GStreamer官方入门课程5：把GStreamer与你的图形界面集成在一起

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我是靠谱客的博主高高红酒，最近开发中收集的这篇文章主要介绍GStreamer官方入门课程5：把GStreamer与你的图形界面集成在一起，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

本教程演示如何将GStreamer集成到像GTK+这样的图形用户界面（GUI）工具包中。基本上，GStreamer负责媒体播放，而GUI工具包处理用户交互。最有趣的部分是两个库必须交互的部分：指示GStreamer将视频输出到GTK+窗口，并将用户操作转发到GStreamer。

特别是，你将学到：

如何告诉GStreamer将视频输出到特定窗口（而不是创建自己的窗口）。
如何使用来自GStreamer的信息不断刷新GUI。
如何从GStreamer的多线程中更新GUI，这是大多数GUI工具包禁止的操作。
一种机制，只订阅您感兴趣的消息，而不被通知所有消息。

1. 介绍

我们将使用GTK+工具包构建一个媒体播放器，但是这些概念适用于其他工具包，例如Qt。对GTK+的基本了解将有助于理解本教程。

要点是告诉GStreamer将视频输出到我们选择的窗口。具体的机制取决于操作系统（或者更确切地说，取决于窗口系统），但是GStreamer为平台独立性提供了一个抽象层。这种独立性来自GstVideoOverlay接口，它允许应用程序告诉视频接收器应该接收呈现的窗口的处理程序。

GObject接口

GObject接口（GStreamer使用）是元素可以实现的一组函数。如果是这样的话，那么就说它支持那个特定的接口。例如，视频接收器通常创建自己的窗口来显示视频，但是，如果它们也能够渲染到外部窗口，则可以选择实现GstVideoOverlay接口并提供指定此外部窗口的函数。从应用程序开发人员的角度来看，如果支持某个接口，则可以使用它，而不必考虑是哪种元素实现它。此外，如果您使用playbin，它将自动公开其内部元素支持的一些接口：您可以直接在playbin上使用接口函数，而不知道谁在实现它们！

另一个问题是，GUI工具包通常只允许通过主（或应用程序）线程操作图形化“小部件”，而GStreamer通常生成多个线程来处理不同的任务。从回调中调用GTK+函数通常会失败，因为回调在调用线程中执行，而调用线程不需要是主线程。这个问题可以通过在回调中的GStreamer总线上发布一条消息来解决：消息将由主线程接收，然后主线程将做出相应的反应。

最后，到目前为止，我们已经注册了一个handle_message，每当消息出现在总线上时，都会调用该函数，这迫使我们解析每个消息，以查看它是否对我们感兴趣。在本教程中，将使用不同的方法为每种消息注册一个回调，这样就减少了解析和代码的总量。

2. GTK+的媒体播放器+

让我们写一个非常简单的媒体播放器基于playbin，这一次，与一个图形用户界面！

将此代码复制到名为basic-tutorial-5.c的文本文件中（或在GStreamer安装中找到它）。

basic-tutorial-5.c

#include <string.h>

#include <gtk/gtk.h>
#include <gst/gst.h>
#include <gst/video/videooverlay.h>

#include <gdk/gdk.h>
#if defined (GDK_WINDOWING_X11)
#include <gdk/gdkx.h>
#elif defined (GDK_WINDOWING_WIN32)
#include <gdk/gdkwin32.h>
#elif defined (GDK_WINDOWING_QUARTZ)
#include <gdk/gdkquartz.h>
#endif

/* Structure to contain all our information, so we can pass it around */
typedef struct _CustomData {
  GstElement *playbin;           /* Our one and only pipeline */

  GtkWidget *slider;              /* Slider widget to keep track of current position */
  GtkWidget *streams_list;        /* Text widget to display info about the streams */
  gulong slider_update_signal_id; /* Signal ID for the slider update signal */

  GstState state;                 /* Current state of the pipeline */
  gint64 duration;                /* Duration of the clip, in nanoseconds */
} CustomData;

