* Boston, MA 02111-1307, USA.
*/
+/* First, include the header file for the plugin, to bring in the
+ * object definition and other useful things.
+ */
#include "example.h"
-/* elementfactory information */
+/* The ElementDetails structure gives a human-readable description
+ * of the plugin, as well as author and version data.
+ */
static GstElementDetails example_details = {
"An example plugin",
"Example",
"(C) 2000",
};
-/* Example signals and args */
+/* These are the signals that this element can fire. They are zero-
+ * based because the numbers themselves are private to the object.
+ * LAST_SIGNAL is used for initialization of the signal array.
+ */
enum {
+ ASDF,
/* FILL ME */
LAST_SIGNAL
};
+/* Arguments are identified the same way, but cannot be zero, so you
+ * must leave the ARG_0 entry in as a placeholder.
+ */
enum {
ARG_0,
- ARG_ACTIVE
+ ARG_ACTIVE,
+ /* FILL ME */
};
+/* The PadFactory structures describe what pads the element has or
+ * can have. They can be quite complex, but for this example plugin
+ * they are rather simple.
+ */
static GstPadFactory sink_factory = {
- "sink", /* the name of the pads */
- GST_PAD_FACTORY_SINK, /* type of the pad */
- GST_PAD_FACTORY_ALWAYS, /* ALWAYS/SOMETIMES */
- GST_PAD_FACTORY_CAPS(
- "example_sink", /* the name of the caps */
- "unknown/unknown", /* the mime type of the caps */
- "something", GST_PROPS_INT (1), /* a property */
- "foo", GST_PROPS_BOOLEAN (TRUE) /* another property */
+ "sink", /* The name of the pad */
+ GST_PAD_FACTORY_SINK, /* Direction of the pad */
+ GST_PAD_FACTORY_ALWAYS, /* The pad exists for every instance */
+ GST_PAD_FACTORY_CAPS( /* This factory has specific capabilities */
+ "example_sink", /* The name of the caps */
+ "unknown/unknown", /* The overall MIME/type */
+ "foo", GST_PROPS_INT (1), /* An integer property */
+ "bar", GST_PROPS_BOOLEAN (TRUE) /* A boolean */
+ "baz", GST_PROPS_LIST ( /* A list of values for */
+ GST_PROPS_INT (1),
+ GST_PROPS_INT (3),
+ ),
),
- NULL
+ NULL /* All factories must be NULL-terminated */
};
+/* This factory is much simpler, and defines the source pad. */
static GstPadFactory src_factory = {
"src",
GST_PAD_FACTORY_SRC,
};
-static void gst_example_class_init (GstExampleClass *klass);
-static void gst_example_init (GstExample *example);
+/* A number of functon prototypes are given so we can refer to them later. */
+static void gst_example_class_init (GstExampleClass *klass);
+static void gst_example_init (GstExample *example);
-static void gst_example_chain (GstPad *pad, GstBuffer *buf);
+static void gst_example_chain (GstPad *pad, GstBuffer *buf);
-static void gst_example_set_arg (GtkObject *object,GtkArg *arg,guint id);
-static void gst_example_get_arg (GtkObject *object,GtkArg *arg,guint id);
+static void gst_example_set_arg (GtkObject *object,GtkArg *arg,guint id);
+static void gst_example_get_arg (GtkObject *object,GtkArg *arg,guint id);
-GstPadTemplate *src_template, *sink_template;
+/* These hold the constructed pad templates, which are created during
+ * plugin load, and used during element instantiation.
+ */
+static GstPadTemplate *src_template, *sink_template;
+/* The parent class pointer needs to be kept around for some object
+ * operations.
+ */
static GstElementClass *parent_class = NULL;
-//static guint gst_example_signals[LAST_SIGNAL] = { 0 };
+/* This array holds the ids of the signals registered for this object.
+ * The array indexes are based on the enum up above.
+ */
+static guint gst_example_signals[LAST_SIGNAL] = { 0 };
+
+/* This function is used to register and subsequently return the type
+ * identifier for this object class. On first invocation, it will
+ * register the type, providing the name of the class, struct sizes,
+ * and pointers to the various functions that define the class.
