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playbin之Source插件加载流程源码剖析

news 来源：原创 2025/5/3 4:08:30

之前我们有讲解过uridecodebin的setup_source中会创建source插件，关键函数：

/* create and configure an element that can handle the uri */

source = gen_source_element (decoder);

/*
 * Generate and configure a source element.
 *
 * Returns: (transfer full): a new #GstElement
 */
static GstElement *
gen_source_element (GstURIDecodeBin * decoder)
{
  GObjectClass *source_class;
  GstElement *source;
  GParamSpec *pspec;
  GstQuery *query;
  GstSchedulingFlags flags;
  GError *err = NULL;

  if (!decoder->uri)
    goto no_uri;

  GST_LOG_OBJECT (decoder, "finding source for %s", decoder->uri);

  if (!gst_uri_is_valid (decoder->uri))
    goto invalid_uri;

  if (IS_BLACKLISTED_URI (decoder->uri))
    goto uri_blacklisted;

  source =
      gst_element_make_from_uri (GST_URI_SRC, decoder->uri, "source", &err);
  if (!source)
    goto no_source;

  GST_LOG_OBJECT (decoder, "found source type %s", G_OBJECT_TYPE_NAME (source));

  source_class = G_OBJECT_GET_CLASS (source);

  pspec = g_object_class_find_property (source_class, "connection-speed");
  if (pspec != NULL) {
    guint64 speed = decoder->connection_speed / 1000;
    gboolean wrong_type = FALSE;

    if (G_PARAM_SPEC_TYPE (pspec) == G_TYPE_PARAM_UINT) {
      GParamSpecUInt *pspecuint = G_PARAM_SPEC_UINT (pspec);

      speed = CLAMP (speed, pspecuint->minimum, pspecuint->maximum);
    } else if (G_PARAM_SPEC_TYPE (pspec) == G_TYPE_PARAM_INT) {
      GParamSpecInt *pspecint = G_PARAM_SPEC_INT (pspec);

      speed = CLAMP (speed, pspecint->minimum, pspecint->maximum);
    } else if (G_PARAM_SPEC_TYPE (pspec) == G_TYPE_PARAM_UINT64) {
      GParamSpecUInt64 *pspecuint = G_PARAM_SPEC_UINT64 (pspec);

      speed = CLAMP (speed, pspecuint->minimum, pspecuint->maximum);
    } else if (G_PARAM_SPEC_TYPE (pspec) == G_TYPE_PARAM_INT64) {
      GParamSpecInt64 *pspecint = G_PARAM_SPEC_INT64 (pspec);

      speed = CLAMP (speed, pspecint->minimum, pspecint->maximum);
    } else {
      GST_WARNING_OBJECT (decoder,
          "The connection speed property %" G_GUINT64_FORMAT
          " of type %s is not useful not setting it", speed,
          g_type_name (G_PARAM_SPEC_TYPE (pspec)));
      wrong_type = TRUE;
    }

    if (!wrong_type) {
      g_object_set (source, "connection-speed", speed, NULL);

      GST_DEBUG_OBJECT (decoder,
          "setting connection-speed=%" G_GUINT64_FORMAT " to source element",
          speed);
    }
  }

  pspec = g_object_class_find_property (source_class, "subtitle-encoding");
  if (pspec != NULL && G_PARAM_SPEC_VALUE_TYPE (pspec) == G_TYPE_STRING) {
    GST_DEBUG_OBJECT (decoder,
        "setting subtitle-encoding=%s to source element", decoder->encoding);
    g_object_set (source, "subtitle-encoding", decoder->encoding, NULL);
  }

  /* Sink reference before passing it to signal handler.
   * Language binding might otherwise sink it and then unref.
   * A floating ref is also tricky for a native signal handler in case
   * of a "transfer floating" call followed by unref
   * (e.g. some container_add and then container_remove).
   * Bottom line; best not have a floating ref boldly going into unknown code.
   */
  g_object_ref_sink (source);

  g_signal_emit (decoder, gst_uri_decode_bin_signals[SIGNAL_SOURCE_SETUP],
      0, source);

  decoder->is_stream = IS_STREAM_URI (decoder->uri);

  query = gst_query_new_scheduling ();
  if (gst_element_query (source, query)) {
    gst_query_parse_scheduling (query, &flags, NULL, NULL, NULL);
    if ((flags & GST_SCHEDULING_FLAG_BANDWIDTH_LIMITED))
      decoder->is_stream = TRUE;
  }
  gst_query_unref (query);

