【GStreamer源码分析】playbin播放test.wav加载插件过程分析

playbin播放test.wav加载插件过程分析

一、前言
二、playbin 播放 .wav 音频插件加载一览
三、测试代码
- 3.1 gst_init
- 3.2 gst_element_set_state
四、gst_element_set_state 触发 playbin 加载流程分析
- 4.1 gst_element_factory_make 创建 playbin时流程
- 4.2 gst_element_set_state 是如何触发 playbin 的呢？
- 4.3 playbin 在何时创建 uridecodebin & uridecodebin 又加载了哪些插件
- - 4.3.1 playbin 在何时创建 uridecodebin
  - 4.3.2 uridecodebin 又加载了哪些插件
- 4.3.3 playbin 加载 inputSelector
- 4.3.3 playsink 加载了哪些插件呢？
- - 4.3.3.1 playsink 加载 tee
  - 4.3.3.1 playbin 的 gst_play_sink_reconfigure
五、总结

一、前言

希望通过这篇文章记录一下 playbin 加载 plugin 的机制，熟悉一些 playbin （或者是 GStreamer）的源码

二、playbin 播放 .wav 音频插件加载一览

从上图可以知道 playbin可分为两部分，中间用 input-selector 连接起来

uridecodebin -> input-selector -> playsink

当然，uridecodebin 和 playsink 中也包含很多组件，我们一点一点来分析。

三、测试代码

#include <gst/gst.h>
// In which states all these operations can be performed./* Structure to contain all our information, so we can pass it around */
typedef struct _CustomData {GstElement *playbin;  /* Our one and only element */gboolean playing;      /* Are we in the PLAYING state? */gboolean terminate;    /* Should we terminate execution? */gboolean seek_enabled; /* Is seeking enabled for this media? */gboolean seek_done;    /* Have we performed the seek already? */gint64 duration;       /* How long does this media last, in nanoseconds */
} CustomData;/* Forward definition of the message processing function */
static void handle_message (CustomData *data, GstMessage *msg);int main(int argc, char *argv[]) {CustomData data;GstBus *bus;GstMessage *msg;GstStateChangeReturn ret;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);// return 0;/* 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", "file:///home/joshua/Project/gst_base_tutorial/media/test.wav", 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;}/* Listen to the bus */bus = gst_element_get_bus (data.playbin);do {msg = gst_bus_timed_pop_filtered (bus, 100 * GST_MSECOND,GST_MESSAGE_STATE_CHANGED | GST_MESSAGE_ERROR | GST_MESSAGE_EOS | GST_MESSAGE_DURATION);/* Parse message */if (msg != NULL) {handle_message (&data, msg);} else {/* We got no message, this means 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, GST_FORMAT_TIME, &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, GST_FORMAT_TIME,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 ("\nEnd-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 ("Unexpected message received.\n");break;}gst_message_unref (msg);
}

3.1 gst_init

gst_init (&argc, &argv);
data.playbin = gst_element_factory_make ("playbin", "playbin");
g_object_set (data.playbin, "uri", "file:///home/joshua/Project/gst_base_tutorial/media/test.wav", NULL);

gst_init & gst_element_factory_make，不是这里的重点，就不深究了；
g_object_set 这里可以简单看下 playbin2.c 源码里是怎么实现的。这里需要了解 gobject install property 的一个小知识点，利用 g_object_class_install_property 可以向 plugin 中添加属性

3.2 gst_element_set_state

这里简简单单的一步对于 element state 的设置，却牵一发动全身，不仅让 playbin 把整个 pipeline 搭建了起来，还把 wav的整个流播放了起来。

四、gst_element_set_state 触发 playbin 加载流程分析

4.1 gst_element_factory_make 创建 playbin时流程

playbin 的继承关系

GObject+----GInitiallyUnowned+----GstObject+----GstElement+----GstBin+----GstPipeline+----GstPlayBin