/* This function is called when the GUI toolkit creates the physical window that will hold the video.
 * At this point we can retrieve its handler (which has a different meaning depending on the windowing system)
 * and pass it to GStreamer through the VideoOverlay interface. */
static void realize_cb (GtkWidget *widget, CustomData *data) {
  GdkWindow *window = gtk_widget_get_window (widget);
  guintptr window_handle;

  if (!gdk_window_ensure_native (window))
    g_error ("Couldn't create native window needed for GstVideoOverlay!");

  /* Retrieve window handler from GDK */
#if defined (GDK_WINDOWING_WIN32)
  window_handle = (guintptr)GDK_WINDOW_HWND (window);
#elif defined (GDK_WINDOWING_QUARTZ)
  window_handle = gdk_quartz_window_get_nsview (window);
#elif defined (GDK_WINDOWING_X11)
  window_handle = GDK_WINDOW_XID (window);
#endif
  /* Pass it to playbin, which implements VideoOverlay and will forward it to the video sink */
  gst_video_overlay_set_window_handle (GST_VIDEO_OVERLAY (data->playbin), window_handle);
}

/* This function is called when the PLAY button is clicked */
static void play_cb (GtkButton *button, CustomData *data) {
  gst_element_set_state (data->playbin, GST_STATE_PLAYING);
}

/* This function is called when the PAUSE button is clicked */
static void pause_cb (GtkButton *button, CustomData *data) {
  gst_element_set_state (data->playbin, GST_STATE_PAUSED);
}

/* This function is called when the STOP button is clicked */
static void stop_cb (GtkButton *button, CustomData *data) {
  gst_element_set_state (data->playbin, GST_STATE_READY);
}

/* This function is called when the main window is closed */
static void delete_event_cb (GtkWidget *widget, GdkEvent *event, CustomData *data) {
  stop_cb (NULL, data);
  gtk_main_quit ();
}

/* This function is called everytime the video window needs to be redrawn (due to damage/exposure,
 * rescaling, etc). GStreamer takes care of this in the PAUSED and PLAYING states, otherwise,
 * we simply draw a black rectangle to avoid garbage showing up. */
static gboolean draw_cb (GtkWidget *widget, cairo_t *cr, CustomData *data) {
  if (data->state < GST_STATE_PAUSED) {
    GtkAllocation allocation;

    /* Cairo is a 2D graphics library which we use here to clean the video window.
     * It is used by GStreamer for other reasons, so it will always be available to us. */
    gtk_widget_get_allocation (widget, &allocation);
    cairo_set_source_rgb (cr, 0, 0, 0);
    cairo_rectangle (cr, 0, 0, allocation.width, allocation.height);
    cairo_fill (cr);
  }

  return FALSE;
}

/* This function is called when the slider changes its position. We perform a seek to the
 * new position here. */
static void slider_cb (GtkRange *range, CustomData *data) {
  gdouble value = gtk_range_get_value (GTK_RANGE (data->slider));
  gst_element_seek_simple (data->playbin, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_KEY_UNIT,
      (gint64)(value * GST_SECOND));
}

/* This creates all the GTK+ widgets that compose our application, and registers the callbacks */
static void create_ui (CustomData *data) {
  GtkWidget *main_window;  /* The uppermost window, containing all other windows */
  GtkWidget *video_window; /* The drawing area where the video will be shown */
  GtkWidget *main_box;     /* VBox to hold main_hbox and the controls */
  GtkWidget *main_hbox;    /* HBox to hold the video_window and the stream info text widget */
  GtkWidget *controls;     /* HBox to hold the buttons and the slider */
  GtkWidget *play_button, *pause_button, *stop_button; /* Buttons */

  main_window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  g_signal_connect (G_OBJECT (main_window), "delete-event", G_CALLBACK (delete_event_cb), data);

  video_window = gtk_drawing_area_new ();
  gtk_widget_set_double_buffered (video_window, FALSE);
  g_signal_connect (video_window, "realize", G_CALLBACK (realize_cb), data);
  g_signal_connect (video_window, "draw", G_CALLBACK (draw_cb), data);

