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91 lines
3.2 KiB
Text
91 lines
3.2 KiB
Text
GstObject
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---------
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The base class for the entire GStreamer hierarchy is the GstObject.
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Parentage
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~~~~~~~~~
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A pointer is available to store the current parent of the object. This is one
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of the two fundamental requirements for a hierarchical system such as GStreamer
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(for the other, read up on GstBin). Three functions are provided:
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_set_parent(), _get_parent(), and _unparent(). The third is required because
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there is an explicit check in _set_parent(): an object must not already have a
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parent if you wish to set one. You must unparent the object first. This
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allows for new additions later.
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- GstObject's that can be parented:
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GstElement (inside a bin)
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GstPad (inside an element)
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Naming
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~~~~~~
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- names of objects cannot be changed when they are parented
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- names of objects should be unique across parent
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- set_name() can fail because of this
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- as can gst_element_add_pad()/gst_bin_add_element()
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- gst_object_set_name() only changes the object's name
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- objects also have a name_prefix that is used to prefix the object name
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during debugging and identification
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- there are object-specific set_name's() which also set the name_prefix
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on the object. This is useful for debugging purposes to give the object
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a more identifiable name. Typically a parent will call _set_name_prefix
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on children, taking a lock on them to do so.
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Locking
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~~~~~~~
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The GstObject contains the necessary primitives to lock the object in a
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thread-safe manner. This will be used to provide general thread-safety as
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needed. However, this lock is generic, i.e. it covers the whole object.
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The object LOCK is a very lowlevel lock that should only be held to access
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the object properties for short periods of code.
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All members of the GstObject structure marked as
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/*< public >*/ /* with LOCK */
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are protected by this lock. These members can only be accessed for reading
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or writing while the lock is held. All members should be copied or reffed
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if they are used after releasing the LOCK.
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Note that this does *not* mean that no other thread can modify the object at
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the same time that the lock is held. It only means that any two sections of
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code that obey the lock are guaranteed to not be running simultaneously. "The
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lock is voluntary and cooperative".
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This lock will ideally be used for parentage, flags and naming, which is
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reasonable, since they are the only possible things to protect in the
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GstObject.
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Locking order
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~~~~~~~~~~~~~
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In parent-child situations the lock of the parent must always be taken first
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before taking the lock of the child. It is NOT allowed to hold the child
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lock before taking the parent lock.
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This policy allows for parents to iterate their children and setting properties
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on them.
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Whenever a nested lock needs to be taken on objects not involved in a
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parent-child relation (eg. pads), an explictic locking order has to be defined.
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Path Generation
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~~~~~~~~~~~~~~~
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Due to the base nature of the GstObject, it becomes the only reasonable place
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to put this particular function (_get_path_string). It will generate a string
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describing the parent hierarchy of a given GstObject.
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Flags
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~~~~~
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Each object in the GStreamer object hierarchy can have flags associated with it,
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which are used to describe a state or a feature of the object.
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