从上面的继承关系中我们可以得知，创建playbin时也同时调用 element, bin, pipeline 的init，这就是基于GObject的继承关系；
（plugin_init里面就不分析了，无非就是 plugin的登记）

gst_play_bin_init 中创建了一个 playsink 的对象，并把playsink 加入到bin中，作为playbin的一个子element。从dot图中可以知道这是playbin中sink的最外层，用于存放后面加载的plugin。

// ---/* add sink */playbin->playsink =g_object_new (GST_TYPE_PLAY_SINK, "name", "playsink", "send-event-mode",1, NULL);gst_bin_add (GST_BIN_CAST (playbin), GST_ELEMENT_CAST (playbin->playsink));gst_play_sink_set_flags (playbin->playsink, DEFAULT_FLAGS);
// ---

gst_play_sink_init 中创建 streamsynchronizer，并把streamsynchronizer加入到bin中，作为playsink的一个子element。

  playsink->stream_synchronizer =g_object_new (GST_TYPE_STREAM_SYNCHRONIZER, NULL);gst_bin_add (GST_BIN_CAST (playsink),GST_ELEMENT_CAST (playsink->stream_synchronizer));

4.2 gst_element_set_state 是如何触发 playbin 的呢？

gst_element_set_state

  if (oclass->set_state)result = (oclass->set_state) (element, state);

从这里可以知道， gst_element_set_state 是调用了 oclass->set_state

在 gst_element_class_init 中，我们又看到这样一段赋值，实际上是调用 gst_element_set_state_func

在 gst_element_set_state_func 中，我们又可以定位到比较关键的一行
在 gst_element_change_state 中，我们终于找到了底层的调用func，也就是 oclass->change_state; 注意一下，谁是oclass呢？

oclass = GST_ELEMENT_GET_CLASS (element);

这里的 element 是 playbin，也就是说，这里调用的是 playbin 中的 change_state。（因为 playbin 继承自 GstElement，在gst_play_bin_class_init中，重写了 GstElement 的 change_state 函数）

playbin2.c 中 gst_play_bin_change_state 是一个非常重要的函数，其中 transition 的更迭更是playbin众多业务的触发点，我们下面可以从gst_play_bin_change_state 来分析 playbin的加载流程，也可以看到 GStreamer 设计的巧妙，如何通过设置流的状态来推动整个pipeline的运行。

4.3 playbin 在何时创建 uridecodebin & uridecodebin 又加载了哪些插件

4.3.1 playbin 在何时创建 uridecodebin

从 gst_play_bin_change_state 中可以看到 GstStateChange： GST_STATE_CHANGE_READY_TO_PAUSED 时，调用setup_next_source，在 setup_next_source -> activate_group -> 加载 uridecodebin plugin

// gstplaybin2.ccase GST_STATE_CHANGE_READY_TO_PAUSED:if ((ret =setup_next_source (playbin,GST_STATE_PAUSED)) == GST_STATE_CHANGE_FAILURE)goto failure;if (ret == GST_STATE_CHANGE_SUCCESS)ret = GST_STATE_CHANGE_ASYNC;

activate_group 中加载 uridecodebin ，并把 uridecodebin add 到 playbin 里；

    GST_DEBUG_OBJECT (playbin, "making new uridecodebin");uridecodebin = gst_element_factory_make ("uridecodebin", NULL);if (!uridecodebin)goto no_decodebin;gst_bin_add (GST_BIN_CAST (playbin), uridecodebin);

4.3.2 uridecodebin 又加载了哪些插件

gst_uri_decode_bin_change_state 中可以看到 GstStateChange： GST_STATE_CHANGE_READY_TO_PAUSED 时，调用 setup_source -> gen_source_element -> gst_element_make_from_uri 加载 filesrc

 //gsturidecodebin.ccase GST_STATE_CHANGE_READY_TO_PAUSED:GST_DEBUG ("ready to paused");if (!setup_source (decoder))goto source_failed;ret = GST_STATE_CHANGE_ASYNC;

gst_element_make_from_uri 加载 filesrc。gst_element_make_from_uri内部就不深究了，它可以根据uri来获取协议便于产生不同的src，目前结构： （uridecodebin）【filesrc】