  play_button = gtk_button_new_from_icon_name ("media-playback-start", GTK_ICON_SIZE_SMALL_TOOLBAR);
  g_signal_connect (G_OBJECT (play_button), "clicked", G_CALLBACK (play_cb), data);

  pause_button = gtk_button_new_from_icon_name ("media-playback-pause", GTK_ICON_SIZE_SMALL_TOOLBAR);
  g_signal_connect (G_OBJECT (pause_button), "clicked", G_CALLBACK (pause_cb), data);

  stop_button = gtk_button_new_from_icon_name ("media-playback-stop", GTK_ICON_SIZE_SMALL_TOOLBAR);
  g_signal_connect (G_OBJECT (stop_button), "clicked", G_CALLBACK (stop_cb), data);

  data->slider = gtk_scale_new_with_range (GTK_ORIENTATION_HORIZONTAL, 0, 100, 1);
  gtk_scale_set_draw_value (GTK_SCALE (data->slider), 0);
  data->slider_update_signal_id = g_signal_connect (G_OBJECT (data->slider), "value-changed", G_CALLBACK (slider_cb), data);

  data->streams_list = gtk_text_view_new ();
  gtk_text_view_set_editable (GTK_TEXT_VIEW (data->streams_list), FALSE);

  controls = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 0);
  gtk_box_pack_start (GTK_BOX (controls), play_button, FALSE, FALSE, 2);
  gtk_box_pack_start (GTK_BOX (controls), pause_button, FALSE, FALSE, 2);
  gtk_box_pack_start (GTK_BOX (controls), stop_button, FALSE, FALSE, 2);
  gtk_box_pack_start (GTK_BOX (controls), data->slider, TRUE, TRUE, 2);

  main_hbox = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 0);
  gtk_box_pack_start (GTK_BOX (main_hbox), video_window, TRUE, TRUE, 0);
  gtk_box_pack_start (GTK_BOX (main_hbox), data->streams_list, FALSE, FALSE, 2);

  main_box = gtk_box_new (GTK_ORIENTATION_VERTICAL, 0);
  gtk_box_pack_start (GTK_BOX (main_box), main_hbox, TRUE, TRUE, 0);
  gtk_box_pack_start (GTK_BOX (main_box), controls, FALSE, FALSE, 0);
  gtk_container_add (GTK_CONTAINER (main_window), main_box);
  gtk_window_set_default_size (GTK_WINDOW (main_window), 640, 480);

  gtk_widget_show_all (main_window);
}

/* This function is called periodically to refresh the GUI */
static gboolean refresh_ui (CustomData *data) {
  gint64 current = -1;

  /* We do not want to update anything unless we are in the PAUSED or PLAYING states */
  if (data->state < GST_STATE_PAUSED)
    return TRUE;

  /* 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");
    } else {
      /* Set the range of the slider to the clip duration, in SECONDS */
      gtk_range_set_range (GTK_RANGE (data->slider), 0, (gdouble)data->duration / GST_SECOND);
    }
  }

  if (gst_element_query_position (data->playbin, GST_FORMAT_TIME, &current)) {
    /* Block the "value-changed" signal, so the slider_cb function is not called
     * (which would trigger a seek the user has not requested) */
    g_signal_handler_block (data->slider, data->slider_update_signal_id);
    /* Set the position of the slider to the current pipeline positoin, in SECONDS */
    gtk_range_set_value (GTK_RANGE (data->slider), (gdouble)current / GST_SECOND);
    /* Re-enable the signal */
    g_signal_handler_unblock (data->slider, data->slider_update_signal_id);
  }
  return TRUE;
}

/* This function is called when new metadata is discovered in the stream */
static void tags_cb (GstElement *playbin, gint stream, CustomData *data) {
  /* We are possibly in a GStreamer working thread, so we notify the main
   * thread of this event through a message in the bus */
  gst_element_post_message (playbin,
    gst_message_new_application (GST_OBJECT (playbin),
      gst_structure_new_empty ("tags-changed")));
}

