So far, we've looked at how to create a pipeline to do media processing and how to make it run. Most application developers will be interested in providing feedback to the user on media progress. Media players, for example, will want to show a slider showing the progress in the song, and usually also a label indicating stream length. Transcoding applications will want to show a progress bar on how much percent of the task is done. GStreamer has built-in support for doing all this using a concept known as querying. Since seeking is very similar, it will be discussed here as well. Seeking is done using the concept of events.
Querying is defined as requesting a specific stream-property related
to progress tracking. This includes getting the length of a stream (if
available) or getting the current position. Those stream properties
can be retrieved in various formats such as time, audio samples, video
frames or bytes. The function most commonly used for this is
gst_element_query ()
, although some convenience
wrappers are provided as well (such as
gst_element_query_position ()
and
gst_element_query_duration ()
). You can generally
query the pipeline directly, and it'll figure out the internal details
for you, like which element to query.
Internally, queries will be sent to the sinks, and "dispatched" backwards until one element can handle it; that result will be sent back to the function caller. Usually, that is the demuxer, although with live sources (from a webcam), it is the source itself.
#include <gst/gst.h> static gboolean cb_print_position (GstElement *pipeline) { GstFormat fmt = GST_FORMAT_TIME; gint64 pos, len; if (gst_element_query_position (pipeline, &fmt, &pos) && gst_element_query_duration (pipeline, &fmt, &len)) { g_print ("Time: %" GST_TIME_FORMAT " / %" GST_TIME_FORMAT "\r", GST_TIME_ARGS (pos), GST_TIME_ARGS (len)); } /* call me again */ return TRUE; } gint main (gint argc, gchar *argv[]) { GstElement *pipeline; [..] /* run pipeline */ g_timeout_add (200, (GSourceFunc) cb_print_position, pipeline); g_main_loop_run (loop); [..] }