GstMessage

GstMessage — Lightweight objects to signal the application of pipeline events

Synopsis


#include <gst/gst.h>


                    GstMessage;
enum                GstMessageType;
#define             GST_MESSAGE_SRC                     (message)
#define             GST_MESSAGE_TIMESTAMP               (message)
#define             GST_MESSAGE_TYPE                    (message)
#define             GST_MESSAGE_TYPE_NAME               (message)
#define             GST_MESSAGE_TRACE_NAME
GQuark              gst_message_type_to_quark           (GstMessageType type);
const gchar*        gst_message_type_get_name           (GstMessageType type);
#define             gst_message_copy                    (msg)
const GstStructure* gst_message_get_structure           (GstMessage *message);
#define             gst_message_make_writable           (msg)
GstMessage*         gst_message_new_application         (GstObject *src,
                                                         GstStructure *structure);
GstMessage*         gst_message_new_clock_provide       (GstObject *src,
                                                         GstClock *clock,
                                                         gboolean ready);
GstMessage*         gst_message_new_clock_lost          (GstObject *src,
                                                         GstClock *clock);
GstMessage*         gst_message_new_custom              (GstMessageType type,
                                                         GstObject *src,
                                                         GstStructure *structure);
GstMessage*         gst_message_new_element             (GstObject *src,
                                                         GstStructure *structure);
GstMessage*         gst_message_new_eos                 (GstObject *src);
GstMessage*         gst_message_new_error               (GstObject *src,
                                                         GError *error,
                                                         gchar *debug);
GstMessage*         gst_message_new_info                (GstObject *src,
                                                         GError *error,
                                                         gchar *debug);
GstMessage*         gst_message_new_new_clock           (GstObject *src,
                                                         GstClock *clock);
GstMessage*         gst_message_new_segment_done        (GstObject *src,
                                                         GstFormat format,
                                                         gint64 position);
GstMessage*         gst_message_new_segment_start       (GstObject *src,
                                                         GstFormat format,
                                                         gint64 position);
GstMessage*         gst_message_new_state_changed       (GstObject *src,
                                                         GstState oldstate,
                                                         GstState newstate,
                                                         GstState pending);
GstMessage*         gst_message_new_tag                 (GstObject *src,
                                                         GstTagList *tag_list);
GstMessage*         gst_message_new_buffering           (GstObject *src,
                                                         gint percent);
GstMessage*         gst_message_new_warning             (GstObject *src,
                                                         GError *error,
                                                         gchar *debug);
GstMessage*         gst_message_new_duration            (GstObject *src,
                                                         GstFormat format,
                                                         gint64 duration);
GstMessage*         gst_message_new_state_dirty         (GstObject *src);
GstMessage*         gst_message_new_async_start         (GstObject *src,
                                                         gboolean new_base_time);
GstMessage*         gst_message_new_async_done          (GstObject *src);
GstMessage*         gst_message_new_latency             (GstObject *src);
void                gst_message_parse_clock_lost        (GstMessage *message,
                                                         GstClock **clock);
void                gst_message_parse_clock_provide     (GstMessage *message,
                                                         GstClock **clock,
                                                         gboolean *ready);
void                gst_message_parse_error             (GstMessage *message,
                                                         GError **gerror,
                                                         gchar **debug);
void                gst_message_parse_info              (GstMessage *message,
                                                         GError **gerror,
                                                         gchar **debug);
void                gst_message_parse_new_clock         (GstMessage *message,
                                                         GstClock **clock);
void                gst_message_parse_segment_done      (GstMessage *message,
                                                         GstFormat *format,
                                                         gint64 *position);
void                gst_message_parse_segment_start     (GstMessage *message,
                                                         GstFormat *format,
                                                         gint64 *position);
void                gst_message_parse_state_changed     (GstMessage *message,
                                                         GstState *oldstate,
                                                         GstState *newstate,
                                                         GstState *pending);
void                gst_message_parse_tag               (GstMessage *message,
                                                         GstTagList **tag_list);
void                gst_message_parse_buffering         (GstMessage *message,
                                                         gint *percent);
void                gst_message_parse_warning           (GstMessage *message,
                                                         GError **gerror,
                                                         gchar **debug);
void                gst_message_parse_duration          (GstMessage *message,
                                                         GstFormat *format,
                                                         gint64 *duration);
void                gst_message_parse_async_start       (GstMessage *message,
                                                         gboolean *new_base_time);
GstMessage*         gst_message_ref                     (GstMessage *msg);
#define             gst_message_unref                   (msg)


Description

Messages are implemented as a subclass of GstMiniObject with a generic GstStructure as the content. This allows for writing custom messages without requiring an API change while allowing a wide range of different types of messages.

