GStreamer的Decodebin插件

Decodebin是playbin的实际自动插件后端，想要做更深层次的解码定制工作，可以使用Decodebin来实现，下面是官方的例程代码。

#include 


[.. my_bus_callback goes here ..]



GstElement *pipeline, *audio;

static void
cb_newpad (GstElement *decodebin,
       GstPad     *pad,
       gpointer    data)
{
  GstCaps *caps;
  GstStructure *str;
  GstPad *audiopad;

  /* only link once */
  audiopad = gst_element_get_static_pad (audio, "sink");
  if (GST_PAD_IS_LINKED (audiopad)) {
    g_object_unref (audiopad);
    return;
  }

  /* check media type */
  caps = gst_pad_query_caps (pad, NULL);
  str = gst_caps_get_structure (caps, 0);
  if (!g_strrstr (gst_structure_get_name (str), "audio")) {
    gst_caps_unref (caps);
    gst_object_unref (audiopad);
    return;
  }
  gst_caps_unref (caps);

  /* link'n'play */
  gst_pad_link (pad, audiopad);

  g_object_unref (audiopad);
}

gint
main (gint   argc,
      gchar *argv[])
{
  GMainLoop *loop;
  GstElement *src, *dec, *conv, *sink;
  GstPad *audiopad;
  GstBus *bus;

  /* init GStreamer */
  gst_init (&argc, &argv);
  loop = g_main_loop_new (NULL, FALSE);

  /* make sure we have input */
  if (argc != 2) {
    g_print ("Usage: %s \n", argv[0]);
    return -1;
  }

  /* setup */
  pipeline = gst_pipeline_new ("pipeline");

  bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
  gst_bus_add_watch (bus, my_bus_callback, loop);
  gst_object_unref (bus);

  //创建一个filesrc元素，命名为source
  src = gst_element_factory_make ("filesrc", "source");
  //设置属性名，及文件源
  g_object_set (G_OBJECT (src), "location", argv[1], NULL);
  //创建一个decodebin元素
  dec = gst_element_factory_make ("decodebin", "decoder");
  //将dec的pad-added操作与cb_newpad函数做信号连接
  //pad类似是元素上的端口
  g_signal_connect (dec, "pad-added", G_CALLBACK (cb_newpad), NULL);
  //将src(filesrc)，dec(decodebin)两个元素加入到pipeline上
  gst_bin_add_many (GST_BIN (pipeline), src, dec, NULL);
  //将两个元素连接
  gst_element_link (src, dec);

  /* create audio output */
  audio = gst_bin_new ("audiobin");
  //创建一个audioconvert作为音频输出转换操作
  conv = gst_element_factory_make ("audioconvert", "aconv");
  //获取转换接口的pad
  audiopad = gst_element_get_static_pad (conv, "sink");
  //创建alsasink作为输出
  sink = gst_element_factory_make ("alsasink", "sink");
  //给bin添加元素
  gst_bin_add_many (GST_BIN (audio), conv, sink, NULL);
  //将conv与sink连接，audioconvert--->alsasink
  gst_element_link (conv, sink);
  //给audio添加conv的pad
  gst_element_add_pad (audio,
      gst_ghost_pad_new ("sink", audiopad));
  //减少audiopad的引用计数，当计数为0则销毁
  //永远不要在保持LOCK的情况下调用unref方法，因为这可能会死锁dispose函数
  gst_object_unref (audiopad);
  //将audio加入到pipeline
  gst_bin_add (GST_BIN (pipeline), audio);

  /* run */
  gst_element_set_state (pipeline, GST_STATE_PLAYING);
  g_main_loop_run (loop);

  /* cleanup */
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (GST_OBJECT (pipeline));

  return 0;
}