  GST_LOG_OBJECT (decoder, "source is stream: %d", decoder->is_stream);

  decoder->need_queue = IS_QUEUE_URI (decoder->uri);
  GST_LOG_OBJECT (decoder, "source needs queue: %d", decoder->need_queue);

  return source;

  /* ERRORS */
no_uri:
  {
    GST_ELEMENT_ERROR (decoder, RESOURCE, NOT_FOUND,
        (_("No URI specified to play from.")), (NULL));
    return NULL;
  }
invalid_uri:
  {
    GST_ELEMENT_ERROR (decoder, RESOURCE, NOT_FOUND,
        (_("Invalid URI \"%s\"."), decoder->uri), (NULL));
    g_clear_error (&err);
    return NULL;
  }
uri_blacklisted:
  {
    GST_ELEMENT_ERROR (decoder, RESOURCE, FAILED,
        (_("This stream type cannot be played yet.")), (NULL));
    return NULL;
  }
no_source:
  {
    /* whoops, could not create the source element, dig a little deeper to
     * figure out what might be wrong. */
    if (err != NULL && err->code == GST_URI_ERROR_UNSUPPORTED_PROTOCOL) {
      gchar *prot;

      prot = gst_uri_get_protocol (decoder->uri);
      if (prot == NULL)
        goto invalid_uri;

      gst_element_post_message (GST_ELEMENT_CAST (decoder),
          gst_missing_uri_source_message_new (GST_ELEMENT (decoder), prot));

      GST_ELEMENT_ERROR (decoder, CORE, MISSING_PLUGIN,
          (_("No URI handler implemented for \"%s\"."), prot), (NULL));

      g_free (prot);
    } else {
      GST_ELEMENT_ERROR (decoder, RESOURCE, NOT_FOUND,
          ("%s", (err) ? err->message : "URI was not accepted by any element"),
          ("No element accepted URI '%s'", decoder->uri));
    }

    g_clear_error (&err);
    return NULL;
  }
}

关键函数：

（1） gst_uri_is_valid (decoder->uri) 对uri进行校验

static void
gst_uri_protocol_check_internal (const gchar * uri, gchar ** endptr)
{
  gchar *check = (gchar *) uri;

  g_assert (uri != NULL);
  g_assert (endptr != NULL);

  if (g_ascii_isalpha (*check)) {
    check++;
    while (g_ascii_isalnum (*check) || *check == '+'
        || *check == '-' || *check == '.')
      check++;
  }

  *endptr = check;
}

（2）IS_BLACKLISTED_URI (decoder->uri)黑名单校验

/* blacklisted URIs, we know they will always fail. */
static const gchar *blacklisted_uris[] = { NULL };

（3）decoder->is_stream = IS_STREAM_URI (decoder->uri);

/* list of URIs that we consider to be streams and that need buffering.
 * We have no mechanism yet to figure this out with a query. */
static const gchar *stream_uris[] = { "http://", "https://", "mms://",
  "mmsh://", "mmsu://", "mmst://", "fd://", "myth://", "ssh://",
  "ftp://", "sftp://",
  NULL
};

（4）decoder->need_queue = IS_QUEUE_URI (decoder->uri);

/* list of URIs that need a queue because they are pretty bursty */
static const gchar *queue_uris[] = { "cdda://", NULL };

最关键函数：

source = gst_element_make_from_uri (GST_URI_SRC, decoder->uri, "source", &err);

/**
 * gst_element_make_from_uri:
 * @type: Whether to create a source or a sink
 * @uri: URI to create an element for
 * @elementname: (allow-none): Name of created element, can be %NULL.
 * @error: (allow-none): address where to store error information, or %NULL.
 *
 * Creates an element for handling the given URI.
 *
 * Returns: (transfer floating): a new element or %NULL if none
 * could be created
 */
GstElement *
gst_element_make_from_uri (const GstURIType type, const gchar * uri,
    const gchar * elementname, GError ** error)
{
  GList *possibilities, *walk;
  gchar *protocol;
  GstElement *ret = NULL;

  g_return_val_if_fail (gst_is_initialized (), NULL);
  g_return_val_if_fail (GST_URI_TYPE_IS_VALID (type), NULL);
  g_return_val_if_fail (error == NULL || *error == NULL, NULL);

  if (!gst_uri_is_valid (uri)) {
    g_set_error (error, GST_URI_ERROR, GST_URI_ERROR_BAD_URI,
        _("Invalid URI: %s"), uri);
    return NULL;
  }