Messages are posted by objects in the pipeline and are passed to the application using the GstBus.

The basic use pattern of posting a message on a GstBus is as follows:

Example 11. Posting a GstMessage

   gst_bus_post (bus, gst_message_new_eos());
  


A GstElement usually posts messages on the bus provided by the parent container using gst_element_post_message().

Last reviewed on 2005-11-09 (0.9.4)

Details

GstMessage

typedef struct {
  GstMiniObject mini_object;

  GstMessageType type;
  guint64 timestamp;
  GstObject *src;

  GstStructure *structure;
} GstMessage;

A GstMessage.

GstMiniObject mini_object; the parent structure
GstMessageType type; the GstMessageType of the message
guint64 timestamp; the timestamp of the message
GstObject *src; the src of the message
GstStructure *structure; the GstStructure containing the message info.

enum GstMessageType

typedef enum
{
  GST_MESSAGE_UNKNOWN           = 0,
  GST_MESSAGE_EOS               = (1 << 0),
  GST_MESSAGE_ERROR             = (1 << 1),
  GST_MESSAGE_WARNING           = (1 << 2),
  GST_MESSAGE_INFO              = (1 << 3),
  GST_MESSAGE_TAG               = (1 << 4),
  GST_MESSAGE_BUFFERING         = (1 << 5),
  GST_MESSAGE_STATE_CHANGED     = (1 << 6),
  GST_MESSAGE_STATE_DIRTY       = (1 << 7),
  GST_MESSAGE_STEP_DONE         = (1 << 8),
  GST_MESSAGE_CLOCK_PROVIDE     = (1 << 9),
  GST_MESSAGE_CLOCK_LOST        = (1 << 10),
  GST_MESSAGE_NEW_CLOCK         = (1 << 11),
  GST_MESSAGE_STRUCTURE_CHANGE  = (1 << 12),
  GST_MESSAGE_STREAM_STATUS     = (1 << 13),
  GST_MESSAGE_APPLICATION       = (1 << 14),
  GST_MESSAGE_ELEMENT           = (1 << 15),
  GST_MESSAGE_SEGMENT_START     = (1 << 16),
  GST_MESSAGE_SEGMENT_DONE      = (1 << 17),
  GST_MESSAGE_DURATION          = (1 << 18),
  GST_MESSAGE_LATENCY           = (1 << 19),
  GST_MESSAGE_ASYNC_START       = (1 << 20),
  GST_MESSAGE_ASYNC_DONE        = (1 << 21),
  GST_MESSAGE_ANY               = ~0
} GstMessageType;

The different message types that are available.

GST_MESSAGE_UNKNOWN an undefined message
GST_MESSAGE_EOS end-of-stream reached in a pipeline. The application will only receive this message in the PLAYING state and every time it sets a pipeline to PLAYING that is in the EOS state. The application can perform a seek in the pipeline to a new position.
GST_MESSAGE_ERROR an error occured. Whe the application receives an error message it should stop playback of the pipeline and not assume that more data will be played.
GST_MESSAGE_WARNING a warning occured.
GST_MESSAGE_INFO an info message occured
GST_MESSAGE_TAG a tag was found.
GST_MESSAGE_BUFFERING the pipeline is buffering. When the application receives a buffering message in the PLAYING state for a non-live pipeline it must PAUSE the pipeline until the buffering completes, when the percentage field in the message is 100%. For live pipelines, no action must be performed and the buffering percentage can be used to infor the user about the progress.
GST_MESSAGE_STATE_CHANGED a state change happened
GST_MESSAGE_STATE_DIRTY an element changed state in a streaming thread. This message is deprecated.
GST_MESSAGE_STEP_DONE a framestep finished. This message is not yet implemented.
GST_MESSAGE_CLOCK_PROVIDE an element notifies its capability of providing a clock.
GST_MESSAGE_CLOCK_LOST The current clock as selected by the pipeline became unusable. The pipeline will select a new clock on the next PLAYING state change.
GST_MESSAGE_NEW_CLOCK a new clock was selected in the pipeline.
GST_MESSAGE_STRUCTURE_CHANGE the structure of the pipeline changed.
GST_MESSAGE_STREAM_STATUS status about a stream, emitted when it starts, stops, errors, etc..
GST_MESSAGE_APPLICATION message posted by the application, possibly via an application-specific element.
GST_MESSAGE_ELEMENT element-specific message, see the specific element's documentation
GST_MESSAGE_SEGMENT_START pipeline started playback of a segment.
GST_MESSAGE_SEGMENT_DONE pipeline completed playback of a segment.
GST_MESSAGE_DURATION The duration of a pipeline changed.
GST_MESSAGE_LATENCY Posted by elements when their latency changes. The pipeline will calculate and distribute a new latency. Since: 0.10.12
GST_MESSAGE_ASYNC_START Posted by elements when they start an ASYNC state change. Since: 0.10.13
GST_MESSAGE_ASYNC_DONE Posted by elements when they complete an ASYNC state change. Since: 0.10.13
GST_MESSAGE_ANY mask for all of the above messages.

