user_data – user data passed to
func.
- async_signal_func(message: Message, data: Any = None) bool#
A helper
BusFuncthat can be used to convert all asynchronous messages into signals.- Parameters:
message – the
Messagereceiveddata – user data
- create_watch() Source | None#
Create watch for this bus. The
Sourcewill be dispatched whenever a message is on the bus. After the GSource is dispatched, the message is popped off the bus and unreffed.As with other watches, there can only be one watch on the bus, including any signal watch added with
gst_bus_add_signal_watch.
- disable_sync_message_emission() None#
Instructs GStreamer to stop emitting the “sync-message” signal for this bus. See
enable_sync_message_emission()for more information.In the event that multiple pieces of code have called
enable_sync_message_emission(), the sync-message emissions will only be stopped after all calls toenable_sync_message_emission()were “cancelled” by calling this function. In this way the semantics are exactly the same asref()that which calls enable should also call disable.
- enable_sync_message_emission() None#
Instructs GStreamer to emit the “sync-message” signal after running the bus’s sync handler. This function is here so that code can ensure that they can synchronously receive messages without having to affect what the bin’s sync handler is.
This function may be called multiple times. To clean up, the caller is responsible for calling
disable_sync_message_emission()as many times as this function is called.While this function looks similar to
add_signal_watch(), it is not exactly the same – this function enables synchronous emission of signals when messages arrive;add_signal_watch()adds an idle callback to pop messages off the bus asynchronously. The sync-message signal comes from the thread of whatever object posted the message; the “message” signal is marshalled to the main thread via theMainLoop.
- get_pollfd() PollFD#
Gets the file descriptor from the bus which can be used to get notified about messages being available with functions like
poll(), and allows integration into other event loops based on file descriptors. Whenever a message is available, the POLLIN / %G_IO_IN event is set.Warning: NEVER read or write anything to the returned fd but only use it for getting notifications via
poll()or similar and then use the normal GstBus API, e.g.pop().Added in version 1.14.
- peek() Message | None#
Peeks the message on the top of the bus’ queue. The message will remain on the bus’ message queue.
- poll(events: MessageType, timeout: int) Message | None#
Polls the bus for messages. Will block while waiting for messages to come. You can specify a maximum time to poll with the
timeoutparameter. Iftimeoutis negative, this function will block indefinitely.All messages not in
eventswill be popped off the bus and will be ignored. It is not possible to use message enums beyondGST_MESSAGE_EXTENDEDin theeventsmaskBecause poll is implemented using the “message” signal enabled by
add_signal_watch(), callingpoll()will cause the “message” signal to be emitted for every message that poll sees. Thus a “message” signal handler will see the same messages that this function sees – neither will steal messages from the other.This function will run a
MainLoopfrom the default main context when polling.You should never use this function, since it is pure evil. This is especially true for GUI applications based on Gtk+ or Qt, but also for any other non-trivial application that uses the GLib main loop. As this function runs a GLib main loop, any callback attached to the default GLib main context may be invoked. This could be timeouts, GUI events, I/O events etc.; even if
poll()is called with a 0 timeout. Any of these callbacks may do things you do not expect, e.g. destroy the main application window or some other resource; change other application state; display a dialog and run another main loop until the user clicks it away. In short, using this function may add a lot of complexity to your code through unexpected re-entrancy and unexpected changes to your application’s state.For 0 timeouts use
pop_filtered()instead of this function; for other short timeouts usetimed_pop_filtered(); everything else is better handled by setting up an asynchronous bus watch and doing things from there.- Parameters:
events – a mask of
MessageType, representing the set of message types to poll for (note special handling of extended message types below)timeout – the poll timeout, as a
ClockTime, orGST_CLOCK_TIME_NONEto poll indefinitely.
- pop_filtered(types: MessageType) Message | None#
Gets a message matching
typefrom the bus. Will discard all messages on the bus that do not matchtypeand that have been posted before the first message that does matchtype. If there is no message matchingtypeon the bus, all messages will be discarded. It is not possible to use message enums beyondGST_MESSAGE_EXTENDEDin theeventsmask.- Parameters:
types – message types to take into account
- post(message: Message) bool#
Posts a message on the given bus. Ownership of the message is taken by the bus.
- Parameters:
message – the
Messageto post
- remove_signal_watch() None#
Removes a signal watch previously added with
add_signal_watch().
- set_flushing(flushing: bool) None#
If
flushing, flushes out and unrefs any messages queued in the bus. Releases references to the message origin objects. Will flush future messages untilset_flushing()setsflushingtoFalse.- Parameters:
flushing – whether or not to flush the bus
- set_sync_handler(func: Callable[[Bus, Message, Any], BusSyncReply] | None = None, user_data: Any = None) None#
Sets the synchronous handler on the bus. The function will be called every time a new message is posted on the bus. Note that the function will be called in the same thread context as the posting object. This function is usually only called by the creator of the bus. Applications should handle messages asynchronously using the gst_bus watch and poll functions.
Before 1.16.3 it was not possible to replace an existing handler and clearing an existing handler with
Nonewas not thread-safe.- Parameters:
func – The handler function to install
user_data – User data that will be sent to the handler function.
- sync_signal_handler(message: Message, data: Any = None) BusSyncReply#
A helper
BusSyncHandlerthat can be used to convert all synchronous messages into signals.- Parameters:
message – the
Messagereceiveddata – user data
- timed_pop(timeout: int) Message | None#
Gets a message from the bus, waiting up to the specified timeout.
If
timeoutis 0, this function behaves likepop(). IftimeoutisGST_CLOCK_TIME_NONE, this function will block forever until a message was posted on the bus.- Parameters:
timeout – a timeout
- timed_pop_filtered(timeout: int, types: MessageType) Message | None#
Gets a message from the bus whose type matches the message type mask
types, waiting up to the specified timeout (and discarding any messages that do not match the mask provided).If
timeoutis 0, this function behaves likepop_filtered(). IftimeoutisGST_CLOCK_TIME_NONE, this function will block forever until a matching message was posted on the bus.- Parameters:
timeout – a timeout in nanoseconds, or
CLOCK_TIME_NONEto wait forevertypes – message types to take into account,
ANYfor any type