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188 lines
9.3 KiB
XML
188 lines
9.3 KiB
XML
<chapter id="chapter-porting">
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<title>Porting 0.8 plug-ins to 0.10</title>
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<para>
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This section of the appendix will discuss shortly what changes to
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plugins will be needed to quickly and conveniently port most
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applications from &GStreamer;-0.8 to &GStreamer;-0.10, with references
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to the relevant sections in this Plugin Writer's Guide where needed.
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With this list, it should be possible to port most plugins to
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&GStreamer;-0.10 in less than a day. Exceptions are elements that will
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require a base class in 0.10 (sources, sinks), in which case it may take
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a lot longer, depending on the coder's skills (however, when using the
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<classname>GstBaseSink</classname> and <classname>GstBaseSrc</classname>
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base-classes, it shouldn't be all too bad), and elements requiring
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the deprecated bytestream interface, which should take 1-2 days with
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random access. The scheduling parts of muxers will also need a rewrite,
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which will take about the same amount of time.
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</para>
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<sect1 id="section-porting-objects">
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<title>List of changes</title>
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<itemizedlist>
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<listitem>
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<para>
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Discont events have been replaced by newsegment events. In 0.10, it is
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essential that you send a newsegment event downstream before you send
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your first buffer (in 0.8 the scheduler would invent discont events if
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you forgot them, in 0.10 this is no longer the case).
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</para>
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</listitem>
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<listitem>
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<para>
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In 0.10, buffers have caps attached to them. Elements should allocate
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new buffers with <function>gst_pad_alloc_buffer ()</function>. See
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<xref linkend="chapter-negotiation"/> for more details.
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</para>
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</listitem>
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<listitem>
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<para>
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Most functions returning an object or an object property have
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been changed to return its own reference rather than a constant
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reference of the one owned by the object itself. The reason for
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this change is primarily thread-safety. This means effectively
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that return values of functions such as
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<function>gst_element_get_pad ()</function>,
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<function>gst_pad_get_name ()</function>,
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<function>gst_pad_get_parent ()</function>,
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<function>gst_object_get_parent ()</function>,
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and many more like these
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have to be free'ed or unreferenced after use. Check the API
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references of each function to know for sure whether return
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values should be free'ed or not.
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</para>
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</listitem>
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<listitem>
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<para>
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In 0.8, scheduling could happen in any way. Source elements could
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be <function>_get ()</function>-based or <function>_loop
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()</function>-based, and any other element could be <function>_chain
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()</function>-based or <function>_loop ()</function>-based, with
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no limitations. Scheduling in 0.10 is simpler for the scheduler,
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and the element is expected to do some more work. Pads get
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assigned a scheduling mode, based on which they can either
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operate in random access-mode, in pipeline driving mode or in
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push-mode. all this is documented in detail in <xref
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linkend="chapter-scheduling"/>. As a result of this, the bytestream
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object no longer exists. Elements requiring byte-level access should
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now use random access on their sinkpads.
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</para>
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</listitem>
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<listitem>
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<para>
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Negotiation is asynchronous. This means that downstream negotiation
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is done as data comes in and upstream negotiation is done whenever
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renegotiation is required. All details are described in
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<xref linkend="chapter-negotiation"/>.
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</para>
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</listitem>
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<listitem>
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<para>
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For as far as possible, elements should try to use existing base
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classes in 0.10. Sink and source elements, for example, could derive
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from <classname>GstBaseSrc</classname> and
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<classname>GstBaseSink</classname>. Audio sinks or sources could even
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derive from audio-specific base classes. All existing base classes
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have been discussed in <xref linkend="chapter-other-base"/> and the
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next few chapters.
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</para>
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</listitem>
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<listitem>
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<para>
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In 0.10, event handling and buffers are separated once again. This
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means that in order to receive events, one no longer has to set the
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<classname>GST_FLAG_EVENT_AWARE</classname> flag, but can simply
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set an event handling function on the element's sinkpad(s), using
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the function <function>gst_pad_set_event_function ()</function>. The
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<function>_chain ()</function>-function will only receive buffers.
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</para>
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</listitem>
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<listitem>
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<para>
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Although core will wrap most threading-related locking for you (e.g.
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it takes the stream lock before calling your data handling
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functions), you are still responsible for locking around certain
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functions, e.g. object properties. Be sure to lock properly here,
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since applications will change those properties in a different thread
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than the thread which does the actual data passing! You can use the
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<function>GST_OBJECT_LOCK ()</function> and <function>GST_OBJECT_UNLOCK
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()</function> helpers in most cases, fortunately, which grabs the
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default property lock of the element.
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</para>
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</listitem>
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<listitem>
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<para>
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<classname>GstValueFixedList</classname> and all
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<function>*_fixed_list_* ()</function> functions were renamed to
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<classname>GstValueArray</classname> and <function>*_array_*
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()</function>.
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</para>
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</listitem>
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<listitem>
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<para>
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The semantics of <symbol>GST_STATE_PAUSED</symbol> and
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<symbol>GST_STATE_PLAYING</symbol> have changed for elements that
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are not sink elements. Non-sink elements need to be able to accept
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and process data already in the <symbol>GST_STATE_PAUSED</symbol>
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state now (i.e. when prerolling the pipeline). More details can be
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found in <xref linkend="chapter-statemanage-states"/>.
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</para>
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</listitem>
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<listitem>
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<para>
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If your plugin's state change function hasn't been superseded by
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virtual start() and stop() methods of one of the new base classes,
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then your plugin's state change functions may need to be changed in
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order to safely handle concurrent access by multiple threads. Your
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typical state change function will now first handle upwards state
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changes, then chain up to the state change function of the parent
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class (usually GstElementClass in these cases), and only then handle
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downwards state changes. See the vorbis decoder plugin in
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gst-plugins-base for an example.
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</para>
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<para>
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The reason for this is that in the case of downwards state changes
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you don't want to destroy allocated resources while your plugin's
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chain function (for example) is still accessing those resources in
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another thread. Whether your chain function might be running or not
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depends on the state of your plugin's pads, and the state of those
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pads is closely linked to the state of the element. Pad states are
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handled in the GstElement class's state change function, including
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proper locking, that's why it is essential to chain up before
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destroying allocated resources.
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</para>
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<para>
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As already mentioned above, you should really rewrite your plugin
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to derive from one of the new base classes though, so you don't have
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to worry about these things, as the base class will handle it for you.
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There are no base classes for decoders and encoders yet, so the above
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paragraphs about state changes definitively apply if your plugin is a
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decoder or an encoder.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>gst_pad_set_link_function ()</function>, which used to set
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a function that would be called when a format was negotiated between
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two <classname>GstPad</classname>s, now sets a function that is
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called when two elements are linked together in an application. For
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all practical purposes, you most likely want to use the function
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<function>gst_pad_set_setcaps_function ()</function>, nowadays, which
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sets a function that is called when the format streaming over a pad
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changes (so similar to <function>_set_link_function ()</function> in
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&GStreamer;-0.8).
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</para>
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<para>
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If the element is derived from a <classname>GstBase</classname> class,
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then override the <function>set_caps ()</function>.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>gst_pad_use_explicit_caps ()</function> has been replaced by
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<function>gst_pad_use_fixed_caps ()</function>. You can then set the
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fixed caps to use on a pad with <function>gst_pad_set_caps ()</function>.
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</para>
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</listitem>
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</itemizedlist>
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</sect1>
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</chapter>
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