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Original commit message from CVS: * check/gst/gstobject.c: * docs/design/part-gstelement.txt: * docs/design/part-gstobject.txt: * gst/gstobject.c: * gst/gstobject.h: add _get/_set_name_prefix() for debugging. Update docs. Finish unique name unit test
91 lines
4 KiB
Text
91 lines
4 KiB
Text
gstelement
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name
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pads
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state
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loopfunction
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threadstate
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manager
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ghostpads
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GstElement
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==========
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The Element is the most important object in the entire GStreamer system, as it
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defines the structure of the pipeline. Elements include sources, filters,
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sinks, and containers (Bins). They may be an intrinsic part of the core
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GStreamer library, or may be loaded from a plugin. In some cases they're even
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fabricated from completely different systems (see the LADSPA plugin). They
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are generally created from a GstElementFactory, which will be covered in
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another chapter, but for the intrinsic types they can be created with specific
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functions.
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Elements contains GstPads (also covered in another chapter), which are
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subsequently used to connect the Elements together to form a pipeline capable
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of passing and processing data. They have a parent, which must be another
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Element. This allows deeply nested pipelines, and the possibility of
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"black-box" meta-elements.
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Name
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----
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All elements are named, and while they should ideally be unique in any given
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pipeline, they do not have to be. The only guaranteed unique name for an
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element is its complete path in the object hierarchy. In other words, an
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element's name is unique inside its parent. (This follows from GstObject's
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name explanation)
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This uniqueness is guaranteed through all functions where either parentage
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or name of an element is changed.
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Functions are provided
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to set and get the name of the element. _set_name() creates a local copy of
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the string passed. _get_name() returns the actual element's pointer.
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Therefore, the argument to _set_name() is the responsibility of the caller to
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free if necessary, but the return from _get_name() is definitely not to be
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messed with by the caller. Accordingly, _get_name() returns an const gchar *.
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Providing a new name to an element causes it to free its internal copy of the name and make a copy of the new name.
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This means that you must consider the pointer returned by _get_name() to be short-lived. If you must make use of the
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name beyond the immediate scope, it is suggested that you make yourself a copy of it. If you know for a fact neither
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the pointer nor its contents will change, you may retain the original pointer. If you get odd results when using the
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returned string, that's the first thing to check.
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Pads
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----
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GstPads are the property of a given GstElement. They provide the connection capability, with allowing arbitrary
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structure in the graph. For any Element but a source or sink, there will be at least 2 Pads owned by the Element.
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These pads are stored in a single GList within the Element. Several counters are kept in order to allow quicker
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determination of the type and properties of a given Element.
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Pads may be added to an element with _add_pad. Retrieval is via _get_pad(), which operates on the name of the Pad (the
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unique key). This means that all Pads owned by a given Element must have unique names (FIXME we don't verify this at
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_add time). A pointer to the GList of pads may be obtained with _get_pad_list. As with the element's name,
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precaution must be taken with all these pointers, as they are the same pointer that the Element uses internally. One
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must be especially careful not to manipulate the list of pads.
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gst_element_add_pad(element,pads):
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Sets the element as the parent of the pad, then adds the pad to the element's list of pads, keeping the counts
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of total, src, and sink pads up to date. Emits the "new_pad" signal with the pad as argument. Fails if either
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the element or pad are either NULL or not what they claim to be. Should fail if the pad already has a parent.
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Should fail if the pad is already owned by the element. Should fail if there's already a pad by that name in
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the the list of pads.
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pad = gst_element_get_pad(element,"padname"):
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Searches through the list of pads
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Ghost Pads
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----------
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State
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-----
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Elements use state to determine what the are capable of doing at any given moment. The states are defined as follows:
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NULL No state is held for the element
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READY Devices are open
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PLAYING
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PAUSED
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