GST_MESSAGE_SRC()

#define GST_MESSAGE_SRC(message)	(GST_MESSAGE(message)->src)

Get the object that posted message.

message : a GstMessage

GST_MESSAGE_TIMESTAMP()

#define GST_MESSAGE_TIMESTAMP(message)	(GST_MESSAGE(message)->timestamp)

Get the timestamp of message. This is the timestamp when the message was created.

message : a GstMessage

GST_MESSAGE_TYPE()

#define GST_MESSAGE_TYPE(message)	(GST_MESSAGE(message)->type)

Get the GstMessageType of message.

message : a GstMessage

GST_MESSAGE_TYPE_NAME()

#define GST_MESSAGE_TYPE_NAME(message)	gst_message_type_get_name(GST_MESSAGE_TYPE(message))

Get a constant string representation of the GstMessageType of message.

message : a GstMessage

Since 0.10.4


GST_MESSAGE_TRACE_NAME

#define GST_MESSAGE_TRACE_NAME	"GstMessage"

The name used for memory allocation tracing


gst_message_type_to_quark ()

GQuark              gst_message_type_to_quark           (GstMessageType type);

Get the unique quark for the given message type.

type : the message type
Returns : the quark associated with the message type

gst_message_type_get_name ()

const gchar*        gst_message_type_get_name           (GstMessageType type);

Get a printable name for the given message type. Do not modify or free.

type : the message type
Returns : a reference to the static name of the message.

gst_message_copy()

#define         gst_message_copy(msg)		GST_MESSAGE (gst_mini_object_copy (GST_MINI_OBJECT (msg)))

Creates a copy of the message. Returns a copy of the message.

MT safe

msg : the message to copy

gst_message_get_structure ()

const GstStructure* gst_message_get_structure           (GstMessage *message);

Access the structure of the message.

message : The GstMessage.
Returns : The structure of the message. The structure is still owned by the message, which means that you should not free it and that the pointer becomes invalid when you free the message. MT safe.

gst_message_make_writable()

#define         gst_message_make_writable(msg)	GST_MESSAGE (gst_mini_object_make_writable (GST_MINI_OBJECT (msg)))

Checks if a message is writable. If not, a writable copy is made and returned. Returns a message (possibly a duplicate) that is writable.

MT safe

msg : the message to make writable

gst_message_new_application ()

GstMessage*         gst_message_new_application         (GstObject *src,
                                                         GstStructure *structure);

Create a new application-typed message. GStreamer will never create these messages; they are a gift from us to you. Enjoy.

src : The object originating the message.
structure : The structure for the message. The message will take ownership of the structure.
Returns : The new application message. MT safe.

gst_message_new_clock_provide ()

GstMessage*         gst_message_new_clock_provide       (GstObject *src,
                                                         GstClock *clock,
                                                         gboolean ready);

Create a clock provide message. This message is posted whenever an element is ready to provide a clock or lost its ability to provide a clock (maybe because it paused or became EOS).

