文章目录
1.Goal
有时您可能想快速找出文件（或URI）包含哪种媒体，或者您是否能够播放所有媒体。 您可以构建一个管道，将其设置为运行，并查看总线消息，但是GStreamer具有一个实用程序可以为您完成此任务。 本教程显示：
- 如何恢复有关URI的信息
- 如何确定URI是否可播放
2.介绍
GstDiscoverer是在pbutils库（插件基础实用程序）中找到的实用程序对象，该对象接受URI或URI列表，并返回有关它们的信息。 它可以在同步或异步模式下工作。
在同步模式下，只有一个要调用的函数gst_discoverer_discover_uri会阻塞，直到信息准备就绪为止。 由于这种阻塞，通常对于基于GUI的应用程序来说不太有趣，并且使用了异步模式，如本教程中所述。
恢复的信息包括编解码器描述，流拓扑（流和子流的数量）和可用的元数据（如音频语言）。
例如，这是https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm的结果。
[外链图片转存失败,源站可能有防盗链机制,建议将图片保存下来直接上传(img-S5cXmR95-1623032260818)(:/b1d136ffd4f24504813101ca78ac4a45)]
以下代码尝试发现通过命令行提供的URI，并输出检索到的信息（如果未提供URI，则使用默认值）。
这是gst-discoverer-1.0工具的简化版本（基础教程10：GStreamer工具），该工具仅显示数据，不执行任何播放。
3.The GStreamer Discoverer
3.1 Compile
gcc basic-tutorial-9.c -o basic-tutorial-9 `pkg-config --cflags --libs gstreamer-1.0 gstreamer-pbutils-1.0`
3.2 Code
#include <string.h>
#include <gst/gst.h>
#include <gst/pbutils/pbutils.h>
/* Structure to contain all our information, so we can pass it around */
typedef struct _CustomData {
GstDiscoverer *discoverer;
GMainLoop *loop;
} CustomData;
/* Print a tag in a human-readable format (name: value) */
static void print_tag_foreach (const GstTagList *tags, const gchar *tag, gpointer user_data) {
GValue val = { 0, };
gchar *str;
gint depth = GPOINTER_TO_INT (user_data);
gst_tag_list_copy_value (&val, tags, tag);
if (G_VALUE_HOLDS_STRING (&val))
str = g_value_dup_string (&val);
else
str = gst_value_serialize (&val);
g_print ("%*s%s: %s\n", 2 * depth, " ", gst_tag_get_nick (tag), str);
g_free (str);
g_value_unset (&val);
}
/* Print information regarding a stream */
static void print_stream_info (GstDiscovererStreamInfo *info, gint depth) {
gchar *desc = NULL;
GstCaps *caps;
const GstTagList *tags;
caps = gst_discoverer_stream_info_get_caps (info);
if (caps) {
if (gst_caps_is_fixed (caps))
desc = gst_pb_utils_get_codec_description (caps);
else
desc = gst_caps_to_string (caps);
gst_caps_unref (caps);
}
g_print ("%*s%s: %s\n", 2 * depth, " ", gst_discoverer_stream_info_get_stream_type_nick (info), (desc ? desc : ""));
if (desc) {
g_free (desc);
desc = NULL;
}
tags = gst_discoverer_stream_info_get_tags (info);
if (tags) {
g_print ("%*sTags:\n", 2 * (depth + 1), " ");
gst_tag_list_foreach (tags, print_tag_foreach, GINT_TO_POINTER (depth + 2));
}
}
/* Print information regarding a stream and its substreams, if any */
static void print_topology (GstDiscovererStreamInfo *info, gint depth) {
GstDiscovererStreamInfo *next;
if (!info)
return;
print_stream_info (info, depth);
next = gst_discoverer_stream_info_get_next (info);
if (next) {
print_topology (next, depth + 1);
gst_discoverer_stream_info_unref (next);
} else if (GST_IS_DISCOVERER_CONTAINER_INFO (info)) {
GList *tmp, *streams;
streams = gst_discoverer_container_info_get_streams (GST_DISCOVERER_CONTAINER_INFO (info));
for (tmp = streams; tmp; tmp = tmp->next) {
GstDiscovererStreamInfo *tmpinf = (GstDiscovererStreamInfo *) tmp->data;