 //gsturidecodebin.csource =gst_element_make_from_uri (GST_URI_SRC, decoder->uri, "source", &err);

setup_source -> make_decoder，加载 decodebin，目前 pipeline = （uridecodebin）【filesrc - decodebin】

    /* now create the decoder element */decodebin = gst_element_factory_make ("decodebin", NULL);GST_LOG_OBJECT(decoder, "making new decodebin end");if (!decodebin)goto no_decodebin;

然而decodebin也可以细化，在decodebin init 的过程中，它加载神奇的插件 typefind，目前 pipeline = （uridecodebin）【filesrc - （decodebin）【typefind】】（这个typefind的功能很诡异，后面可以着重看一下）

  //decodebin.c/* we create the typefind element only once */decode_bin->typefind = gst_element_factory_make ("typefind", "typefind");if (!decode_bin->typefind) {g_warning ("can't find typefind element, decodebin will not work");} else {GstPad *pad;GstPad *gpad;GstPadTemplate *pad_tmpl;/* add the typefind element */if (!gst_bin_add (GST_BIN (decode_bin), decode_bin->typefind)) {g_warning ("Could not add typefind element, decodebin will not work");gst_object_unref (decode_bin->typefind);decode_bin->typefind = NULL;}

再看 gst_decode_bin_change_state 中可以看到 GstStateChange： GST_STATE_CHANGE_READY_TO_PAUSED，这里绑定 “have-type” 的信号，找到合适类型的时候会触发 type_found 回调，type_found -> analyze_new_pad ->connect_pad

      /* connect a signal to find out when the typefind element found* a type */dbin->have_type_id =g_signal_connect (dbin->typefind, "have-type",G_CALLBACK (type_found), dbin);

connect_pad 中这里，加载 wavparse 插件，并放入decodebin中，目前 pipeline = （uridecodebin）【filesrc +（decodebin）【typefind + wavparse 】】，至此 uridecodebin 里的 plugin 全部加载完毕。

    /* 2.1. Try to create an element */if ((element = gst_element_factory_create (factory, NULL)) == NULL) {GST_WARNING_OBJECT (dbin, "Could not create an element from %s",gst_plugin_feature_get_name (GST_PLUGIN_FEATURE (factory)));g_string_append_printf (error_details,"Could not create an element from %s\n",gst_plugin_feature_get_name (GST_PLUGIN_FEATURE (factory)));continue;}
/// --------------------------------/* ... add it ... */if (!(gst_bin_add (GST_BIN_CAST (dbin), element))) {GST_WARNING_OBJECT (dbin, "Couldn't add %s to the bin",GST_ELEMENT_NAME (element));remove_error_filter (dbin, element, NULL);g_string_append_printf (error_details, "Couldn't add %s to the bin\n",GST_ELEMENT_NAME (element));gst_object_unref (element);continue;}

4.3.3 playbin 加载 inputSelector

1.playbin 中绑定处理 uridecodebin 的 “padd-added” 信号，这里 uridecodebin 最右边的已经没有控件了，需要 request a new pad，playbin 这里处理了这个信号，并加载了 input-selector

  /* connect pads and other things */group->pad_added_id = g_signal_connect (uridecodebin, "pad-added",G_CALLBACK (pad_added_cb), group);

    /* no combiner, create one */GST_DEBUG_OBJECT (playbin, "creating new input selector");if (custom_combiner)combine->combiner = custom_combiner;elsecombine->combiner = gst_element_factory_make ("input-selector", NULL);

4.3.3 playsink 加载了哪些插件呢？

从外部的大框架上来看，playbin = uridecodebin + inputseletor + playsink。因为 uridecodebin 内部的控件加载特别多，所以 playsink的内部也不简单。