/* This function is called when an error message is posted on the bus */
static void error_cb (GstBus *bus, GstMessage *msg, CustomData *data) {
  GError *err;
  gchar *debug_info;

  /* Print error details on the screen */
  gst_message_parse_error (msg, &err, &debug_info);
  g_printerr ("Error received from element %s: %sn", GST_OBJECT_NAME (msg->src), err->message);
  g_printerr ("Debugging information: %sn", debug_info ? debug_info : "none");
  g_clear_error (&err);
  g_free (debug_info);

  /* Set the pipeline to READY (which stops playback) */
  gst_element_set_state (data->playbin, GST_STATE_READY);
}

/* This function is called when an End-Of-Stream message is posted on the bus.
 * We just set the pipeline to READY (which stops playback) */
static void eos_cb (GstBus *bus, GstMessage *msg, CustomData *data) {
  g_print ("End-Of-Stream reached.n");
  gst_element_set_state (data->playbin, GST_STATE_READY);
}

/* This function is called when the pipeline changes states. We use it to
 * keep track of the current state. */
static void state_changed_cb (GstBus *bus, GstMessage *msg, CustomData *data) {
  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)) {
    data->state = new_state;
    g_print ("State set to %sn", gst_element_state_get_name (new_state));
    if (old_state == GST_STATE_READY && new_state == GST_STATE_PAUSED) {
      /* For extra responsiveness, we refresh the GUI as soon as we reach the PAUSED state */
      refresh_ui (data);
    }
  }
}

/* Extract metadata from all the streams and write it to the text widget in the GUI */
static void analyze_streams (CustomData *data) {
  gint i;
  GstTagList *tags;
  gchar *str, *total_str;
  guint rate;
  gint n_video, n_audio, n_text;
  GtkTextBuffer *text;

  /* Clean current contents of the widget */
  text = gtk_text_view_get_buffer (GTK_TEXT_VIEW (data->streams_list));
  gtk_text_buffer_set_text (text, "", -1);

  /* Read some properties */
  g_object_get (data->playbin, "n-video", &n_video, NULL);
  g_object_get (data->playbin, "n-audio", &n_audio, NULL);
  g_object_get (data->playbin, "n-text", &n_text, NULL);

  for (i = 0; i < n_video; i++) {
    tags = NULL;
    /* Retrieve the stream's video tags */
    g_signal_emit_by_name (data->playbin, "get-video-tags", i, &tags);
    if (tags) {
      total_str = g_strdup_printf ("video stream %d:n", i);
      gtk_text_buffer_insert_at_cursor (text, total_str, -1);
      g_free (total_str);
      gst_tag_list_get_string (tags, GST_TAG_VIDEO_CODEC, &str);
      total_str = g_strdup_printf ("  codec: %sn", str ? str : "unknown");
      gtk_text_buffer_insert_at_cursor (text, total_str, -1);
      g_free (total_str);
      g_free (str);
      gst_tag_list_free (tags);
    }
  }

  for (i = 0; i < n_audio; i++) {
    tags = NULL;
    /* Retrieve the stream's audio tags */
    g_signal_emit_by_name (data->playbin, "get-audio-tags", i, &tags);
    if (tags) {
      total_str = g_strdup_printf ("naudio stream %d:n", i);
      gtk_text_buffer_insert_at_cursor (text, total_str, -1);
      g_free (total_str);
      if (gst_tag_list_get_string (tags, GST_TAG_AUDIO_CODEC, &str)) {
        total_str = g_strdup_printf ("  codec: %sn", str);
        gtk_text_buffer_insert_at_cursor (text, total_str, -1);
        g_free (total_str);
        g_free (str);
      }
      if (gst_tag_list_get_string (tags, GST_TAG_LANGUAGE_CODE, &str)) {
        total_str = g_strdup_printf ("  language: %sn", str);
        gtk_text_buffer_insert_at_cursor (text, total_str, -1);
        g_free (total_str);
        g_free (str);
      }
      if (gst_tag_list_get_uint (tags, GST_TAG_BITRATE, &rate)) {
        total_str = g_strdup_printf ("  bitrate: %dn", rate);
        gtk_text_buffer_insert_at_cursor (text, total_str, -1);
        g_free (total_str);
      }
      gst_tag_list_free (tags);
    }
  }

