gstreamer/subprojects/gst-editing-services/docs/design/metadata.txt

Metadata
~~~~~~~~

Summary
~~~~~~~

1. Basic ideas
2. Problems
3. Ways of solving problems
4. Use-cases
5. API draft

1. Basic ideas
~~~~~~~~~~~~~~

If we look at entities that are present in GES we can see that almost all of
them need some sort of metadata:
    * GESTimeline
    * GESTimelineLayer
    * GESTimelineObject
    * GESTrackObject
    * Yet to be implemented GESProject

For all those classes to be able to contain metadatas and to avoid code
duplication as much as possible, we should have an interface to handle Metadata.
Let's call the interface GESMetaContainer for now (name to be defined).

2. Problems
~~~~~~~~~~~

    1) We must be able to discover all metadata items that are
       attached to object
    2) We must be able to hold metadata of any type user wants
    3) Some metadatas are read only, others are writable
    4) User should be able to query metadata easily using various criteria
    5) Metadatas should be serializable
    6) User should be able to define read only metadatas with a default value
    7) User should be able to define metadatas that have a specific type which can not
       be changed when setting a new value


3. Possible solution
~~~~~~~~~~~~~~~~~~~~~

    1) To implement metadata GstStructure will be used. It allows to get list of
       all available tags in specified list by calling "gst_structure_foreach".
    2) We will have methods to register metas

4. Use-cases
~~~~~~~~~~~~

    UC-1. Hold tag information about file source asset.
      - TS: I think some of them are TrackObject specific... so we should be
        able to get them from the 2 types of objects
    UC-2. Hold descriptions of operations
    UC-3. Hold information about projects (title, author, description)
    UC-4. Hold user comments about any of TimelineLayer/Timeline/Project/TimelineObjects
    UC-5. Hold application specific settings (i.e. layer height, folding state
          in Pitivi)
    UC-6. Serialize a timeline, project and keep metadatas

5. API
~~~~~~

We have a GESMetdata class that controls metadata.

gboolean
ges_meta_container_set_boolean     (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        gboolean value);

gboolean
ges_meta_container_set_int         (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        gint value);

gboolean
ges_meta_container_set_uint        (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        guint value);

gboolean
ges_meta_container_set_int64       (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        gint64 value);

gboolean
ges_meta_container_set_uint64      (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        guint64 value);

gboolean
ges_meta_container_set_float       (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        gfloat value);

gboolean
ges_meta_container_set_double      (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        gdouble value);

gboolean
ges_meta_container_set_date        (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        const GDate* value);

gboolean
ges_meta_container_set_date_time   (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        const GstDateTime* value);

gboolean
ges_meta_container_set_string      (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        const gchar* value);

gboolean
ges_meta_container_set_meta            (GESMetaContainer * container,
                                        const gchar* meta_item,
                                        const GValue *value);

gboolean
ges_meta_container_register_meta_boolean (GESMetaContainer *container,
                                          GESMetaFlag flags,
                                          const gchar* meta_item,
                                          gboolean value);

gboolean
ges_meta_container_register_meta_int     (GESMetaContainer *container,
                                          GESMetaFlag flags,
                                          const gchar* meta_item,
                                          gint value);

gboolean
ges_meta_container_register_meta_uint    (GESMetaContainer *container,
                                          GESMetaFlag flags,
                                          const gchar* meta_item,
                                          guint value);

gboolean
ges_meta_container_register_meta_int64   (GESMetaContainer *container,
                                          GESMetaFlag flags,
                                          const gchar* meta_item,
                                          gint64 value);

gboolean
ges_meta_container_register_meta_uint64  (GESMetaContainer *container,
                                          GESMetaFlag flags,
                                          const gchar* meta_item,
                                          guint64 value);

gboolean
ges_meta_container_register_meta_float   (GESMetaContainer *container,
                                          GESMetaFlag flags,
                                          const gchar* meta_item,
                                          gfloat value);

gboolean
ges_meta_container_register_meta_double  (GESMetaContainer *container,
                                          GESMetaFlag flags,
                                          const gchar* meta_item,
                                          gdouble value);

gboolean
ges_meta_container_register_meta_date    (GESMetaContainer *container,
                                          GESMetaFlag flags,
                                          const gchar* meta_item,
                                          const GDate* value);

gboolean
ges_meta_container_register_meta_date_time  (GESMetaContainer *container,
                                             GESMetaFlag flags,
                                             const gchar* meta_item,
                                             const GstDateTime* value);

gboolean
ges_meta_container_register_meta_string     (GESMetaContainer *container,
                                             GESMetaFlag flags,
                                             const gchar* meta_item,
                                             const gchar* value);

gboolean
ges_meta_container_register_meta            (GESMetaContainer *container,
                                             GESMetaFlag flags,
                                             const gchar* meta_item,
                                             const GValue * value);

gboolean
ges_meta_container_check_meta_registered    (GESMetaContainer *container,
                                             const gchar * meta_item,
                                             GESMetaFlag * flags,
                                             GType * type);

gboolean
ges_meta_container_get_boolean     (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        gboolean* dest);

gboolean
ges_meta_container_get_int         (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        gint* dest);

gboolean
ges_meta_container_get_uint        (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        guint* dest);

gboolean
ges_meta_container_get_int64       (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        gint64* dest);

gboolean
ges_meta_container_get_uint64      (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        guint64* dest);

gboolean
ges_meta_container_get_float       (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        gfloat* dest);

gboolean
ges_meta_container_get_double      (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        gdouble* dest);

gboolean
ges_meta_container_get_date        (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        GDate** dest);

gboolean
ges_meta_container_get_date_time   (GESMetaContainer *container,
                                        const gchar* meta_item,
                                        GstDateTime** dest);

const gchar *
ges_meta_container_get_string      (GESMetaContainer * container,
                                        const gchar * meta_item);

const GValue *
ges_meta_container_get_meta            (GESMetaContainer * container,
                                        const gchar * key);

typedef void
(*GESMetaForeachFunc)                  (const GESMetaContainer *container,
                                        const gchar *key,
                                        const GValue *value,
                                        gpointer user_data);

void
ges_meta_container_foreach             (GESMetaContainer *container,
                                        GESMetaForeachFunc func,
                                        gpointer user_data);

gchar *
ges_meta_container_metas_to_string     (GESMetaContainer *container);

gboolean
ges_meta_container_add_metas_from_string (GESMetaContainer *container,
                                          const gchar *str);