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103 lines
4.4 KiB
XML
103 lines
4.4 KiB
XML
<chapter id="chapter-statemanage-states">
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<title>
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What are states?
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</title>
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<para>
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A state describes whether the element instance is initialized, whether it
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is ready to transfer data and whether it is currently handling data. There
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are four states defined in &GStreamer;: <classname>GST_STATE_NULL</classname>,
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<classname>GST_STATE_READY</classname>, <classname>GST_STATE_PAUSED</classname>
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and <classname>GST_STATE_PLAYING</classname>.
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</para>
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<para>
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<classname>GST_STATE_NULL</classname> (from now on referred to as
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<quote>NULL</quote>) is the default state of an element. In this state, it
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has not allocated any runtime resources, it has not loaded any runtime
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libraries and it can obviously not handle data.
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</para>
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<para>
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<classname>GST_STATE_READY</classname> (from now on referred to as
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<quote>READY</quote>) is the next state that an element can be in. In the
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READY state, an element has all default resources (runtime-libraries,
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runtime-memory) allocated. However, it has not yet allocated or defined
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anything that is stream-specific. When going from NULL to READY state
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(<classname>GST_STATE_NULL_TO_READY</classname>), an element should
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allocate any non-stream-specific resources and should load runtime-loadable
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libraries (if any). When going the other way around (from READY to NULL,
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<classname>GST_STATE_READY_TO_NULL</classname>), an element should unload
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these libraries and free all allocated resources. Examples of such
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resources are hardware devices. Note that files are generally streams,
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and these should thus be considered as stream-specific resources; therefore,
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they should <emphasis>not</emphasis> be allocated in this state.
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</para>
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<para>
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<classname>GST_STATE_PAUSED</classname> (from now on referred to as
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<quote>PAUSED</quote>) is a state in which an element is by all means able
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to handle data; the only 'but' here is that it doesn't actually handle
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any data. When going from the READY state into the PAUSED state
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(<classname>GST_STATE_READY_TO_PAUSED</classname>), the element will
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usually not do anything at all: all stream-specific info is generally
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handled in the <function>_link ()</function>, which is called during caps
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negotiation. Exceptions to this rule are, for example, files: these are
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considered stream-specific data (since one file is one stream), and should
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thus be opened in this state change. When going from the PAUSED back to
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READY (<classname>GST_STATE_PAUSED_TO_READY</classname>), all
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stream-specific data should be discarded.
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</para>
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<para>
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<classname>GST_STATE_PLAYING</classname> (from now on referred to as
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<quote>PLAYING</quote>) is the highest state that an element can be in. It
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is similar to PAUSED, except that now, data is actually passing over the
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pipeline. The transition from PAUSED to PLAYING
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(<classname>GST_STATE_PAUSED_TO_PLAYING</classname>) should be as small
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as possible and would ideally cause no delay at all. The same goes for the
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reverse transition (<classname>GST_STATE_PLAYING_TO_PAUSED</classname>).
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</para>
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<sect1 id="section-statemanage-filters">
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<title>
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Mangaging filter state
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</title>
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<para>
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An element can be notified of state changes through a virtual function
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pointer. Inside this function, the element can initialize any sort of
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specific data needed by the element, and it can optionally fail to
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go from one state to another.
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</para>
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<programlisting>
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static GstElementStateReturn
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gst_my_filter_change_state (GstElement *element);
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static void
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gst_my_filter_class_init (GstMyFilterClass *klass)
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{
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GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
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element_class->change_state = gst_my_filter_change_state;
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}
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static GstElementStateReturn
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gst_my_filter_change_state (GstElement *element)
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{
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GstMyFilter *filter = GST_MY_FILTER (element);
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switch (GST_STATE_TRANSITION (element)) {
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case GST_STATE_NULL_TO_READY:
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if (!gst_my_filter_allocate_memory (filter))
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return GST_STATE_FAILURE;
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break;
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case GST_STATE_READY_TO_NULL:
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gst_my_filter_free_memory (filter);
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break;
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default:
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break;
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}
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if (GST_ELEMENT_CLASS (parent_class)->change_state)
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return GST_ELEMENT_CLASS (parent_class)->change_state (element);
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return GST_STATE_SUCCESS;
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}
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</programlisting>
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</sect1>
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</chapter>
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