print_topology (tmpinf, depth + 1);
}
gst_discoverer_stream_info_list_free (streams);
}
}
/* This function is called every time the discoverer has information regarding
* one of the URIs we provided.*/
static void on_discovered_cb (GstDiscoverer *discoverer, GstDiscovererInfo *info, GError *err, CustomData *data) {
GstDiscovererResult result;
const gchar *uri;
const GstTagList *tags;
GstDiscovererStreamInfo *sinfo;
uri = gst_discoverer_info_get_uri (info);
result = gst_discoverer_info_get_result (info);
switch (result) {
case GST_DISCOVERER_URI_INVALID:
g_print ("Invalid URI '%s'\n", uri);
break;
case GST_DISCOVERER_ERROR:
g_print ("Discoverer error: %s\n", err->message);
break;
case GST_DISCOVERER_TIMEOUT:
g_print ("Timeout\n");
break;
case GST_DISCOVERER_BUSY:
g_print ("Busy\n");
break;
case GST_DISCOVERER_MISSING_PLUGINS:{
const GstStructure *s;
gchar *str;
s = gst_discoverer_info_get_misc (info);
str = gst_structure_to_string (s);
g_print ("Missing plugins: %s\n", str);
g_free (str);
break;
}
case GST_DISCOVERER_OK:
g_print ("Discovered '%s'\n", uri);
break;
}
if (result != GST_DISCOVERER_OK) {
g_printerr ("This URI cannot be played\n");
return;
}
/* If we got no error, show the retrieved information */
g_print ("\nDuration: %" GST_TIME_FORMAT "\n", GST_TIME_ARGS (gst_discoverer_info_get_duration (info)));
tags = gst_discoverer_info_get_tags (info);
if (tags) {
g_print ("Tags:\n");
gst_tag_list_foreach (tags, print_tag_foreach, GINT_TO_POINTER (1));
}
g_print ("Seekable: %s\n", (gst_discoverer_info_get_seekable (info) ? "yes" : "no"));
g_print ("\n");
sinfo = gst_discoverer_info_get_stream_info (info);
if (!sinfo)
return;
g_print ("Stream information:\n");
print_topology (sinfo, 1);
gst_discoverer_stream_info_unref (sinfo);
g_print ("\n");
}
/* This function is called when the discoverer has finished examining
* all the URIs we provided.*/
static void on_finished_cb (GstDiscoverer *discoverer, CustomData *data) {
g_print ("Finished discovering\n");
g_main_loop_quit (data->loop);
}
int main (int argc, char **argv) {
CustomData data;
GError *err = NULL;
gchar *uri = "https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm";
/* if a URI was provided, use it instead of the default one */
if (argc > 1) {
uri = argv[1];
}
/* Initialize cumstom data structure */
memset (&data, 0, sizeof (data));
/* Initialize GStreamer */
gst_init (&argc, &argv);
g_print ("Discovering '%s'\n", uri);
/* Instantiate the Discoverer */
data.discoverer = gst_discoverer_new (5 * GST_SECOND, &err);
if (!data.discoverer) {
g_print ("Error creating discoverer instance: %s\n", err->message);
g_clear_error (&err);
return -1;
}
/* Connect to the interesting signals */
g_signal_connect (data.discoverer, "discovered", G_CALLBACK (on_discovered_cb), &data);
g_signal_connect (data.discoverer, "finished", G_CALLBACK (on_finished_cb), &data);
/* Start the discoverer process (nothing to do yet) */