+ */
GtkType
gst_example_get_type(void)
{
sizeof(GstExampleClass),
(GtkClassInitFunc)gst_example_class_init,
(GtkObjectInitFunc)gst_example_init,
- (GtkArgSetFunc)gst_example_set_arg,
- (GtkArgGetFunc)gst_example_get_arg,
+ (GtkArgSetFunc)NULL, /* These last three are depracated */
+ (GtkArgGetFunc)NULL,
(GtkClassInitFunc)NULL,
};
example_type = gtk_type_unique(GST_TYPE_ELEMENT,&example_info);
return example_type;
}
+/* In order to create an instance of an object, the class must be
+ * initialized by this function. GtkObject will take care of running
+ * it, based on the pointer to the function provided above.
+ */
static void
gst_example_class_init (GstExampleClass *klass)
{
+ /* Class pointers are needed to supply pointers to the private
+ * implementations of parent class methods.
+ */
GtkObjectClass *gtkobject_class;
GstElementClass *gstelement_class;
+ /* Since the example class contains the parent classes, you can simply
+ * cast the pointer to get access to the parent classes.
+ */
gtkobject_class = (GtkObjectClass*)klass;
gstelement_class = (GstElementClass*)klass;
+ /* The parent class is needed for class method overrides. */
parent_class = gtk_type_class(GST_TYPE_ELEMENT);
+ /* Here we add an argument to the object. This argument is an integer,
+ * and can be both read and written.
+ */
gtk_object_add_arg_type("GstExample::active", GTK_TYPE_INT,
GTK_ARG_READWRITE, ARG_ACTIVE);
+ /* The last thing is to provide the functions that implement get and set
+ * of arguments.
+ */
gtkobject_class->set_arg = gst_example_set_arg;
gtkobject_class->get_arg = gst_example_get_arg;
}
+/* This function is responsible for initializing a specific instance of
+ * the plugin.
+ */
static void
gst_example_init(GstExample *example)
{
+ /* First we create the sink pad, which is the input to the element.
+ * We will use the sink_template constructed in the plugin_init function
+ * (below) to quickly generate the pad we need.
+ */
example->sinkpad = gst_pad_new_from_template (sink_template, "sink");
- gst_element_add_pad(GST_ELEMENT(example),example->sinkpad);
+ /* Setting the chain function allows us to supply the function that will
+ * actually be performing the work. Without this, the element would do
+ * nothing, with undefined results (assertion failures and such).
+ */
gst_pad_set_chain_function(example->sinkpad,gst_example_chain);
+ /* We then must add this pad to the element's list of pads. The base
+ * element class manages the list of pads, and provides accessors to it.
+ */
+ gst_element_add_pad(GST_ELEMENT(example),example->sinkpad);
+ /* The src pad, the output of the element, is created and registered
+ * in the same way, with the exception of the chain function. Source
+ * pads don't have chain functions, because they can't accept buffers,
+ * they only produce them.
+ */
example->srcpad = gst_pad_new_from_template (src_template, "src");
gst_element_add_pad(GST_ELEMENT(example),example->srcpad);
+ /* Initialization of element's private variables. */
example->active = FALSE;
}
+/* The chain function is the heart of the element. It's where all the
+ * work is done. It is passed a pointer to the pad in question, as well
+ * as the buffer provided by the peer element.
+ */
static void
gst_example_chain (GstPad *pad, GstBuffer *buf)
{
GstExample *example;
+ GstBuffer *outbuf;
+ /* Some of these checks are of dubious value, since if there were not
+ * already true, the chain function would never be called.
+ */
g_return_if_fail(pad != NULL);
g_return_if_fail(GST_IS_PAD(pad));
g_return_if_fail(buf != NULL);
- //g_return_if_fail(GST_IS_BUFFER(buf));
+ g_return_if_fail(GST_IS_BUFFER(buf));
+ /* We need to get a pointer to the element this pad belogs to. */
example = GST_EXAMPLE(gst_pad_get_parent (pad));
+ /* A few more sanity checks to make sure that the element that owns
+ * this pad is the right kind of element, in case something got confused.
+ */
g_return_if_fail(example != NULL);
g_return_if_fail(GST_IS_EXAMPLE(example));
+ /* If we are supposed to be doing something, here's where it happens. */
if (example->active) {
- /* DO STUFF */
- }
+ /* In this example we're going to copy the buffer to another one,
+ * so we need to allocate a new buffer first.