  GST_DEBUG ("type:%d, uri:%s, elementname:%s", type, uri, elementname);

  protocol = gst_uri_get_protocol (uri);
  possibilities = get_element_factories_from_uri_protocol (type, protocol);

  if (!possibilities) {
    GST_DEBUG ("No %s for URI '%s'", type == GST_URI_SINK ? "sink" : "source",
        uri);
    /* The error message isn't great, but we don't expect applications to
     * show that error to users, but call the missing plugins functions */
    g_set_error (error, GST_URI_ERROR, GST_URI_ERROR_UNSUPPORTED_PROTOCOL,
        _("No URI handler for the %s protocol found"), protocol);
    g_free (protocol);
    return NULL;
  }
  g_free (protocol);

  possibilities = g_list_sort (possibilities, (GCompareFunc) sort_by_rank);
  walk = possibilities;
  while (walk) {
    GstElementFactory *factory = walk->data;
    GError *uri_err = NULL;

    ret = gst_element_factory_create (factory, elementname);
    if (ret != NULL) {
      GstURIHandler *handler = GST_URI_HANDLER (ret);

      if (gst_uri_handler_set_uri (handler, uri, &uri_err))
        break;

      GST_WARNING ("%s didn't accept URI '%s': %s", GST_OBJECT_NAME (ret), uri,
          uri_err->message);

      if (error != NULL && *error == NULL)
        g_propagate_error (error, uri_err);
      else
        g_error_free (uri_err);

      gst_object_unref (ret);
      ret = NULL;
    }
    walk = walk->next;
  }
  gst_plugin_feature_list_free (possibilities);

  GST_LOG_OBJECT (ret, "created %s for URL '%s'",
      type == GST_URI_SINK ? "sink" : "source", uri);

  /* if the first handler didn't work, but we found another one that works */
  if (ret != NULL)
    g_clear_error (error);

  return ret;
}

（1）protocol = gst_uri_get_protocol (uri) 获取协议，例如http/https/ftp等

（2）possibilities = get_element_factories_from_uri_protocol (type, protocol)，从注册表中搜索可匹配上的source的lists

类型：GST_URI_SRC 协议：protocols

static GList *
get_element_factories_from_uri_protocol (const GstURIType type,
    const gchar * protocol)
{
  GList *possibilities;
  SearchEntry entry;

  g_return_val_if_fail (protocol, NULL);

  entry.type = type;
  entry.protocol = protocol;
  possibilities = gst_registry_feature_filter (gst_registry_get (),
      search_by_entry, FALSE, &entry);

  return possibilities;
}

（3）对possibilities排序，从中选择最终匹配的source

  possibilities = g_list_sort (possibilities, (GCompareFunc) sort_by_rank);
  walk = possibilities;
  while (walk) {
    GstElementFactory *factory = walk->data;
    GError *uri_err = NULL;

    ret = gst_element_factory_create (factory, elementname);
    if (ret != NULL) {
      GstURIHandler *handler = GST_URI_HANDLER (ret);

      if (gst_uri_handler_set_uri (handler, uri, &uri_err))
        break;

      GST_WARNING ("%s didn't accept URI '%s': %s", GST_OBJECT_NAME (ret), uri,
          uri_err->message);

      if (error != NULL && *error == NULL)
        g_propagate_error (error, uri_err);
      else
        g_error_free (uri_err);

      gst_object_unref (ret);
      ret = NULL;
    }
    walk = walk->next;
  }

a）根据rank排序

b）对每一个source尝试gst_uri_handler_set_uri ，返回成功则选择该source插件

    ret = gst_element_factory_create (factory, elementname);
    if (ret != NULL) {
      GstURIHandler *handler = GST_URI_HANDLER (ret);
      if (gst_uri_handler_set_uri (handler, uri, &uri_err))
        break;

c) gst_uri_handler_set_uri主要是从，soruce中获取接口iface = GST_URI_HANDLER_GET_INTERFACE (handler)，对比source支持的get_protocols是否与当前uri匹配，匹配则成功。