gst_discoverer_start (data.discoverer);
/* Add a request to process asynchronously the URI passed through the command line */
if (!gst_discoverer_discover_uri_async (data.discoverer, uri)) {
g_print ("Failed to start discovering URI '%s'\n", uri);
g_object_unref (data.discoverer);
return -1;
}
/* Create a GLib Main Loop and set it to run, so we can wait for the signals */
data.loop = g_main_loop_new (NULL, FALSE);
g_main_loop_run (data.loop);
/* Stop the discoverer process */
gst_discoverer_stop (data.discoverer);
/* Free resources */
g_object_unref (data.discoverer);
g_main_loop_unref (data.loop);
return 0;
}
4.解析
这些是使用GstDiscoverer的主要步骤
/* Instantiate the Discoverer */
data.discoverer = gst_discoverer_new (5 * GST_SECOND, &err);
if (!data.discoverer) {
g_print ("Error creating discoverer instance: %s\n", err->message);
g_clear_error (&err);
return -1;
}
gst_discoverer_new创建一个新的Discoverer对象。 第一个参数是每个文件的超时时间，以纳秒为单位（为简单起见，请使用GST_SECOND宏）。
/* Connect to the interesting signals */
g_signal_connect (data.discoverer, "discovered", G_CALLBACK (on_discovered_cb), &data);
g_signal_connect (data.discoverer, "finished", G_CALLBACK (on_finished_cb), &data);
链接感兴趣的signal。
/* Start the discoverer process (nothing to do yet) */
gst_discoverer_start (data.discoverer);
gst_discoverer_start启动发现过程，但我们尚未提供任何要发现的URI。 接下来完成：
/* Add a request to process asynchronously the URI passed through the command line */
if (!gst_discoverer_discover_uri_async (data.discoverer, uri)) {
g_print ("Failed to start discovering URI '%s'\n", uri);
g_object_unref (data.discoverer);
return -1;
}
gst_discoverer_discover_uri_async将提供的URI放入队列中进行发现。可以使用此功能将多个URI排队。当它们的发现过程完成时，将触发已注册的回调函数。
/* Create a GLib Main Loop and set it to run, so we can wait for the signals */
data.loop = g_main_loop_new (NULL, FALSE);
g_main_loop_run (data.loop);
通常的GLib主循环被实例化并执行。当从on_finished_cb回调调用g_main_loop_quit时，我们将退出循环。
/* Stop the discoverer process */
gst_discoverer_stop (data.discoverer);
discoverer完成操作后，请使用gst_discoverer_stop停止它，并使用g_object_unref取消引用它。
现在让我们回顾一下我们已经注册的回调on_discovered_cb：
/* This function is called every time the discoverer has information regarding
* one of the URIs we provided.*/
static void on_discovered_cb (GstDiscoverer *discoverer, GstDiscovererInfo *info, GError *err, CustomData *data) {
GstDiscovererResult result;
const gchar *uri;
const GstTagList *tags;
GstDiscovererStreamInfo *sinfo;
uri = gst_discoverer_info_get_uri (info);
result = gst_discoverer_info_get_result (info);
在Discoverer已完成对一个URI的处理时该函数被调用，并为我们提供了包含所有信息的GstDiscovererInfo结构。
第一步是使用gst_discoverer_info_get_uri检索此调用引用的特定URI（有时会运行多个发现进程，在本示例中不是这种情况），并使用gst_discoverer_info_get_result()检索发现结果。
switch (result) {
case GST_DISCOVERER_URI_INVALID:
g_print ("Invalid URI '%s'\n", uri);
break;
case GST_DISCOVERER_ERROR:
g_print ("Discoverer error: %s\n", err->message);
break;
case GST_DISCOVERER_TIMEOUT:
g_print ("Timeout\n");
break;