This message is mainly used internally to manage the clock selection.

src : The object originating the message.
clock : The clock it provides
ready : TRUE if the sender can provide a clock
Returns : The new provide clock message. MT safe.

gst_message_new_clock_lost ()

GstMessage*         gst_message_new_clock_lost          (GstObject *src,
                                                         GstClock *clock);

Create a clock lost message. This message is posted whenever the clock is not valid anymore.

If this message is posted by the pipeline, the pipeline will select a new clock again when it goes to PLAYING. It might therefore be needed to set the pipeline to PAUSED and PLAYING again.

src : The object originating the message.
clock : the clock that was lost
Returns : The new clock lost message. MT safe.

gst_message_new_custom ()

GstMessage*         gst_message_new_custom              (GstMessageType type,
                                                         GstObject *src,
                                                         GstStructure *structure);

Create a new custom-typed message. This can be used for anything not handled by other message-specific functions to pass a message to the app. The structure field can be NULL.

type : The GstMessageType to distinguish messages
src : The object originating the message.
structure : The structure for the message. The message will take ownership of the structure.
Returns : The new message. MT safe.

gst_message_new_element ()

GstMessage*         gst_message_new_element             (GstObject *src,
                                                         GstStructure *structure);

Create a new element-specific message. This is meant as a generic way of allowing one-way communication from an element to an application, for example "the firewire cable was unplugged". The format of the message should be documented in the element's documentation. The structure field can be NULL.

src : The object originating the message.
structure : The structure for the message. The message will take ownership of the structure.
Returns : The new element message. MT safe.

gst_message_new_eos ()

GstMessage*         gst_message_new_eos                 (GstObject *src);

Create a new eos message. This message is generated and posted in the sink elements of a GstBin. The bin will only forward the EOS message to the application if all sinks have posted an EOS message.

src : The object originating the message.
Returns : The new eos message. MT safe.

gst_message_new_error ()

GstMessage*         gst_message_new_error               (GstObject *src,
                                                         GError *error,
                                                         gchar *debug);

Create a new error message. The message will copy error and debug. This message is posted by element when a fatal event occured. The pipeline will probably (partially) stop. The application receiving this message should stop the pipeline.

src : The object originating the message.
error : The GError for this message.
debug : A debugging string for something or other.
Returns : The new error message. MT safe.

gst_message_new_info ()

GstMessage*         gst_message_new_info                (GstObject *src,
                                                         GError *error,
                                                         gchar *debug);

Create a new info message. The message will make copies of error and debug.

src : The object originating the message.
error : The GError for this message.
debug : A debugging string for something or other.
Returns : The new info message.

Since 0.10.12 MT safe.


gst_message_new_new_clock ()

GstMessage*         gst_message_new_new_clock           (GstObject *src,
                                                         GstClock *clock);

Create a new clock message. This message is posted whenever the pipeline selectes a new clock for the pipeline.

src : The object originating the message.
clock : the new selected clock
Returns : The new new clock message. MT safe.

gst_message_new_segment_done ()

GstMessage*         gst_message_new_segment_done        (GstObject *src,
                                                         GstFormat format,
                                                         gint64 position);

Create a new segment done message. This message is posted by elements that finish playback of a segment as a result of a segment seek. This message is received by the application after all elements that posted a segment_start have posted the segment_done.

src : The object originating the message.
format : The format of the position being done
position : The position of the segment being done
Returns : The new segment done message. MT safe.

gst_message_new_segment_start ()

GstMessage*         gst_message_new_segment_start       (GstObject *src,
                                                         GstFormat format,
                                                         gint64 position);

Create a new segment message. This message is posted by elements that start playback of a segment as a result of a segment seek. This message is not received by the application but is used for maintenance reasons in container elements.

src : The object originating the message.
format : The format of the position being played
position : The position of the segment being played
Returns : The new segment start message. MT safe.

gst_message_new_state_changed ()

GstMessage*         gst_message_new_state_changed       (GstObject *src,
                                                         GstState oldstate,
                                                         GstState newstate,
                                                         GstState pending);

Create a state change message. This message is posted whenever an element changed its state.

src : the object originating the message
oldstate : the previous state
newstate : the new (current) state
pending : the pending (target) state
Returns : The new state change message. MT safe.

gst_message_new_tag ()

GstMessage*         gst_message_new_tag                 (GstObject *src,
                                                         GstTagList *tag_list);

Create a new tag message. The message will take ownership of the tag list. The message is posted by elements that discovered a new taglist.

src : The object originating the message.
tag_list : The tag list for the message.
Returns : The new tag message. MT safe.

gst_message_new_buffering ()

GstMessage*         gst_message_new_buffering           (GstObject *src,
                                                         gint percent);

Create a new buffering message. This message can be posted by an element that needs to buffer data before it can continue processing. percent should be a value between 0 and 100. A value of 100 means that the buffering completed.