/**
 * gst_uri_handler_set_uri:
 * @handler: A #GstURIHandler
 * @uri: URI to set
 * @error: (allow-none): address where to store a #GError in case of
 *    an error, or %NULL
 *
 * Tries to set the URI of the given handler.
 *
 * Returns: %TRUE if the URI was set successfully, else %FALSE.
 */
gboolean
gst_uri_handler_set_uri (GstURIHandler * handler, const gchar * uri,
    GError ** error)
{
  GstURIHandlerInterface *iface;
  gboolean ret;
  gchar *protocol;

  g_return_val_if_fail (GST_IS_URI_HANDLER (handler), FALSE);
  g_return_val_if_fail (error == NULL || *error == NULL, FALSE);

  iface = GST_URI_HANDLER_GET_INTERFACE (handler);
  g_return_val_if_fail (iface != NULL, FALSE);
  g_return_val_if_fail (iface->set_uri != NULL, FALSE);

  if (!gst_uri_is_valid (uri)) {
    g_set_error (error, GST_URI_ERROR, GST_URI_ERROR_BAD_URI,
        _("Invalid URI: %s"), uri);
    return FALSE;
  }

  protocol = gst_uri_get_protocol (uri);

  if (iface->get_protocols) {
    const gchar *const *protocols;
    const gchar *const *p;
    gboolean found_protocol = FALSE;

    protocols = iface->get_protocols (G_OBJECT_TYPE (handler));
    if (protocols != NULL) {
      for (p = protocols; *p != NULL; ++p) {
        if (g_ascii_strcasecmp (protocol, *p) == 0) {
          found_protocol = TRUE;
          break;
        }
      }

      if (!found_protocol) {
        g_set_error (error, GST_URI_ERROR, GST_URI_ERROR_UNSUPPORTED_PROTOCOL,
            _("URI scheme '%s' not supported"), protocol);
        g_free (protocol);
        return FALSE;
      }
    }
  }

  ret = iface->set_uri (handler, uri, error);

  g_free (protocol);

  return ret;
}

备注：

Source插件需要实现接口GstURIHandlerInterface

/**
 * GstURIHandlerInterface:
 * @parent: The parent interface type
 * @get_type: Method to tell whether the element handles source or sink URI.
 * @get_protocols: Method to return the list of protocols handled by the element.
 * @get_uri: Method to return the URI currently handled by the element.
 * @set_uri: Method to set a new URI.
 *
 * Any #GstElement using this interface should implement these methods.
 */
struct _GstURIHandlerInterface {
  GTypeInterface parent;

  /* vtable */
  /*< public >*/
  /* querying capabilities */
  GstURIType             (* get_type)           (GType type);
  const gchar * const *  (* get_protocols)      (GType type);

  /* using the interface */
  gchar *                (* get_uri)            (GstURIHandler * handler);
  gboolean               (* set_uri)            (GstURIHandler * handler,
                                                 const gchar   * uri,
                                                 GError       ** error);
};

souphttpsrc的例子：

G_DEFINE_TYPE_WITH_CODE (GstSoupHTTPSrc, gst_soup_http_src, GST_TYPE_PUSH_SRC,
    G_IMPLEMENT_INTERFACE (GST_TYPE_URI_HANDLER,
        gst_soup_http_src_uri_handler_init));

static void
gst_soup_http_src_uri_handler_init (gpointer g_iface, gpointer iface_data)
{
  GstURIHandlerInterface *iface = (GstURIHandlerInterface *) g_iface;

  iface->get_type = gst_soup_http_src_uri_get_type;
  iface->get_protocols = gst_soup_http_src_uri_get_protocols;
  iface->get_uri = gst_soup_http_src_uri_get_uri;
  iface->set_uri = gst_soup_http_src_uri_set_uri;
}

（6）那我们继续看下gst_element_make_from_uri中的关键步骤 possibilities = get_element_factories_from_uri_protocol (type, protocol)->gst_registry_feature_filter

  possibilities = gst_registry_feature_filter (gst_registry_get (),
      search_by_entry, FALSE, &entry);

