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