When percent is < 100 the application should PAUSE a PLAYING pipeline. When percent is 100, the application can set the pipeline (back) to PLAYING. The application must be prepared to receive BUFFERING messages in the PREROLLING state and may only set the pipeline to PLAYING after receiving a message with percent set to 100, which can happen after the pipeline completed prerolling.

src : The object originating the message.
percent : The buffering percent
Returns : The new buffering message.

Since 0.10.11 MT safe.


gst_message_new_warning ()

GstMessage*         gst_message_new_warning             (GstObject *src,
                                                         GError *error,
                                                         gchar *debug);

Create a new warning message. The message will make copies of error and debug.

src : The object originating the message.
error : The GError for this message.
debug : A debugging string for something or other.
Returns : The new warning message. MT safe.

gst_message_new_duration ()

GstMessage*         gst_message_new_duration            (GstObject *src,
                                                         GstFormat format,
                                                         gint64 duration);

Create a new duration message. This message is posted by elements that know the duration of a stream in a specific format. This message is received by bins and is used to calculate the total duration of a pipeline. Elements may post a duration message with a duration of GST_CLOCK_TIME_NONE to indicate that the duration has changed and the cached duration should be discarded. The new duration can then be retrieved via a query.

src : The object originating the message.
format : The format of the duration
duration : The new duration
Returns : The new duration message. MT safe.

gst_message_new_state_dirty ()

GstMessage*         gst_message_new_state_dirty         (GstObject *src);

Create a state dirty message. This message is posted whenever an element changed its state asynchronously and is used internally to update the states of container objects.

src : the object originating the message
Returns : The new state dirty message. MT safe.

gst_message_new_async_start ()

GstMessage*         gst_message_new_async_start         (GstObject *src,
                                                         gboolean new_base_time);

This message is posted by elements when they start an ASYNC state change. new_base_time is set to TRUE when the element lost its state when it was PLAYING.

src : The object originating the message.
new_base_time : if a new base_time should be set on the element
Returns : The new async_start message. MT safe.

Since 0.10.13


gst_message_new_async_done ()

GstMessage*         gst_message_new_async_done          (GstObject *src);

The message is posted when elements completed an ASYNC state change.

src : The object originating the message.
Returns : The new async_done message. MT safe.

Since 0.10.13


gst_message_new_latency ()

GstMessage*         gst_message_new_latency             (GstObject *src);

This message can be posted by elements when their latency requirements have changed.

src : The object originating the message.
Returns : The new latency message. MT safe.

Since 0.10.12


gst_message_parse_clock_lost ()

void                gst_message_parse_clock_lost        (GstMessage *message,
                                                         GstClock **clock);

Extracts the lost clock from the GstMessage. The clock object returned remains valid until the message is freed.

MT safe.

message : A valid GstMessage of type GST_MESSAGE_CLOCK_LOST.
clock : A pointer to hold the lost clock

gst_message_parse_clock_provide ()

void                gst_message_parse_clock_provide     (GstMessage *message,
                                                         GstClock **clock,
                                                         gboolean *ready);

Extracts the clock and ready flag from the GstMessage. The clock object returned remains valid until the message is freed.

MT safe.

message : A valid GstMessage of type GST_MESSAGE_CLOCK_PROVIDE.
clock : A pointer to hold a clock object.
ready : A pointer to hold the ready flag.

gst_message_parse_error ()

void                gst_message_parse_error             (GstMessage *message,
                                                         GError **gerror,
                                                         gchar **debug);

Extracts the GError and debug string from the GstMessage. The values returned in the output arguments are copies; the caller must free them when done.

MT safe.

message : A valid GstMessage of type GST_MESSAGE_ERROR.
gerror : Location for the GError
debug : Location for the debug message, or NULL

gst_message_parse_info ()

void                gst_message_parse_info              (GstMessage *message,
                                                         GError **gerror,
                                                         gchar **debug);

Extracts the GError and debug string from the GstMessage. The values returned in the output arguments are copies; the caller must free them when done.