主要逻辑就是从注册表中匹配对应的source:

static gboolean
search_by_entry (GstPluginFeature * feature, gpointer search_entry)
{
  const gchar *const *protocols;
  GstElementFactory *factory;
  SearchEntry *entry = (SearchEntry *) search_entry;

  if (!GST_IS_ELEMENT_FACTORY (feature))
    return FALSE;
  factory = GST_ELEMENT_FACTORY_CAST (feature);

  if (factory->uri_type != entry->type)
    return FALSE;

  protocols = gst_element_factory_get_uri_protocols (factory);

  if (protocols == NULL) {
    g_warning ("Factory '%s' implements GstUriHandler interface but returned "
        "no supported protocols!", gst_plugin_feature_get_name (feature));
    return FALSE;
  }

  while (*protocols != NULL) {
    if (g_ascii_strcasecmp (*protocols, entry->protocol) == 0)
      return TRUE;
    protocols++;
  }
  return FALSE;
}

/**
 * gst_element_factory_get_uri_protocols:
 * @factory: a #GstElementFactory
 *
 * Gets a %NULL-terminated array of protocols this element supports or %NULL if
 * no protocols are supported. You may not change the contents of the returned
 * array, as it is still owned by the element factory. Use g_strdupv() to
 * make a copy of the protocol string array if you need to.
 *
 * Returns: (transfer none) (array zero-terminated=1): the supported protocols
 *     or %NULL
 */
const gchar *const *
gst_element_factory_get_uri_protocols (GstElementFactory * factory)
{
  g_return_val_if_fail (GST_IS_ELEMENT_FACTORY (factory), NULL);

  return (const gchar * const *) factory->uri_protocols;
}

看下几个常见的source插件的信息：

gst-inspect-1.0 soupsrc

Factory Details:
  Rank                     primary (256)
  Long-name                HTTP client source
  Klass                    Source/Network
  Description              Receive data as a client over the network via HTTP using SOUP
  Author                   Wouter Cloetens <wouter@mind.be>

Plugin Details:
  Name                     soup
  Description              libsoup HTTP client src/sink
  Filename                 /libgstsoup.so
  Version                  1.20
  License                  LGPL
  Source module            gst-plugins-good
  Binary package           GStreamer Good Plug-ins git
  Origin URL               Unknown package origin

GObject
 +----GInitiallyUnowned
       +----GstObject
             +----GstElement
                   +----GstBaseSrc
                         +----GstPushSrc
                               +----GstSoupHTTPSrc

Implemented Interfaces:
  GstURIHandler

Pad Templates:
  SRC template: 'src'
    Availability: Always
    Capabilities:
      ANY

Element has no clocking capabilities.

URI handling capabilities:
  Element can act as source.
  Supported URI protocols:
    http
    https
    icy
    icyx

Pads:
  SRC: 'src'
    Pad Template: 'src'

