Bus

class Bus(**properties: Any)

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

class Bus

classmethod new() → Bus

Creates a new Bus instance.

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 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 when the MainLoop is 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[[...], bool], *user_data: Any) → 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 MainLoop is being run.

The watch can be removed using remove_watch() or by returning False from func. If the watch was added to the default main context it is also possible to remove the watch using remove().

The bus watch will take its own reference to the bus, so it is safe to unref bus using unref() 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: None) → bool

A helper BusFunc that can be used to convert all asynchronous messages into signals.

Parameters:

• message – the Message received

• data – user data

create_watch() → Source | None

Create watch for this bus. The Source will 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 to enable_sync_message_emission() were “cancelled” by calling this function. In this way the semantics are exactly the same as ref() 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 the MainLoop.

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.

have_pending() → bool

Checks if there are pending messages on the bus that should be handled.

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 timeout parameter. If timeout is negative, this function will block indefinitely.

All messages not in events will be popped off the bus and will be ignored. It is not possible to use message enums beyond GST_MESSAGE_EXTENDED in the events mask

Because poll is implemented using the “message” signal enabled by add_signal_watch(), calling poll() 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 MainLoop from 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 use timed_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, or GST_CLOCK_TIME_NONE to poll indefinitely.

pop() → Message | None

Gets a message from the bus.

pop_filtered(types: MessageType) → Message | None

Gets a message matching type from the bus. Will discard all messages on the bus that do not match type and that have been posted before the first message that does match type. If there is no message matching type on the bus, all messages will be discarded. It is not possible to use message enums beyond GST_MESSAGE_EXTENDED in the events mask.

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 Message to post

remove_signal_watch() → None

Removes a signal watch previously added with add_signal_watch().

remove_watch() → bool

Removes an installed bus watch from bus.

Added in version 1.6.

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 until set_flushing() sets flushing to False.

Parameters:

flushing – whether or not to flush the bus

set_sync_handler(func: Callable[[...], BusSyncReply] | None = None, *user_data: Any) → 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 None was 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: None) → BusSyncReply

A helper BusSyncHandler that can be used to convert all synchronous messages into signals.

Parameters:

• message – the Message received

• data – user data

timed_pop(timeout: int) → Message | None

Gets a message from the bus, waiting up to the specified timeout.

If timeout is 0, this function behaves like pop(). If timeout is GST_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 timeout is 0, this function behaves like pop_filtered(). If timeout is GST_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_NONE to wait forever

• types – message types to take into account, ANY for any type

Properties

class Bus

props.enable_async: bool

Enables async message delivery support for bus watches, pop() and similar API. Without this only the synchronous message handlers are called.

This property is used to create the child element buses in Bin.

Signals

class Bus.signals

message(message: Message) → None

A message has been posted on the bus. This signal is emitted from a Source added to the mainloop. this signal will only be emitted when there is a MainLoop running.

Parameters:

message – the message that has been posted asynchronously

sync_message(message: Message) → None

A message has been posted on the bus. This signal is emitted from the thread that posted the message so one has to be careful with locking.

This signal will not be emitted by default, you have to call enable_sync_message_emission() before.

Parameters:

message – the message that has been posted synchronously

Virtual Methods

class Bus

do_message(message: Message) → None

A message has been posted on the bus.

Parameters:

message – the message that has been posted asynchronously

do_sync_message(message: Message) → None

A message has been posted on the bus.

Parameters:

message – the message that has been posted synchronously

Fields

class Bus

object

The parent structure

priv