macOS上的GStreamer主事件循环

问题描述

我正在经历macOS上的官方GStreamer tutorials。在进行first basic tutorial时，我注意到视频没有播放，我只能听到音频。我在网上搜索了问题，并遇到了this的答案，该答案建议添加一个GMainLoop类型的main event loop来轮询事件源并调用事件处理程序。添加循环并调用函数g_main_loop_run之后，问题已解决，我可以看到视频；但是，我仍然不清楚为什么在本教程在Ubuntu上按原样运行（没有事件循环）的情况下为什么在macOS上需要这样的事件循环。

无论如何，我继续教程，来到basic-tutorial-4，在其中添加了消息处理程序以查询管道中的流位置或持续时间，并寻找流中的其他位置（时间）。在这里，我注意到，如果我尝试执行相同的技巧并在将g_main_loop_run的状态设置为playbin之后调用GST_STATE_PLAYING（请参见下面的代码），则视频和音频将按预期播放，但是其余代码似乎已被阻止；即我既无法从总线上获取任何消息，也无法在流中进行查询或查找。如果删除主事件循环，则该教程将按预期工作，并且可以进行查询/查找。但是，由于缺少主事件循环，因此无法播放视频。

如何配置主事件循环以获得所需的行为，即既可以观看视频又可以在流中查询和查找？

/*  Basic Tutorial 4: Time Management */

#include <gst/gst.h>

typedef struct _CustomData {
  GstElement *playbin;
  gboolean playing;
  gboolean terminate;
  gboolean seek_enabled;
  gboolean seek_done;
  gint64 duration;
} CustomData;

static void handle_message(CustomData *data,GstMessage *msg);

int main( int argc,char* argv[] )
{
  CustomData data;
  GstBus *bus;
  GstMessage *msg;
  GstStateChangeReturn ret;
  GMainLoop *main_loop;

  data.playing = FALSE;
  data.terminate = FALSE;
  data.seek_enabled = FALSE;
  data.seek_done = FALSE;
  data.duration = GST_CLOCK_TIME_NONE;
  
  /* Initialize GStreamer */
  gst_init (&argc,&argv);

  /* Create the elements */
  data.playbin = gst_element_factory_make ("playbin","playbin");

  if (!data.playbin) {
    g_printerr ("Not all elements Could be created.\n");
    return -1;
  }

  /* Set the URI to play */
  // g_object_set (data.playbin,"uri","https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm",NULL);
  g_object_set (data.playbin,"file:///Users/dev/media/sintel_trailer-480p.webm",NULL);

  /* Start playing */
  ret = gst_element_set_state (data.playbin,GST_STATE_PLAYING);
  if (ret == GST_STATE_CHANGE_FAILURE) {
    g_printerr ("Unable to set the pipeline to the playing state.\n");
    gst_object_unref (data.playbin);
    return -1;
  }

  /* For video to play on macOS */
  main_loop = g_main_loop_new(NULL,FALSE);
  g_main_loop_run(main_loop);

  bus = gst_element_get_bus(data.playbin);

  /* Listen to the bus */
  do {
    msg = gst_bus_timed_pop_filtered (bus,100 * GST_MSECOND,(GstMessageType) (GST_MESSAGE_STATE_CHANGED | GST_MESSAGE_ERROR | GST_MESSAGE_EOS | GST_MESSAGE_DURATION));

    /* Parse the message */
    if(msg != NULL) {
      handle_message(&data,msg);
    }
    else 
    {
      /* No message means that the timeout expired */
      if(data.playing) {
        gint64 current = -1;

        /* Query the current position of the stream */
        if(!gst_element_query_position(data.playbin,GST_FORMAT_TIME,&current))
        {
          g_printerr("Could not query current position.\n");
        }

        /* If we didn't kNow it yet,query the stream duration */
        if (!GST_CLOCK_TIME_IS_VALID (data.duration)) {
          if (!gst_element_query_duration (data.playbin,&data.duration)) {
            g_printerr ("Could not query current duration.\n");
          }
        }

        /* Print current position and total duration */
        g_print ("Position %" GST_TIME_FORMAT " / %" GST_TIME_FORMAT "\r",GST_TIME_ARGS (current),GST_TIME_ARGS (data.duration));

        /* If seeking is enabled,we have not done it yet,and the time is right,seek */
        if (data.seek_enabled && !data.seek_done && current > 10 * GST_SECOND) {
          g_print ("\nReached 10s,performing seek...\n");
          gst_element_seek_simple (data.playbin,GstSeekFlags (GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_KEY_UNIT),30 * GST_SECOND);
          data.seek_done = TRUE;
        }
      }
    }
  } while(!data.terminate);

  /* Free resources */
  gst_object_unref (bus);
  gst_element_set_state (data.playbin,GST_STATE_NULL);
  gst_object_unref (data.playbin);
  return 0;
}

static void handle_message (CustomData *data,GstMessage *msg) 
{
  GError *err;
  gchar *debug_info;

  switch (GST_MESSAGE_TYPE (msg)) {
    case GST_MESSAGE_ERROR:
      gst_message_parse_error (msg,&err,&debug_info);
      g_printerr ("Error received from element %s: %s\n",GST_OBJECT_NAME (msg->src),err->message);
      g_printerr ("Debugging information: %s\n",debug_info ? debug_info : "none");
      g_clear_error (&err);
      g_free (debug_info);
      data->terminate = TRUE;
      break;
    case GST_MESSAGE_EOS:
      g_print ("End-Of-Stream reached.\n");
      data->terminate = TRUE;
      break;
    case GST_MESSAGE_DURATION:
      /* The duration has changed,mark the current one as invalid */
      data->duration = GST_CLOCK_TIME_NONE;
      break;
    case GST_MESSAGE_STATE_CHANGED:
    {
      GstState old_state,new_state,pending_state;
      gst_message_parse_state_changed(msg,&old_state,&new_state,&pending_state);
      if(GST_MESSAGE_SRC (msg) == GST_OBJECT(data->playbin)) {
        g_print("Pipeline state changed from %s to %s:\n",gst_element_state_get_name(old_state),gst_element_state_get_name(new_state));

        /* Remember whether we are in the PLAYING state or not */
        data->playing = (new_state == GST_STATE_PLAYING);

        if (data->playing) {
          /* We just moved to PLAYING. Check if seeking is possible */
          GstQuery *query;
          gint64 start,end;
          query = gst_query_new_seeking (GST_FORMAT_TIME);
          if (gst_element_query (data->playbin,query)) {
            gst_query_parse_seeking (query,NULL,&data->seek_enabled,&start,&end);
            if (data->seek_enabled) {
              g_print ("Seeking is ENABLED from %" GST_TIME_FORMAT " to %" GST_TIME_FORMAT "\n",GST_TIME_ARGS (start),GST_TIME_ARGS (end));
            } else {
              g_print ("Seeking is disABLED for this stream.\n");
            }
          }
          else {
            g_printerr ("Seeking query Failed.");
          }
          gst_query_unref (query);
        }
      } break;
      default:
      /* We should not reach here */
        g_printerr("Seeking query Failed.");
        break;
    }
    gst_message_unref(msg);
  }
}

解决方法

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