Element Properties:
  automatic-redirect  : Automatically follow HTTP redirects (HTTP Status Code 3xx)
                        flags: readable, writable
                        Boolean. Default: true
  blocksize           : Size in bytes to read per buffer (-1 = default)
                        flags: readable, writable
                        Unsigned Integer. Range: 0 - 4294967295 Default: 4096
  compress            : Allow compressed content encodings
                        flags: readable, writable
                        Boolean. Default: false
  cookies             : HTTP request cookies
                        flags: readable, writable
                        Boxed pointer of type "GStrv"
  do-timestamp        : Apply current stream time to buffers
                        flags: readable, writable
                        Boolean. Default: false
  extra-headers       : Extra headers to append to the HTTP request
                        flags: readable, writable
                        Boxed pointer of type "GstStructure"
  http-log-level      : Set log level for soup's HTTP session log
                        flags: readable, writable
                        Enum "SoupLoggerLogLevel" Default: 2, "headers"
                           (0): none             - SOUP_LOGGER_LOG_NONE
                           (1): minimal          - SOUP_LOGGER_LOG_MINIMAL
                           (2): headers          - SOUP_LOGGER_LOG_HEADERS
                           (3): body             - SOUP_LOGGER_LOG_BODY
  iradio-mode         : Enable internet radio mode (ask server to send shoutcast/icecast metadata interleaved with the actual stream data)
                        flags: readable, writable
                        Boolean. Default: true
  is-live             : Act like a live source
                        flags: readable, writable
                        Boolean. Default: false
  keep-alive          : Use HTTP persistent connections
                        flags: readable, writable
                        Boolean. Default: true
  location            : Location to read from
                        flags: readable, writable
                        String. Default: null
  method              : The HTTP method to use (GET, HEAD, OPTIONS, etc)
                        flags: readable, writable
                        String. Default: null
  name                : The name of the object
                        flags: readable, writable, 0x2000
                        String. Default: "souphttpsrc0"
  num-buffers         : Number of buffers to output before sending EOS (-1 = unlimited)
                        flags: readable, writable
                        Integer. Range: -1 - 2147483647 Default: -1
  parent              : The parent of the object
                        flags: readable, writable, 0x2000
                        Object of type "GstObject"
  proxy               : HTTP proxy server URI
                        flags: readable, writable
                        String. Default: ""
  proxy-id            : HTTP proxy URI user id for authentication
                        flags: readable, writable
                        String. Default: null
  proxy-pw            : HTTP proxy URI user password for authentication
                        flags: readable, writable
                        String. Default: null
  retries             : Maximum number of retries until giving up (-1=infinite)
                        flags: readable, writable
                        Integer. Range: -1 - 2147483647 Default: 3
  ssl-ca-file         : Location of a SSL anchor CA file to use
                        flags: readable, writable
                        String. Default: null
  ssl-strict          : Strict SSL certificate checking
                        flags: readable, writable
                        Boolean. Default: true
  ssl-use-system-ca-file: Use system CA file
                        flags: readable, writable
                        Boolean. Default: true
  timeout             : Value in seconds to timeout a blocking I/O (0 = No timeout).
                        flags: readable, writable
                        Unsigned Integer. Range: 0 - 3600 Default: 15
  tls-database        : TLS database with anchor certificate authorities used to validate the server certificate
                        flags: readable, writable
                        Object of type "GTlsDatabase"
  tls-interaction     : A GTlsInteraction object to be used when the connection or certificate database need to interact with the user.
                        flags: readable, writable
                        Object of type "GTlsInteraction"
  typefind            : Run typefind before negotiating (deprecated, non-functional)
                        flags: readable, writable, deprecated
                        Boolean. Default: false
  user-agent          : Value of the User-Agent HTTP request header field
                        flags: readable, writable
                        String. Default: "GStreamer souphttpsrc 1.20.6.1 "
  user-id             : HTTP location URI user id for authentication
                        flags: readable, writable
                        String. Default: null
  user-pw             : HTTP location URI user password for authentication
                        flags: readable, writable
                        String. Default: null

gst-inspect-1.0 filesrc

Factory Details:
  Rank                     primary (256)
  Long-name                File Source
  Klass                    Source/File
  Description              Read from arbitrary point in a file
  Author                   Erik Walthinsen <omega@cse.ogi.edu>

Plugin Details:
  Name                     coreelements
  Description              GStreamer core elements
  Filename                 /libgstcoreelements.so
  Version                  1.20
  License                  LGPL
  Source module            gstreamer
  Binary package           GStreamer git
  Origin URL               Unknown package origin

GObject
 +----GInitiallyUnowned
       +----GstObject
             +----GstElement
                   +----GstBaseSrc
                         +----GstFileSrc

Implemented Interfaces:
  GstURIHandler

Pad Templates:
  SRC template: 'src'
    Availability: Always
    Capabilities:
      ANY

Element has no clocking capabilities.

URI handling capabilities:
  Element can act as source.
  Supported URI protocols:
    file

Pads:
  SRC: 'src'
    Pad Template: 'src'

Element Properties:
  blocksize           : Size in bytes to read per buffer (-1 = default)
                        flags: readable, writable
                        Unsigned Integer. Range: 0 - 4294967295 Default: 4096
  do-timestamp        : Apply current stream time to buffers
                        flags: readable, writable
  location            : Location of the file to read
                        flags: readable, writable, changeable only in NULL or READY state
                        String. Default: null
  name                : The name of the object
                        flags: readable, writable, 0x2000
                        String. Default: "filesrc0"
  num-buffers         : Number of buffers to output before sending EOS (-1 = unlimited)
                        flags: readable, writable
                        Integer. Range: -1 - 2147483647 Default: -1
  parent              : The parent of the object
                        flags: readable, writable, 0x2000
                        Object of type "GstObject"
  typefind            : Run typefind before negotiating (deprecated, non-functional)
                        flags: readable, writable, deprecated
                        Boolean. Default: false