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4b986a4a94
Add a GST_MESSAGE_REQUEST_STATE that can be posted by element when they would like to have the application change the state of the pipeline. the primary use case is to pause the pipeline when an audio mixer is mixing a higher priority stream but it can also be used for other purposes. Add some docs and a unit test. Implement the REQUEST_STATE message in gst-launch. API: gst_message_new_request_state() API: gst_message_parse_request_state() API: GST_MESSAGE_REQUEST_STATE
135 lines
4.4 KiB
Text
135 lines
4.4 KiB
Text
Messages
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--------
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Messages are refcounted lightweight objects to signal the application
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of 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 without
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requiring an API change while allowing a wide range of different types
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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 part-gstbus.txt and part-gstpipeline.txt).
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Message types
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-------------
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GST_MESSAGE_EOS:
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Posted by sink elements. This message is posted to the application when
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all the sinks in a pipeline posted an EOS message. When performing a seek,
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the EOS state of the pipeline and sinks is undone.
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GST_MESSAGE_ERROR:
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An element in the pipeline got into an error state. The message carries
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a GError and a debug string describing the error. This usually means that
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part of the pipeline is not streaming anymore.
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GST_MESSAGE_WARNING:
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An element in the pipeline encountered a condition that made it produce a
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warning. This could be a recoverable decoding error or some other non fatal
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event. The pipeline continues streaming after a warning.
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GST_MESSAGE_INFO:
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An element produced an informational message.
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GST_MESSAGE_TAG:
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An element decoded metadata about the stream. The message carries a GstTagList
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with the tag information.
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GST_MESSAGE_BUFFERING:
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An element is buffering data and that could potentially take some time. This
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message is typically emited by elements that perform some sort of network
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buffering. While the pipeline is buffering it should remain in the PAUSED
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state. When the buffering is finished, it can resume PLAYING.
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GST_MESSAGE_STATE_CHANGED:
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An element changed state in the pipeline. The message carries the old, new
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and pending state of the element.
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GST_MESSAGE_STATE_DIRTY:
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An internal message used to instruct a pipeline hierarchy that a state
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recalculation must be performed because of an ASYNC state change completed.
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This message is not used anymore.
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GST_MESSAGE_STEP_DONE:
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An element stepping frames has finished. This is currently not used.
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GST_MESSAGE_CLOCK_PROVIDE:
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An element notifies it capability of providing a clock for the pipeline.
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GST_MESSAGE_CLOCK_LOST:
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The current clock as selected by the pipeline became unusable. The pipeline
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will select a new clock on the next PLAYING state change.
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GST_MESSAGE_NEW_CLOCK:
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A new clock was selected for the pipeline.
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GST_MESSAGE_STRUCTURE_CHANGE:
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The pipeline changed of structure, This means elements were added or removed or
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pads were linked or unlinked. This messages is not yet used.
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GST_MESSAGE_STREAM_STATUS:
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Posted by an element when it start/stop/pauses a streaming task. It
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contains information about the reason why the stream state changed along
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with the thread id. The application can use this information to detect
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failures in streaming threads. It can also be used to adjust streaming
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thread priorities by the application.
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GST_MESSAGE_APPLICATION:
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The application posted a message. This message must be used when the
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application posts a message on the bus.
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GST_MESSAGE_ELEMENT:
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Element-specific message, see the specific element's documentation
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GST_MESSAGE_SEGMENT_START:
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An element started playback of a new segment. This message is not forwarded
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the the application but is used internally to schedule SEGMENT_DONE messages.
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GST_MESSAGE_SEGMENT_DONE:
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An element or bin completed playback of a segment. This message is only posted
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on the bus if a SEGMENT seek is performed on a pipeline.
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GST_MESSAGE_DURATION:
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An element posts this message when it has detected or updated the stream duration.
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GST_MESSAGE_ASYNC_START:
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Posted by sinks when they start an asynchronous state change.
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GST_MESSAGE_ASYNC_DONE:
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Posted by sinks when they receive the first data buffer and complete the
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asynchronous state change.
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GST_MESSAGE_LATENCY:
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Posted by elements when the latency in a pipeline changed and a new global
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latency should be calculated by the pipeline or application.
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GST_MESSAGE_REQUEST_STATE:
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Posted by elements when they want to change the state of the pipeline they
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are in. A typical use case would be an audio sink that requests the pipeline
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to pause in order to play a higher priority stream.
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