case GST_DISCOVERER_BUSY:
g_print ("Busy\n");
break;
case GST_DISCOVERER_MISSING_PLUGINS:{
const GstStructure *s;
gchar *str;
s = gst_discoverer_info_get_misc (info);
str = gst_structure_to_string (s);
g_print ("Missing plugins: %s\n", str);
g_free (str);
break;
}
case GST_DISCOVERER_OK:
g_print ("Discovered '%s'\n", uri);
break;
}
if (result != GST_DISCOVERER_OK) {
g_printerr ("This URI cannot be played\n");
return;
}
如代码所示，GST_DISCOVERER_OK以外的任何结果都意味着存在某种问题，并且该URI无法播放。原因可能会有所不同，但是枚举值非常明确（GST_DISCOVERER_BUSY仅在同步模式下会发生，本示例中未使用）。
如果没有发生错误，则可以使用不同的gst_discoverer_info_get_*方法（例如，例如gst_discoverer_info_get_duration）从GstDiscovererInfo结构中检索信息。
由列表组成的信息位，例如tags和stream info，需要一些额外的解析：
tags = gst_discoverer_info_get_tags (info);
if (tags) {
g_print ("Tags:\n");
gst_tag_list_foreach (tags, print_tag_foreach, GINT_TO_POINTER (1));
}
tags是附加到媒体的元数据（标签）。可以使用gst_tag_list_foreach检查它们，该函数将为找到的每个tag调用print_tag_foreach（例如，也可以手动遍历列表，或者可以使用gst_tag_list_get_string搜索特定的tag）。print_tag_foreach的代码几乎是不言自明的。
sinfo = gst_discoverer_info_get_stream_info (info);
if (!sinfo)
return;
g_print ("Stream information:\n");
print_topology (sinfo, 1);
gst_discoverer_stream_info_unref (sinfo);
gst_discoverer_info_get_stream_info返回一个GstDiscovererStreamInfo结构，该结构在print_topology函数中进行了解析，然后被gst_discoverer_stream_info_unref丢弃。
/* Print information regarding a stream and its substreams, if any */
static void print_topology (GstDiscovererStreamInfo *info, gint depth) {
GstDiscovererStreamInfo *next;
if (!info)
return;
print_stream_info (info, depth);
next = gst_discoverer_stream_info_get_next (info);
if (next) {
print_topology (next, depth + 1);
gst_discoverer_stream_info_unref (next);
} else if (GST_IS_DISCOVERER_CONTAINER_INFO (info)) {
GList *tmp, *streams;
streams = gst_discoverer_container_info_get_streams (GST_DISCOVERER_CONTAINER_INFO (info));
for (tmp = streams; tmp; tmp = tmp->next) {
GstDiscovererStreamInfo *tmpinf = (GstDiscovererStreamInfo *) tmp->data;
print_topology (tmpinf, depth + 1);
}
gst_discoverer_stream_info_list_free (streams);
}
}
print_stream_info函数的代码也很不言自明：它也使用print_tag_foreach打印流的功能，然后打印相关的caps。
然后，print_topology寻找下一个要显示的元素。 如果gst_discoverer_stream_info_get_next返回一个非NULL流信息，则它指向我们的后代，并且应该显示该信息。 否则，如果我们是容器，则对通过gst_discoverer_container_info_get_streams获得的每个子代递归调用print_topology。 否则，我们是最终的流，不需要递归（公认的Discoverer API的这一部分有些晦涩）。
Ref
- gst_discoverer_new
GstDiscoverer *gst_discoverer_new(GstClockTime timout, GError **error))
创建具有timeout的GstDiscoverer。
timeout以纳秒为单位，范围为1s1h(GST_SECOND3600*GST_SECOND)
- gst_discoverer_start
- gst_discoverer_discover_uri_async
- gst_discoverer_info_get_uri
- gst_discoverer_info_get_result
- gst-discoverer_info_get_misc
- gst_structure_to_string
- gst_discoverer_info_get_*
- gst_tag_list_foreach
- gst_discoverer_stream_info_get_next
- gst_discoverer_container_info_get_streams
5.讨论
本教程显示：
- 如何使用GstDiscoverer恢复有关URI的信息
- 如何通过查看使用gst_discoverer_info_get_result获得的返回码来确定URI是否可播放