| Query |
| ----- |
| |
| NOTE: this is implemented as proposed |
| |
| Purpose |
| ~~~~~~~ |
| |
| Queries are used to get information about the stream. |
| A query is started on a specific pad and travels up or downstream. |
| |
| Types of queries |
| ~~~~~~~~~~~~~~~~ |
| |
| - get length of stream |
| - get position in stream |
| - get seeking capabilities |
| - get latency |
| - convert one value to another |
| - query supported formats |
| - query internal links. |
| |
| Current implementation |
| ~~~~~~~~~~~~~~~~~~~~~~ |
| |
| The current implementation of query requires pads to implement the |
| following functions: |
| |
| gboolean (*GstPadConvertFunction) (GstPad *pad, |
| GstFormat src_format, gint64 src_value, |
| GstFormat *dest_format, gint64 *dest_value); |
| gboolean (*GstPadQueryFunction) (GstPad *pad, GstQueryType type, |
| GstFormat *format, gint64 *value); |
| GList* (*GstPadIntLinkFunction) (GstPad *pad); |
| const GstFormat* (*GstPadFormatsFunction) (GstPad *pad); |
| const GstEventMask* (*GstPadEventMaskFunction) (GstPad *pad); |
| const GstQueryType* (*GstPadQueryTypeFunction) (GstPad *pad); |
| |
| Most of these functions are not very extensible in particular, |
| the queryfunction can only return one value. |
| |
| |
| Requirements |
| ~~~~~~~~~~~~ |
| |
| - multiple return values, grouped together when they make sense. |
| - one pad function to perform the query |
| - extensible queries. |
| |
| Proposition |
| ~~~~~~~~~~~ |
| |
| - define GstQuery extending GstMiniObject and containing a GstStructure (see GstMessage) |
| - define standard query types (see proposed types) |
| - define methods to create a parse the results in the GstQuery. |
| - define pad method: |
| |
| gboolean (*GstPadQueryFunction) (GstPad *pad, GstQuery *query); |
| |
| pad returns result in query structure and TRUE as result or FALSE when |
| query is not supported. |
| |
| Proposed types |
| ~~~~~~~~~~~~~~ |
| |
| - GST_QUERY_SEEKING: |
| |
| get info on how seeking can be done |
| - getrange, with/without offset/size |
| - ranges where seeking is efficient (for caching network sources) |
| - flags describing seeking behaviour (forward, backward, segments, |
| play backwards, ...) |
| |
| - GST_QUERY_POSITION: |
| |
| get info on current position of the stream in stream_time. |
| |
| - GST_QUERY_DURATION: |
| |
| get info on the total duration of the stream. |
| |
| - GST_QUERY_LATENCY: |
| |
| get amount of latency introduced in the pipeline. |
| |
| - GST_QUERY_SEGMENT: |
| |
| get info about the currently configured playback segment. |
| |
| - GST_QUERY_CONVERT: |
| |
| convert format/value to another format/value pair. |
| |
| - GST_QUERY_FORMATS: |
| |
| return list of supported formats that can be used for GST_QUERY_CONVERT. |
| |