+ outbuf = gst_buffer_new();
+
+ /* We need to copy the size and offset of the buffer at a minimum. */
+ GST_BUFFER_SIZE (outbuf) = GST_BUFFER_SIZE (buf);
+ GST_BUFFER_OFFSET (outbuf) = GST_BUFFER_OFFSET (buf);
+
+ /* Then allocate the memory for the new buffer */
+ GST_BUFFER_DATA (outbuf) = (guchar *)g_malloc (GST_BUFFER_SIZE (outbuf));
+
+ /* Then copy the data in the incoming buffer into the new buffer. */
+ memcpy (GST_BUFFER_DATA (outbuf), GST_BUFFER_DATA (buf), GST_BUFFER_SIZE (outbuf));
- gst_pad_push(example->srcpad,buf);
+ /* When we're done with the buffer, we push it on to the next element
+ * in the pipeline, through the element's source pad, which is stored
+ * in the element's structure.
+ */
+ gst_pad_push(example->srcpad,outbuf);
+
+ /* If we're not doing something, just send the original incoming buffer. */
+ } else {
+ gst_pad_push(example->srcpad,buf);
+ }
}
+/* Arguments are part of the Gtk+ object system, and these functions
+ * enable the element to respond to various arguments.
+ */
static void
gst_example_set_arg (GtkObject *object,GtkArg *arg,guint id)
{
GstExample *example;
- /* it's not null if we got it, but it might not be ours */
+ /* It's not null if we got it, but it might not be ours */
g_return_if_fail(GST_IS_EXAMPLE(object));
+
+ /* Get a pointer of the right type. */
example = GST_EXAMPLE(object);
- switch(id) {
+ /* Check the argument id to see which argument we're setting. */
+ switch (id) {
case ARG_ACTIVE:
+ /* Here we simply copy the value of the argument to our private
+ * storage. More complex operations can be done, but beware that
+ * they may occur at any time, possibly even while your chain function
+ * is running, if you are using threads.
+ */
example->active = GTK_VALUE_INT(*arg);
g_print("example: set active to %d\n",example->active);
break;
}
}
+/* The set function is simply the inverse of the get fuction. */
static void
gst_example_get_arg (GtkObject *object,GtkArg *arg,guint id)
{
GstExample *example;
- /* it's not null if we got it, but it might not be ours */
+ /* It's not null if we got it, but it might not be ours */
g_return_if_fail(GST_IS_EXAMPLE(object));
example = GST_EXAMPLE(object);
}
}
+/* This is the entry into the plugin itself. When the plugin loads,
+ * this function is called to register everything that the plugin provides.
+ */
GstPlugin*
plugin_init (GModule *module)
{
GstPlugin *plugin;
GstElementFactory *factory;
+ /* First we try to create a new Plugin structure. */
plugin = gst_plugin_new("example");
+ /* If we get a NULL back, chances are we're already loaded. */
g_return_val_if_fail(plugin != NULL, NULL);
+ /* We need to create an ElementFactory for each element we provide.
+ * This consists of the name of the element, the GtkType identifier,
+ * and a pointer to the details structure at the top of the file.
+ */
factory = gst_elementfactory_new("example", GST_TYPE_EXAMPLE, &example_details);
g_return_val_if_fail(factory != NULL, NULL);
+ /* The pad templates can be easily generated from the factories above,
+ * and then added to the list of padtemplates for the elementfactory.
+ * Note that the generated padtemplates are stored in static global
+ * variables, for the gst_example_init function to use later on.
+ */
sink_template = gst_padtemplate_new (&sink_factory);
gst_elementfactory_add_padtemplate (factory, sink_template);
src_template = gst_padtemplate_new (&src_factory);
gst_elementfactory_add_padtemplate (factory, src_template);
+ /* The very last thing is to register the elementfactory with the plugin. */
gst_plugin_add_factory (plugin, factory);
+ /* Now we can return the pointer to the newly created Plugin object. */
return plugin;
+
+ /* At this point, the GStreamer core registers the plugin, its
+ * elementfactories, padtemplates, etc., for use in you application.
+ */
}
#endif /* __cplusplus */
-/* Definition of structure storing data for this element. */
+/* This is the definition of the element's object structure. */
typedef struct _GstExample GstExample;
+/* The structure itself is derived from GstElement, as can be seen by the
+ * fact that there's a complete instance of the GstElement structure at
+ * the beginning of the object. This allows the element to be cast to
+ * an Element or even an Object.