4.3.3.1 playsink 加载 tee

1.playbin 中绑定处理 uridecodebin 的 “no-more-pads” 信号，这个代表不能产生更多的动态插件，会触发回调 no_more_pads_cb -> gst_play_sink_request_pad -> make tee plugin，目前 playsink = tee + streamsynchronizer

        GST_LOG_OBJECT (playsink, "creating tee");/* create tee when needed. This element will feed the audio sink chain* and the vis chain. */playsink->audio_tee = gst_element_factory_make ("tee", "audiotee");if (playsink->audio_tee == NULL) {post_missing_element_message (playsink, "tee");GST_ELEMENT_ERROR (playsink, CORE, MISSING_PLUGIN,(_("Missing element '%s' - check your GStreamer installation."),"tee"), (NULL));res = NULL;break;}playsink->audio_tee_sink =gst_element_get_static_pad (playsink->audio_tee, "sink");gst_bin_add (GST_BIN_CAST (playsink), playsink->audio_tee);

4.3.3.1 playbin 的 gst_play_sink_reconfigure

下面的 plugin非常多，且创建和加载前后顺序不一致，这里就从 gst_play_sink_reconfigure 进行往下分析吧。

no_more_pads_cb -> gst_play_sink_reconfigure

gboolean
gst_play_sink_reconfigure (GstPlaySink * playsink)
{GST_LOG_OBJECT (playsink, "Triggering reconfiguration");GST_PLAY_SINK_LOCK (playsink);video_set_blocked (playsink, TRUE);audio_set_blocked (playsink, TRUE);text_set_blocked (playsink, TRUE);GST_PLAY_SINK_UNLOCK (playsink);return TRUE;
}

audio_set_blocked -> sinkpad_blocked_cb -> gst_play_sink_do_reconfigure -> gen_audio_chain

    if (!playsink->audiochain) {GST_DEBUG_OBJECT (playsink, "creating new audio chain");playsink->audiochain = gen_audio_chain (playsink, raw);}

make “autoaudiosink”, autoaudiosink 内部会加载 fakesink， plusesink，不细谈了，这里用的是 plusesink

    /* only try fallback if no specific sink was chosen */if (chain->sink == NULL) {GST_DEBUG_OBJECT (playsink, "trying autoaudiosink");elem = gst_element_factory_make ("autoaudiosink", "audiosink");chain->sink = try_element (playsink, elem, TRUE);}

make abin, 作为容器，管理插件用的

chain->chain.bin = gst_bin_new ("abin");bin = GST_BIN_CAST (chain->chain.bin);gst_object_ref_sink (bin);gst_bin_add (bin, chain->sink);

make “queue”

  /* we have to add a queue when we need to decouple for the video sink in* visualisations and for streamsynchronizer */GST_DEBUG_OBJECT (playsink, "adding audio queue");chain->queue = gst_element_factory_make ("queue", "aqueue");if (chain->queue == NULL) {post_missing_element_message (playsink, "queue");GST_ELEMENT_WARNING (playsink, CORE, MISSING_PLUGIN,(_("Missing element '%s' - check your GStreamer installation."),"queue"), ("audio playback and visualizations might not work"));

make “audioconvert”

      GST_DEBUG_OBJECT (playsink, "adding audio filter");chain->filter_conv =gst_element_factory_make ("audioconvert", "filter-convert");if (!chain->filter_conv) {post_missing_element_message (playsink, "audioconvert");GST_ELEMENT_WARNING (playsink, CORE, MISSING_PLUGIN,(_("Missing element '%s' - check your GStreamer installation."),"audioconvert"),("audio playback and visualizations might not work"));} else {gst_bin_add (bin, chain->filter_conv);head = prev = chain->filter_conv;}

五、总结

简单记录一下一段 playbin 源码之旅；
即使现在，我也不能说清楚里面的万分之一，只能把自己走过的路一点点记下来，留个印象。草草收了尾，很多地方不理解，很多地方需要去挖掘，只是暂时没有更多的时间了。
我要去一个新的地方了，希望以后开心一点，可以得到别人的认可。