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42 lines
2 KiB
Markdown
42 lines
2 KiB
Markdown
# GstBus
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The `GstBus` is an object responsible for delivering `GstMessages` in a
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first-in first-out way from the streaming threads to the application.
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Since the application typically only wants to deal with delivery of
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these messages from one thread, the `GstBus` will marshall the messages
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between different threads. This is important since the actual streaming
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of media is done in other threads (streaming threads) than the
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application. It is also important to not block the streaming threads
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while the application deals with the message.
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The `GstBus` provides support for `GSource` based notifications. This makes
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it possible to handle the delivery in the glib mainloop. Different
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`GSources` can be added to the same bin provided they listen to different
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message types.
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A message is posted on the bus with the `gst_bus_post()` method. With
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the `gst_bus_peek()` and `_pop()` methods one can look at or retrieve a
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previously posted message.
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The bus can be polled with the `gst_bus_poll()` method. This method
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blocks up to the specified timeout value until one of the specified
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message types are posted on the bus. The application can then `_pop()`
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these messages from the bus to handle them.
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It is also possible to get messages from the bus without any thread
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marshalling with the `gst_bus_set_sync_handler()` method. This makes
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it possible to react to a message in the same thread that posted it
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on the bus. This should only be used if the application is able
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to deal with messages from different threads.
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If no messages are popped from the bus with either a `GSource` or
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`gst_bus_pop()`, they remain on the bus.
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When a pipeline or bin goes from `READY` into `NULL` state, it will set its
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bus to flushing, ie. the bus will drop all existing and new messages on
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the bus. This is necessary because bus messages hold references to the
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bin/pipeline or its elements, so there are circular references that need
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to be broken if one ever wants to be able to destroy a bin or pipeline
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properly.
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