+ */
struct _GstExample {
GstElement element;
+ /* We need to keep track of our pads, so we do so here. */
GstPad *sinkpad,*srcpad;
- gint8 active;
+ /* We'll use this to decide whether to do anything to the data we get. */
+ gboolean active;
};
-/* Standard definition defining a class for this element. */
+/* The other half of the object is its class. The class also derives from
+ * the same parent, though it must be the class structure this time.
+ * Function pointers for polymophic methods and signals are placed in this
+ * structure. */
typedef struct _GstExampleClass GstExampleClass;
+
struct _GstExampleClass {
GstElementClass parent_class;
};
-/* Standard macros for defining types for this element. */
+/* Five standard preprocessing macros are used in the Gtk+ object system.
+ * The first uses the object's _get_type function to return the GtkType
+ * of the object.
+ */
#define GST_TYPE_EXAMPLE \
(gst_example_get_type())
+/* The second is a checking cast to the correct type. If the object passed
+ * is not the right type, a warning will be generated on stderr.
+ */
#define GST_EXAMPLE(obj) \
(GTK_CHECK_CAST((obj),GST_TYPE_EXAMPLE,GstExample))
+/* The third is a checking cast of the class instead of the object. */
#define GST_EXAMPLE_CLASS(klass) \
(GTK_CHECK_CLASS_CAST((klass),GST_TYPE_EXAMPLE,GstExample))
+/* The last two simply check to see if the passed pointer is an object or
+ * class of the correct type. */
#define GST_IS_EXAMPLE(obj) \
(GTK_CHECK_TYPE((obj),GST_TYPE_EXAMPLE))
#define GST_IS_EXAMPLE_CLASS(obj) \
(GTK_CHECK_CLASS_TYPE((klass),GST_TYPE_EXAMPLE))
-/* Standard function returning type information. */
+/* This is the only prototype needed, because it is used in the above
+ * GST_TYPE_EXAMPLE macro.
+ */
GtkType gst_example_get_type(void);
* Boston, MA 02111-1307, USA.
*/
+/* First, include the header file for the plugin, to bring in the
+ * object definition and other useful things.
+ */
#include "example.h"
-/* elementfactory information */
+/* The ElementDetails structure gives a human-readable description
+ * of the plugin, as well as author and version data.
+ */
static GstElementDetails example_details = {
"An example plugin",
"Example",
"(C) 2000",
};
-/* Example signals and args */
+/* These are the signals that this element can fire. They are zero-
+ * based because the numbers themselves are private to the object.
+ * LAST_SIGNAL is used for initialization of the signal array.
+ */
enum {
+ ASDF,
/* FILL ME */
LAST_SIGNAL
};
+/* Arguments are identified the same way, but cannot be zero, so you
+ * must leave the ARG_0 entry in as a placeholder.
+ */
enum {
ARG_0,
- ARG_ACTIVE
+ ARG_ACTIVE,
+ /* FILL ME */
};
+/* The PadFactory structures describe what pads the element has or
+ * can have. They can be quite complex, but for this example plugin
+ * they are rather simple.