MT safe.

message : A valid GstMessage of type GST_MESSAGE_INFO.
gerror : Location for the GError
debug : Location for the debug message, or NULL

Since 0.10.12


gst_message_parse_new_clock ()

void                gst_message_parse_new_clock         (GstMessage *message,
                                                         GstClock **clock);

Extracts the new clock from the GstMessage. The clock object returned remains valid until the message is freed.

MT safe.

message : A valid GstMessage of type GST_MESSAGE_NEW_CLOCK.
clock : A pointer to hold the selected new clock

gst_message_parse_segment_done ()

void                gst_message_parse_segment_done      (GstMessage *message,
                                                         GstFormat *format,
                                                         gint64 *position);

Extracts the position and format from the segment start message.

MT safe.

message : A valid GstMessage of type GST_MESSAGE_SEGMENT_DONE.
format : Result location for the format, or NULL
position : Result location for the position, or NULL

gst_message_parse_segment_start ()

void                gst_message_parse_segment_start     (GstMessage *message,
                                                         GstFormat *format,
                                                         gint64 *position);

Extracts the position and format from the segment start message.

MT safe.

message : A valid GstMessage of type GST_MESSAGE_SEGMENT_START.
format : Result location for the format, or NULL
position : Result location for the position, or NULL

gst_message_parse_state_changed ()

void                gst_message_parse_state_changed     (GstMessage *message,
                                                         GstState *oldstate,
                                                         GstState *newstate,
                                                         GstState *pending);

Extracts the old and new states from the GstMessage.

MT safe.

message : a valid GstMessage of type GST_MESSAGE_STATE_CHANGED
oldstate : the previous state, or NULL
newstate : the new (current) state, or NULL
pending : the pending (target) state, or NULL

gst_message_parse_tag ()

void                gst_message_parse_tag               (GstMessage *message,
                                                         GstTagList **tag_list);

Extracts the tag list from the GstMessage. The tag list returned in the output argument is a copy; the caller must free it when done.

MT safe.

message : A valid GstMessage of type GST_MESSAGE_TAG.
tag_list : Return location for the tag-list.

gst_message_parse_buffering ()

void                gst_message_parse_buffering         (GstMessage *message,
                                                         gint *percent);

Extracts the buffering percent from the GstMessage. see also gst_message_new_buffering().

message : A valid GstMessage of type GST_MESSAGE_BUFFERING.
percent : Return location for the percent.

Since 0.10.11 MT safe.


gst_message_parse_warning ()

void                gst_message_parse_warning           (GstMessage *message,
                                                         GError **gerror,
                                                         gchar **debug);

Extracts the GError and debug string from the GstMessage. The values returned in the output arguments are copies; the caller must free them when done.

MT safe.

message : A valid GstMessage of type GST_MESSAGE_WARNING.
gerror : Location for the GError
debug : Location for the debug message, or NULL

gst_message_parse_duration ()

void                gst_message_parse_duration          (GstMessage *message,
                                                         GstFormat *format,
                                                         gint64 *duration);

Extracts the duration and format from the duration message. The duration might be GST_CLOCK_TIME_NONE, which indicates that the duration has changed. Applications should always use a query to retrieve the duration of a pipeline.

MT safe.

message : A valid GstMessage of type GST_MESSAGE_DURATION.
format : Result location for the format, or NULL
duration : Result location for the duration, or NULL

gst_message_parse_async_start ()

void                gst_message_parse_async_start       (GstMessage *message,
                                                         gboolean *new_base_time);

Extract the new_base_time from the async_start message.

MT safe.

message : A valid GstMessage of type GST_MESSAGE_ASYNC_DONE.
new_base_time : Result location for the new_base_time or NULL

Since 0.10.13


gst_message_ref ()

GstMessage*         gst_message_ref                     (GstMessage *msg);

Convenience macro to increase the reference count of the message.

msg : the message to ref
Returns : msg (for convenience when doing assignments)

gst_message_unref()

#define         gst_message_unref(msg)		gst_mini_object_unref (GST_MINI_OBJECT (msg))

Convenience macro to decrease the reference count of the message, possibly freeing it.

msg : the message to unref

See Also

GstBus, GstMiniObject, GstElement