  for (i = 0; i < n_text; i++) {
    tags = NULL;
    /* Retrieve the stream's subtitle tags */
    g_signal_emit_by_name (data->playbin, "get-text-tags", i, &tags);
    if (tags) {
      total_str = g_strdup_printf ("nsubtitle stream %d:n", i);
      gtk_text_buffer_insert_at_cursor (text, total_str, -1);
      g_free (total_str);
      if (gst_tag_list_get_string (tags, GST_TAG_LANGUAGE_CODE, &str)) {
        total_str = g_strdup_printf ("  language: %sn", str);
        gtk_text_buffer_insert_at_cursor (text, total_str, -1);
        g_free (total_str);
        g_free (str);
      }
      gst_tag_list_free (tags);
    }
  }
}

/* This function is called when an "application" message is posted on the bus.
 * Here we retrieve the message posted by the tags_cb callback */
static void application_cb (GstBus *bus, GstMessage *msg, CustomData *data) {
  if (g_strcmp0 (gst_structure_get_name (gst_message_get_structure (msg)), "tags-changed") == 0) {
    /* If the message is the "tags-changed" (only one we are currently issuing), update
     * the stream info GUI */
    analyze_streams (data);
  }
}

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

  /* Initialize GTK */
  gtk_init (&argc, &argv);

  /* Initialize GStreamer */
  gst_init (&argc, &argv);

  /* Initialize our data structure */
  memset (&data, 0, sizeof (data));
  data.duration = GST_CLOCK_TIME_NONE;

  /* 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);

  /* Connect to interesting signals in playbin */
  g_signal_connect (G_OBJECT (data.playbin), "video-tags-changed", (GCallback) tags_cb, &data);
  g_signal_connect (G_OBJECT (data.playbin), "audio-tags-changed", (GCallback) tags_cb, &data);
  g_signal_connect (G_OBJECT (data.playbin), "text-tags-changed", (GCallback) tags_cb, &data);

  /* Create the GUI */
  create_ui (&data);

  /* Instruct the bus to emit signals for each received message, and connect to the interesting signals */
  bus = gst_element_get_bus (data.playbin);
  gst_bus_add_signal_watch (bus);
  g_signal_connect (G_OBJECT (bus), "message::error", (GCallback)error_cb, &data);
  g_signal_connect (G_OBJECT (bus), "message::eos", (GCallback)eos_cb, &data);
  g_signal_connect (G_OBJECT (bus), "message::state-changed", (GCallback)state_changed_cb, &data);
  g_signal_connect (G_OBJECT (bus), "message::application", (GCallback)application_cb, &data);
  gst_object_unref (bus);

  /* 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;
  }

  /* Register a function that GLib will call every second */
  g_timeout_add_seconds (1, (GSourceFunc)refresh_ui, &data);

  /* Start the GTK main loop. We will not regain control until gtk_main_quit is called. */
  gtk_main ();

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

3. 代码详解

关于本教程的结构，我们不再使用前向函数定义：函数将在使用之前定义。此外，为了解释清楚，代码片段的显示顺序并不总是与程序顺序匹配。使用行号定位完整代码中的代码段。

#include <gdk/gdk.h>
#if defined (GDK_WINDOWING_X11)
#include <gdk/gdkx.h>
#elif defined (GDK_WINDOWING_WIN32)
#include <gdk/gdkwin32.h>
#elif defined (GDK_WINDOWING_QUARTZ)
#include <gdk/gdkquartzwindow.h>
#endif

首先值得注意的是，我们不再完全独立于平台。我们需要为我们将要使用的窗口系统包含适当的GDK头。幸运的是，没有那么多受支持的窗口系统，所以这三行代码通常就足够了：X11用于Linux，Win32用于Windows，Quartz用于Mac OSX。