+ */
static GstPadFactory sink_factory = {
- "sink", /* the name of the pads */
- GST_PAD_FACTORY_SINK, /* type of the pad */
- GST_PAD_FACTORY_ALWAYS, /* ALWAYS/SOMETIMES */
- GST_PAD_FACTORY_CAPS(
- "example_sink", /* the name of the caps */
- "unknown/unknown", /* the mime type of the caps */
- "something", GST_PROPS_INT (1), /* a property */
- "foo", GST_PROPS_BOOLEAN (TRUE) /* another property */
+ "sink", /* The name of the pad */
+ GST_PAD_FACTORY_SINK, /* Direction of the pad */
+ GST_PAD_FACTORY_ALWAYS, /* The pad exists for every instance */
+ GST_PAD_FACTORY_CAPS( /* This factory has specific capabilities */
+ "example_sink", /* The name of the caps */
+ "unknown/unknown", /* The overall MIME/type */
+ "foo", GST_PROPS_INT (1), /* An integer property */
+ "bar", GST_PROPS_BOOLEAN (TRUE) /* A boolean */
+ "baz", GST_PROPS_LIST ( /* A list of values for */
+ GST_PROPS_INT (1),
+ GST_PROPS_INT (3),
+ ),
),
- NULL
+ NULL /* All factories must be NULL-terminated */
};
+/* This factory is much simpler, and defines the source pad. */
static GstPadFactory src_factory = {
"src",
GST_PAD_FACTORY_SRC,
};
-static void gst_example_class_init (GstExampleClass *klass);
-static void gst_example_init (GstExample *example);
+/* A number of functon prototypes are given so we can refer to them later. */
+static void gst_example_class_init (GstExampleClass *klass);
+static void gst_example_init (GstExample *example);
-static void gst_example_chain (GstPad *pad, GstBuffer *buf);
+static void gst_example_chain (GstPad *pad, GstBuffer *buf);
-static void gst_example_set_arg (GtkObject *object,GtkArg *arg,guint id);
-static void gst_example_get_arg (GtkObject *object,GtkArg *arg,guint id);
+static void gst_example_set_arg (GtkObject *object,GtkArg *arg,guint id);
+static void gst_example_get_arg (GtkObject *object,GtkArg *arg,guint id);
-GstPadTemplate *src_template, *sink_template;
+/* These hold the constructed pad templates, which are created during
+ * plugin load, and used during element instantiation.
+ */
+static GstPadTemplate *src_template, *sink_template;
+/* The parent class pointer needs to be kept around for some object
+ * operations.
+ */
static GstElementClass *parent_class = NULL;
-//static guint gst_example_signals[LAST_SIGNAL] = { 0 };
+/* This array holds the ids of the signals registered for this object.
+ * The array indexes are based on the enum up above.
+ */
+static guint gst_example_signals[LAST_SIGNAL] = { 0 };
+
+/* This function is used to register and subsequently return the type
+ * identifier for this object class. On first invocation, it will
+ * register the type, providing the name of the class, struct sizes,
+ * and pointers to the various functions that define the class.
+ */
GtkType
gst_example_get_type(void)
{
sizeof(GstExampleClass),
(GtkClassInitFunc)gst_example_class_init,
(GtkObjectInitFunc)gst_example_init,
- (GtkArgSetFunc)gst_example_set_arg,
- (GtkArgGetFunc)gst_example_get_arg,
+ (GtkArgSetFunc)NULL, /* These last three are depracated */
+ (GtkArgGetFunc)NULL,
(GtkClassInitFunc)NULL,
};
example_type = gtk_type_unique(GST_TYPE_ELEMENT,&example_info);
return example_type;
}
+/* In order to create an instance of an object, the class must be
+ * initialized by this function. GtkObject will take care of running
+ * it, based on the pointer to the function provided above.
+ */
static void
gst_example_class_init (GstExampleClass *klass)
{
+ /* Class pointers are needed to supply pointers to the private
+ * implementations of parent class methods.
+ */
GtkObjectClass *gtkobject_class;
GstElementClass *gstelement_class;
+ /* Since the example class contains the parent classes, you can simply
+ * cast the pointer to get access to the parent classes.
+ */
gtkobject_class = (GtkObjectClass*)klass;
gstelement_class = (GstElementClass*)klass;
+ /* The parent class is needed for class method overrides. */
parent_class = gtk_type_class(GST_TYPE_ELEMENT);
+ /* Here we add an argument to the object. This argument is an integer,
+ * and can be both read and written.
+ */
gtk_object_add_arg_type("GstExample::active", GTK_TYPE_INT,
GTK_ARG_READWRITE, ARG_ACTIVE);
+ /* The last thing is to provide the functions that implement get and set
+ * of arguments.
+ */
gtkobject_class->set_arg = gst_example_set_arg;
gtkobject_class->get_arg = gst_example_get_arg;
}
+/* This function is responsible for initializing a specific instance of
+ * the plugin.
+ */
static void
gst_example_init(GstExample *example)
{
+ /* First we create the sink pad, which is the input to the element.
+ * We will use the sink_template constructed in the plugin_init function
+ * (below) to quickly generate the pad we need.
+ */
example->sinkpad = gst_pad_new_from_template (sink_template, "sink");
- gst_element_add_pad(GST_ELEMENT(example),example->sinkpad);
+ /* Setting the chain function allows us to supply the function that will
+ * actually be performing the work. Without this, the element would do
+ * nothing, with undefined results (assertion failures and such).
