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Messages
--------
Messages are refcounted lightweight objects to signal the application
of pipeline events.
Messages are implemented as a subclass of GstMiniObject with a generic
GstStructure as the content. This allows for writing custom messages without
requiring an API change while allowing a wide range of different types
of messages.
Messages are posted by objects in the pipeline and are passed to the
application using the GstBus (See also part-gstbus.txt and part-gstpipeline.txt).
Message types
~~~~~~~~~~~~~
GST_MESSAGE_EOS:
Posted by sink elements. This message is posted when all the sinks in a
pipeline have posted an EOS message. When performing a flushing seek, the
EOS state of the pipeline (and hence, its sinks) is resetted.
GST_MESSAGE_ERROR:
An element in the pipeline got into an error state. The message carries
a GError and a debug string describing the error. This usually means that
part of the pipeline is not streaming anymore.
GST_MESSAGE_WARNING:
An element in the pipeline encountered a condition that made it produce a
warning. This could be a recoverable decoding error or some other non fatal
event. The pipeline continues streaming after a warning.
GST_MESSAGE_INFO:
An element produced an informational message.
GST_MESSAGE_TAG:
An element decoded metadata about the stream. The message carries a GstTagList
with the tag information.
GST_MESSAGE_BUFFERING:
An element is buffering data and that could potentially take some time. This
message is typically emited by elements that perform some sort of network
buffering. While the pipeline is buffering it should remain in the PAUSED
state. When the buffering is finished, it can resume PLAYING.
GST_MESSAGE_STATE_CHANGED:
An element changed state in the pipeline. The message carries the old, new
and pending state of the element.
GST_MESSAGE_STATE_DIRTY:
An internal message used to instruct a pipeline hierarchy that a state
recalculation must be performed because of an ASYNC state change completed.
This message is not used anymore.
GST_MESSAGE_STEP_DONE:
An element stepping frames has finished. This is currently not used.
GST_MESSAGE_CLOCK_PROVIDE:
An element notifies its capability of providing a clock for the pipeline.
GST_MESSAGE_CLOCK_LOST:
The current clock, as selected by the pipeline, became unusable. The pipeline
will select a new clock on the next PLAYING state change.
GST_MESSAGE_NEW_CLOCK:
A new clock was selected for the pipeline.
GST_MESSAGE_STRUCTURE_CHANGE:
The pipeline changed its structure, This means elements were added or removed or
pads were linked or unlinked. This messages is not yet used.
GST_MESSAGE_STREAM_STATUS:
Posted by an element when it starts/stops/pauses a streaming task. It
contains information about the reason why the stream state changed along
with the thread id. The application can use this information to detect
failures in streaming threads and/or to adjust streaming thread priorities.
GST_MESSAGE_APPLICATION:
The application posted a message. This message must be used when the
application posts a message on the bus.
GST_MESSAGE_ELEMENT:
Element-specific message. See the specific element's documentation
GST_MESSAGE_SEGMENT_START:
An element started playback of a new segment. This message is not forwarded
to applications but is used internally to schedule SEGMENT_DONE messages.
GST_MESSAGE_SEGMENT_DONE:
An element or bin completed playback of a segment. This message is only posted
on the bus if a SEGMENT seek is performed on a pipeline.
GST_MESSAGE_DURATION_CHANGED:
An element posts this message when it has detected or updated the stream duration.
GST_MESSAGE_ASYNC_START:
Posted by sinks when they start an asynchronous state change.
GST_MESSAGE_ASYNC_DONE:
Posted by sinks when they receive the first data buffer and complete the
asynchronous state change.
GST_MESSAGE_LATENCY:
Posted by elements when the latency in a pipeline changed and a new global
latency should be calculated by the pipeline or application.
GST_MESSAGE_REQUEST_STATE:
Posted by elements when they want to change the state of the pipeline they
are in. A typical use case would be an audio sink that requests the pipeline
to pause in order to play a higher priority stream.
GST_MESSAGE_STEP_START:
A Stepping operation has started.
GST_MESSAGE_QOS:
A buffer was dropped or an element changed its processing strategy for
Quality of Service reasons.
GST_MESSAGE_PROGRESS:
A progress message was posted. Progress messages inform the application about
the state of asynchronous operations.
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