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63 lines
2.3 KiB
Text
63 lines
2.3 KiB
Text
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-*- outline -*-
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* Creating Elements Without Factories
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** The purpose of factories
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On a typical GStreamer system, there are approximately 6.022*10^23
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plugins. GStreamer knows about all of them because of the registry. The
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goal is to avoid initializing each one of them, when maybe for your
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application you only need one or two.
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The problem becomes, how do you create an instance of the plugin? The
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normal way to instantiate a class is via g_object_new (TYPE, ARGS...).
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In the case that the plugin isn't loaded, you don't know its type, and
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can't even get it from the type name.
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Element factories exist to solve this problem by associating names (like
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"sinesrc" or "identity") with certain types that are provided by the
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plugin. Then when the user asks for "sinesrc", the appropriate plugin is
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loaded, its types are initialized, and then gst_element_factory_create
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creates the object for you.
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** Why not factories?
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To review, factories (1) allow plugins to remain unloaded if not
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necessary, and (2) make it easy to create elements.
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If you are writing an application that has custom elements (as is the
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case with most serious applications), you will probably have the plugin
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loaded up already, and you will have access to the type of the element.
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To muck about creating a plugin for the app, registering the element
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with the plugin, and then creating it with the element factory API
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actually takes more work than the normal way.
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** g_object_new
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So you want to avoid factories. To create objects with a simple
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g_object_new call is our strategy. However, to preserve the same
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semantics as gst_element_factory_create, we need to know what else is
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needed to initialize a GStreamer element.
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The other things that gst_element_factory_create does are as follows:
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*** Sets the ->elementfactory member on the element class
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Note that anything trying to get the factory won't work (e.g.
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gst_element_get_factory). Thankfully this is less of a problem after the
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0.7 plugin system changes.
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*** Initializes the name of the element
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To do this ourselves, we either call gst_object_set_name, or when we
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set the "name" property when creating the object.
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** Summary
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To create a GStreamer element when you know the type, you can just use
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g_object_new (get_type_of_my_element (),
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"name", the_name_you_want_possibly_null,
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... any other properties ...
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NULL);
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