本教程主要由回调函数组成，这些函数将从GStreamer或GTK+调用，因此让我们回顾一下主函数，它注册了所有这些回调。

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

  /* Initialize GTK */
  gtk_init (&argc, &argv);

  /* Initialize GStreamer */
  gst_init (&argc, &argv);

  /* Initialize our data structure */
  memset (&data, 0, sizeof (data));
  data.duration = GST_CLOCK_TIME_NONE;

  /* 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);

标准GStreamer初始化和playbin管道创建，以及GTK+初始化。没什么新鲜事。

 /* Connect to interesting signals in playbin */
  g_signal_connect (G_OBJECT (data.playbin), "video-tags-changed", (GCallback) tags_cb, &data);
  g_signal_connect (G_OBJECT (data.playbin), "audio-tags-changed", (GCallback) tags_cb, &data);
  g_signal_connect (G_OBJECT (data.playbin), "text-tags-changed", (GCallback) tags_cb, &data);

当流中出现新标记（元数据）时，我们有兴趣得到通知。为了简单起见，我们将处理来自同一回调标记的所有类型的标记（视频、音频和文本）。

/* Create the GUI */
create_ui (&data);

所有GTK+小部件的创建和信号注册都在这个函数中进行。它只包含与GTK相关的函数调用，因此我们将跳过它的定义。它所注册的信号传递用户命令，如下在查看回调时所示。

/* Instruct the bus to emit signals for each received message, and connect to the interesting signals */
  bus = gst_element_get_bus (data.playbin);
  gst_bus_add_signal_watch (bus);
  g_signal_connect (G_OBJECT (bus), "message::error", (GCallback)error_cb, &data);
  g_signal_connect (G_OBJECT (bus), "message::eos", (GCallback)eos_cb, &data);
  g_signal_connect (G_OBJECT (bus), "message::state-changed", (GCallback)state_changed_cb, &data);
  g_signal_connect (G_OBJECT (bus), "message::application", (GCallback)application_cb, &data);
  gst_object_unref (bus);

在回放教程1：Playbin用法中，gst_bus_add_watch()用于注册接收发送到GStreamer总线的每条消息的函数。我们可以通过使用信号来实现更精细的粒度，这允许我们只注册感兴趣的消息。通过调用gst_bus_add_signal_watch()，我们指示总线在每次收到消息时发出信号。此信号的名称为message::detail，其中detail是触发信号发射的消息。例如，当总线接收到EOS消息时，它发出一个名为message::eos的信号。

本教程使用信号的详细信息仅注册到我们关心的消息。如果我们已经注册到消息信号，我们将收到每个消息的通知，就像gst_bus_add_watch()那样。

请记住，要使总线监视工作（无论是gst_bus_add_watch()还是gst_bus_add_signal_watch()），必须有GLib主循环运行。在本例中，它隐藏在GTK+主循环中。

/* Register a function that GLib will call every second */
g_timeout_add_seconds (1, (GSourceFunc)refresh_ui, &data);

在将控制权转移到GTK+之前，我们使用g_timeout_add_seconds()来注册另一个回调，这次有一个超时，因此它每秒都被调用一次。我们将使用它从刷新ui函数刷新GUI。

之后，我们完成了设置，可以启动GTK+主循环。当有趣的事情发生时，我们会从回拨中恢复控制。让我们回顾一下回调。每个回调都有不同的签名，这取决于调用它的人。您可以在信号文档中查找签名（参数的含义和返回值）。

/* This function is called when the GUI toolkit creates the physical window that will hold the video.
 * At this point we can retrieve its handler (which has a different meaning depending on the windowing system)
 * and pass it to GStreamer through the VideoOverlay interface. */
static void realize_cb (GtkWidget *widget, CustomData *data) {
  GdkWindow *window = gtk_widget_get_window (widget);
  guintptr window_handle;

  if (!gdk_window_ensure_native (window))
    g_error ("Couldn't create native window needed for GstVideoOverlay!");