+ */
gst_pad_set_chain_function(example->sinkpad,gst_example_chain);
+ /* We then must add this pad to the element's list of pads. The base
+ * element class manages the list of pads, and provides accessors to it.
+ */
+ gst_element_add_pad(GST_ELEMENT(example),example->sinkpad);
+ /* The src pad, the output of the element, is created and registered
+ * in the same way, with the exception of the chain function. Source
+ * pads don't have chain functions, because they can't accept buffers,
+ * they only produce them.
+ */
example->srcpad = gst_pad_new_from_template (src_template, "src");
gst_element_add_pad(GST_ELEMENT(example),example->srcpad);
+ /* Initialization of element's private variables. */
example->active = FALSE;
}
+/* The chain function is the heart of the element. It's where all the
+ * work is done. It is passed a pointer to the pad in question, as well
+ * as the buffer provided by the peer element.
+ */
static void
gst_example_chain (GstPad *pad, GstBuffer *buf)
{
GstExample *example;
+ GstBuffer *outbuf;
+ /* Some of these checks are of dubious value, since if there were not
+ * already true, the chain function would never be called.
+ */
g_return_if_fail(pad != NULL);
g_return_if_fail(GST_IS_PAD(pad));
g_return_if_fail(buf != NULL);
- //g_return_if_fail(GST_IS_BUFFER(buf));
+ g_return_if_fail(GST_IS_BUFFER(buf));
+ /* We need to get a pointer to the element this pad belogs to. */
example = GST_EXAMPLE(gst_pad_get_parent (pad));
+ /* A few more sanity checks to make sure that the element that owns
+ * this pad is the right kind of element, in case something got confused.
+ */
g_return_if_fail(example != NULL);
g_return_if_fail(GST_IS_EXAMPLE(example));
+ /* If we are supposed to be doing something, here's where it happens. */
if (example->active) {
- /* DO STUFF */
- }
+ /* In this example we're going to copy the buffer to another one,
+ * so we need to allocate a new buffer first.
+ outbuf = gst_buffer_new();
+
+ /* We need to copy the size and offset of the buffer at a minimum. */
+ GST_BUFFER_SIZE (outbuf) = GST_BUFFER_SIZE (buf);
+ GST_BUFFER_OFFSET (outbuf) = GST_BUFFER_OFFSET (buf);
+
+ /* Then allocate the memory for the new buffer */
+ GST_BUFFER_DATA (outbuf) = (guchar *)g_malloc (GST_BUFFER_SIZE (outbuf));
+
+ /* Then copy the data in the incoming buffer into the new buffer. */
+ memcpy (GST_BUFFER_DATA (outbuf), GST_BUFFER_DATA (buf), GST_BUFFER_SIZE (outbuf));
- gst_pad_push(example->srcpad,buf);
+ /* When we're done with the buffer, we push it on to the next element
+ * in the pipeline, through the element's source pad, which is stored
+ * in the element's structure.
+ */
+ gst_pad_push(example->srcpad,outbuf);
+
+ /* If we're not doing something, just send the original incoming buffer. */
+ } else {
+ gst_pad_push(example->srcpad,buf);
+ }
}
+/* Arguments are part of the Gtk+ object system, and these functions
+ * enable the element to respond to various arguments.
+ */
static void
gst_example_set_arg (GtkObject *object,GtkArg *arg,guint id)
{
GstExample *example;
- /* it's not null if we got it, but it might not be ours */
+ /* It's not null if we got it, but it might not be ours */
g_return_if_fail(GST_IS_EXAMPLE(object));
+
+ /* Get a pointer of the right type. */
example = GST_EXAMPLE(object);
- switch(id) {
+ /* Check the argument id to see which argument we're setting. */
+ switch (id) {
case ARG_ACTIVE:
+ /* Here we simply copy the value of the argument to our private
+ * storage. More complex operations can be done, but beware that
+ * they may occur at any time, possibly even while your chain function
+ * is running, if you are using threads.