  /* Retrieve window handler from GDK */
#if defined (GDK_WINDOWING_WIN32)
  window_handle = (guintptr)GDK_WINDOW_HWND (window);
#elif defined (GDK_WINDOWING_QUARTZ)
  window_handle = gdk_quartz_window_get_nsview (window);
#elif defined (GDK_WINDOWING_X11)
  window_handle = GDK_WINDOW_XID (window);
#endif
  /* Pass it to playbin, which implements VideoOverlay and will forward it to the video sink */
  gst_video_overlay_set_window_handle (GST_VIDEO_OVERLAY (data->playbin), window_handle);
}

代码注释可以自己说话。在应用程序生命周期的这一点上，我们知道GStreamer应该在其中呈现视频的窗口的句柄（无论是X11的XID、窗口的HWND还是Quartz的NSView）。我们只需从窗口系统中检索它，然后使用gst-video-overlay-set-window-handle()通过GstVideoOverlay接口将其传递给playbin。playbin将定位视频接收器并将处理程序传递给它，因此它不会创建自己的窗口并使用此窗口。

这里看不到更多；playbin和GstVideoOverlay真的简化了这个过程！

/* This function is called when the PLAY button is clicked */
static void play_cb (GtkButton *button, CustomData *data) {
  gst_element_set_state (data->playbin, GST_STATE_PLAYING);
}

/* This function is called when the PAUSE button is clicked */
static void pause_cb (GtkButton *button, CustomData *data) {
  gst_element_set_state (data->playbin, GST_STATE_PAUSED);
}

/* This function is called when the STOP button is clicked */
static void stop_cb (GtkButton *button, CustomData *data) {
  gst_element_set_state (data->playbin, GST_STATE_READY);
}

这三个小回调与GUI中的PLAY、PAUSE和STOP按钮相关。他们只是将管道设置为相应的状态。注意，在STOP状态下，我们将管道设置为READY。我们可以将管道一直降到NULL状态，但是，转换会慢一些，因为需要释放和重新获取一些资源（如音频设备）。

/* This function is called when the main window is closed */
static void delete_event_cb (GtkWidget *widget, GdkEvent *event, CustomData *data) {
  stop_cb (NULL, data);
  gtk_main_quit ();
}

gtk_main_quit()最终将调用main中的gtk_main_run()来终止，在本例中，它将完成程序。在这里，我们在主窗口关闭时调用它，在管道停止后（只是为了整洁）。

/* This function is called everytime the video window needs to be redrawn (due to damage/exposure,
 * rescaling, etc). GStreamer takes care of this in the PAUSED and PLAYING states, otherwise,
 * we simply draw a black rectangle to avoid garbage showing up. */
static gboolean draw_cb (GtkWidget *widget, cairo_t *cr, CustomData *data) {
  if (data->state < GST_STATE_PAUSED) {
    GtkAllocation allocation;

    /* Cairo is a 2D graphics library which we use here to clean the video window.
     * It is used by GStreamer for other reasons, so it will always be available to us. */
    gtk_widget_get_allocation (widget, &allocation);
    cairo_set_source_rgb (cr, 0, 0, 0);
    cairo_rectangle (cr, 0, 0, allocation.width, allocation.height);
    cairo_fill (cr);
  }

  return FALSE;
}

当有数据流（处于暂停和播放状态）时，视频接收器负责刷新视频窗口的内容。但在其他情况下，它不会，所以我们必须这样做。在本例中，我们只是用一个黑色矩形填充窗口。

/* This function is called when the slider changes its position. We perform a seek to the
 * new position here. */
static void slider_cb (GtkRange *range, CustomData *data) {
  gdouble value = gtk_range_get_value (GTK_RANGE (data->slider));
  gst_element_seek_simple (data->playbin, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_KEY_UNIT,
      (gint64)(value * GST_SECOND));
}

这是一个例子，说明了由于GStreamer和GTK+的协作，可以非常容易地实现复杂的GUI元素，如seeker bar（或允许搜索的滑块）。如果滑块已被拖动到新位置，请告诉GStreamer使用gst_element_seek_simple()搜索到该位置（如基本教程4：时间管理中所示）。滑块已设置，因此其值表示秒。

值得一提的是，执行一些限制可以获得一些性能（和响应性），也就是说，不响应每个要查找的用户请求。由于seek操作肯定需要一些时间，因此在seek之后等待半秒（例如）再允许另一个seek通常会更好。否则，如果用户疯狂地拖动滑块，应用程序可能看起来没有响应，这将不允许在新的搜索排队之前完成任何搜索。

/* This function is called periodically to refresh the GUI */
static gboolean refresh_ui (CustomData *data) {
  gint64 current = -1;

  /* We do not want to update anything unless we are in the PAUSED or PLAYING states */
  if (data->state < GST_STATE_PAUSED)
    return TRUE;

此功能将移动滑块以反映媒体的当前位置。首先，如果我们没有处于播放状态，我们在这里没有任何事情可做（另外，位置和持续时间查询通常会失败）。

/* 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");
  } else {
    /* Set the range of the slider to the clip duration, in SECONDS */
    gtk_range_set_range (GTK_RANGE (data->slider), 0, (gdouble)data->duration / GST_SECOND);
  }
}

如果不知道，我们会恢复剪辑的持续时间，这样就可以设置滑块的范围。

if (gst_element_query_position (data->playbin, GST_FORMAT_TIME, &current)) {
  /* Block the "value-changed" signal, so the slider_cb function is not called
   * (which would trigger a seek the user has not requested) */
  g_signal_handler_block (data->slider, data->slider_update_signal_id);
  /* Set the position of the slider to the current pipeline positoin, in SECONDS */
  gtk_range_set_value (GTK_RANGE (data->slider), (gdouble)current / GST_SECOND);
  /* Re-enable the signal */
  g_signal_handler_unblock (data->slider, data->slider_update_signal_id);
}
return TRUE;

我们查询当前的管道位置，并相应地设置滑块的位置。这将触发值改变信号的发射，我们用来知道用户何时拖动滑块。由于我们不希望在用户请求的情况下发生查找，因此我们使用g_signal_handler_block()和g_signal_handler_unblock()禁用此操作期间更改的值信号发射。

从该函数返回TRUE将使它在将来保持调用状态。如果返回FALSE，计时器将被删除。

/* This function is called when new metadata is discovered in the stream */
static void tags_cb (GstElement *playbin, gint stream, CustomData *data) {
  /* We are possibly in a GStreamer working thread, so we notify the main
   * thread of this event through a message in the bus */
  gst_element_post_message (playbin,
    gst_message_new_application (GST_OBJECT (playbin),
      gst_structure_new_empty ("tags-changed")));
}

这是本教程的要点之一。当在媒体中发现新标记时，将调用此函数，即从流式线程（即从应用程序（或主）线程以外的线程）调用此函数。我们在这里要做的是更新GTK+小部件来反映这个新信息，但是GTK+不允许从主线程以外的线程进行操作。

解决方案是让playbin在总线上发布一条消息并返回到调用线程。适当时，主线程将获取此消息并更新GTK。

gst_element_post_message()使GStreamer元素将给定的消息发布到总线。gst_message_new_application()创建应用程序类型的新消息。GStreamer消息有不同的类型，这个特定的类型被保留给应用程序：它将通过总线而不受GStreamer的影响。类型列表可以在GstMessageType文档中找到。

消息可以通过它们的嵌入式GstStructure传递附加信息，GstStructure是一个非常灵活的数据容器。在这里，我们使用gst_structure_new()创建一个新结构，并更改其名称标记，以避免在发送其他应用程序消息时出现混淆。

稍后，在主线程中，总线将接收此消息并发出message::application，我们已将其与application_cb函数关联：

/* This function is called when an "application" message is posted on the bus.
 * Here we retrieve the message posted by the tags_cb callback */
static void application_cb (GstBus *bus, GstMessage *msg, CustomData *data) {
  if (g_strcmp0 (gst_structure_get_name (gst_message_get_structure (msg)), "tags-changed") == 0) {
    /* If the message is the "tags-changed" (only one we are currently issuing), update
     * the stream info GUI */
    analyze_streams (data);
  }
}