+ */
example->active = GTK_VALUE_INT(*arg);
g_print("example: set active to %d\n",example->active);
break;
}
}
+/* The set function is simply the inverse of the get fuction. */
static void
gst_example_get_arg (GtkObject *object,GtkArg *arg,guint id)
{
GstExample *example;
- /* it's not null if we got it, but it might not be ours */
+ /* It's not null if we got it, but it might not be ours */
g_return_if_fail(GST_IS_EXAMPLE(object));
example = GST_EXAMPLE(object);
}
}
+/* This is the entry into the plugin itself. When the plugin loads,
+ * this function is called to register everything that the plugin provides.
+ */
GstPlugin*
plugin_init (GModule *module)
{
GstPlugin *plugin;
GstElementFactory *factory;
+ /* First we try to create a new Plugin structure. */
plugin = gst_plugin_new("example");
+ /* If we get a NULL back, chances are we're already loaded. */
g_return_val_if_fail(plugin != NULL, NULL);
+ /* We need to create an ElementFactory for each element we provide.
+ * This consists of the name of the element, the GtkType identifier,
+ * and a pointer to the details structure at the top of the file.
+ */
factory = gst_elementfactory_new("example", GST_TYPE_EXAMPLE, &example_details);
g_return_val_if_fail(factory != NULL, NULL);
+ /* The pad templates can be easily generated from the factories above,
+ * and then added to the list of padtemplates for the elementfactory.
+ * Note that the generated padtemplates are stored in static global
+ * variables, for the gst_example_init function to use later on.
+ */
sink_template = gst_padtemplate_new (&sink_factory);
gst_elementfactory_add_padtemplate (factory, sink_template);
src_template = gst_padtemplate_new (&src_factory);
gst_elementfactory_add_padtemplate (factory, src_template);
+ /* The very last thing is to register the elementfactory with the plugin. */
gst_plugin_add_factory (plugin, factory);
+ /* Now we can return the pointer to the newly created Plugin object. */
return plugin;
+
+ /* At this point, the GStreamer core registers the plugin, its
+ * elementfactories, padtemplates, etc., for use in you application.
+ */
}
#endif /* __cplusplus */
-/* Definition of structure storing data for this element. */
+/* This is the definition of the element's object structure. */
typedef struct _GstExample GstExample;
+/* The structure itself is derived from GstElement, as can be seen by the
+ * fact that there's a complete instance of the GstElement structure at
+ * the beginning of the object. This allows the element to be cast to
+ * an Element or even an Object.
+ */
struct _GstExample {
GstElement element;
+ /* We need to keep track of our pads, so we do so here. */
GstPad *sinkpad,*srcpad;
- gint8 active;
+ /* We'll use this to decide whether to do anything to the data we get. */
+ gboolean active;
};
-/* Standard definition defining a class for this element. */
+/* The other half of the object is its class. The class also derives from
+ * the same parent, though it must be the class structure this time.
+ * Function pointers for polymophic methods and signals are placed in this
+ * structure. */
typedef struct _GstExampleClass GstExampleClass;
+
struct _GstExampleClass {
GstElementClass parent_class;
};
-/* Standard macros for defining types for this element. */
+/* Five standard preprocessing macros are used in the Gtk+ object system.
+ * The first uses the object's _get_type function to return the GtkType
+ * of the object.
+ */
#define GST_TYPE_EXAMPLE \
(gst_example_get_type())
+/* The second is a checking cast to the correct type. If the object passed
+ * is not the right type, a warning will be generated on stderr.
+ */
#define GST_EXAMPLE(obj) \
(GTK_CHECK_CAST((obj),GST_TYPE_EXAMPLE,GstExample))
+/* The third is a checking cast of the class instead of the object. */
#define GST_EXAMPLE_CLASS(klass) \
(GTK_CHECK_CLASS_CAST((klass),GST_TYPE_EXAMPLE,GstExample))
+/* The last two simply check to see if the passed pointer is an object or
+ * class of the correct type. */
#define GST_IS_EXAMPLE(obj) \
(GTK_CHECK_TYPE((obj),GST_TYPE_EXAMPLE))
#define GST_IS_EXAMPLE_CLASS(obj) \
(GTK_CHECK_CLASS_TYPE((klass),GST_TYPE_EXAMPLE))
-/* Standard function returning type information. */
+/* This is the only prototype needed, because it is used in the above
+ * GST_TYPE_EXAMPLE macro.
+ */
GtkType gst_example_get_type(void);
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