Gstreamer 初探 (五)
（GTK+）
本次教程我们介绍将Gstreamer集成到GTK+中的操作详解，基本上Gstreamer负责媒体流的处理，用户交互的部分留给GTK+来处理，最有趣的部分是Gstreamer如何将视频输出到Gtk+的窗口，而Gtk+将用户操作转发到Gstreamer进行控制
通过这次课程，请留意以下内容：
- 如何将Gstreamer的视频输出到指定的窗口（而不是让其创建自己的窗口）
- 如何将Gstreamer的信息在界面上进行刷新
- 如何从Gstreamer的多个线程中更新界面
- 如何只获取需要的消息，而不是获取所有的消息
介绍
我们将使用Gtk+和Gstreamer构建一个媒体播放器，对Gtk+有一定了解将有助于理解本教程
主要的一点是让Gstreamer的视频流输出到我们使用Gtk+创建的窗口上，当然，这其中具体的机制是依赖于操作系统的（更确切的说依赖于窗口系统），但GStreamer为平*立性提供了一个抽象层，通过GstVideoOverlay接口实现，这允许应用程序告诉sink应该接收渲染的窗口处理程序
GObject interfaces
Gstreamer所使用的GObject 接口是通过element所包含的一组函数，如果有这样的函数，就称作它支持这个特定的接口，例如，sinks通常创建自己的窗口来呈现视频，但如果视频流可以被呈现到外部的窗口，sinks就可以实现GstVideoOverlay这个接口提供函数来提供指定一个外部窗口的方法，从开发者的角度讲，如果element支持某个接口，我们不需要考虑具体是哪个element实现的，而可以直接使用这个接口来完成功能，此外，playbin会自动支持实现一些接口函数以供我们使用
我们需要注意GStreamer的多线程问题，GUI工具包通常只允许通过主(或应用程序)线程操作图形“小部件”，而GStreamer通常生成多个线程来处理不同的任务。从回调内部调用GTK+函数通常会失败，因为回调是在调用线程中执行的，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总线上发布消息来解决:主线程接收消息，主线程将相应地做出反应。
在上一次的教程中，我们通过注册handle_message函数，每当总线上出现一条消息时，就会调用这个函数，这使得我们解析每条消息。在本教程中，使用不同的方法为每种类型的消息注册回调，因此解析更少，代码也更少。
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, ¤t)) {
/* 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: %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);
/* 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 %s\n", 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: %s\n", 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: %s\n", 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: %s\n", 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: %d\n", 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: %s\n", 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;
}
通过 gcc basic-tutorial-5.c -o basic-tutorial-5 pkg-config --cflags --libs gstreamer-video-1.0 gtk+-3.0 gstreamer-1.0 编译代码进行测试
实例打开一个GTK+窗口，并显示一个电影，以及附带的音频。媒体是从Internet获取的，因此窗口可能需要几秒钟才能显示出来，这取决于连接速度。该窗口有一些用于暂停、停止和播放影片的GTK+按钮，以及一个显示流的当前位置的滑块，可以通过拖动来更改它。此外，关于流的信息显示在窗口右边缘的一列上。
没有延迟管理(缓冲)，因此在慢速连接上，电影可能会在几秒钟后停止。之后的教程会解决这个问题
这次教程的结构，函数会在使用之前被定义。另外，为了解释清楚，代码片段呈现的顺序并不总是与程序的顺序匹配。
#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Gtk+在代码中适配了不同平台的窗口系统。我们需要为将要使用的窗口系统包含适当的GDK头文件。受支持的窗口系统并不多: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+中调用，因此我们首先来看下主函数，它注册了所有这些回调函数。
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);
这里没有什么新的内容，标准GStreamer初始化和playbin管道创建，以及GTK+初始化
/* 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);
当流中出现新的标记(元数据)时，我们希望得到通知。为了简单起见，我们将处理来自同一个回调tags_cb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);
通过调用gst_bus_add_signal_watch()，我们指示总线在每次接收到消息时发出一个信号。该信号的名称为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，它会发出一个名称为message:: EOS的信号。
为了让总线监视工作(无论是gst_bus_add_watch()还是gst_bus_add_signal_watch())，必须运行GLib主循环。
在本例中，我们使用了GTK+主循环，它封装了GLib的主循环
/* 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，因此每秒钟都会调用它。我们将使用它从refresh_ui函数刷新GUI。
在此之后，我们就完成了设置，可以开始GTK+主循环了。当有趣的事情发生时，我们将从回调中重新获得控制权。
/* 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);
}
我们知道了GStreamer渲染视频的窗口的句柄(无论是X11的XID，窗口的HWND还是Quartz的NSView)。我们只需从窗口系统检索它，并使用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这个窗口。
playbin和GstVideoOverlay极大地简化了这个过程!
/* 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);
}
这三个回调与GUI中的播放、暂停和停止按钮相关联。它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设置为空状态，但是这样转换就会慢一些，因为需要释放和重新获取一些资源(比如音频设备)。
/* 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，在停止管道之后调用它
/* 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;
}
当有数据流时(处于暂停和播放状态)，视频接收器负责刷新视频窗口的内容。但是在其他情况下，它不会，所以我们必须这样做。在这个例子中，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));
}
这里通过GStreamer和GTK+的协作，实现了滑块定位和媒体流的同步。如果滑块被拖动到一个新的位置，告诉GStreamer使用gst_element_seek_simple()查找那个位置
值得一提的是，通过进行一些限制，可以获得一些性能(和响应能力)，也就是说，不响应每次用户的请求。由于查找操作必然要花费一些时间，在允许另一个查找之前等待半秒(例如)通常更好。否则，如果用户疯狂地拖动滑块，应用程序可能会看起来没有响应
/* 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;
此函数将移动滑块以反映媒体的当前位置。首先，如果我们不在播放状态，我们在这里就没有什么可做的(加上，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);
}
}
我们查询媒体流的持续时间，然后设置滑动条的范围。
if (gst_element_query_position (data->playbin, GST_FORMAT_TIME, ¤t)) {
/* 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;
我们查询当前管道位置，并相应地设置滑块的位置。这将触发值变化信号的发射，我们使用该信号来知道用户何时在拖动滑块。除非用户请求它们，我们不希望去更改播放位置，所以我们在此操作期间使用g_signal_handler_block()和g_signal_handler_unblock()禁用value-changed信号发射。
从这个函数返回TRUE将使它在将来被调用。如果我们返回FALSE，定时器将被删除。
/* 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")));
}
这是本教程的关键点之一。当从一个流线程(即应用程序(或主)线程以外的线程)在媒体中发现新的标记时，将调用此函数。我们在这里要做的是更新一个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在总线上发布消息并返回到调用线程。在适当的时候，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()创建一个应用程序类型的新消息。GStreamer消息具有不同的类型，并且这个特定的类型保留给应用程序:它将通过不受GStreamer影响的总线。类型列表可以在GstMessageType文档中找到。
消息可以通过其嵌入的gst结构传递附加信息，这是一种非常灵活的数据容器。在这里，我们使用gst_structure_new()创建一个新的结构，并将其命名为tag -changed，以避免在希望发送其他应用程序消息时产生混淆。
稍后，一旦进入主线程，总线将接收到此消息并发出message::application信号，我们已经将其关联到application_cb函数:
/* 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);
}
}
一旦我确定它是标记更改的消息，我们调用analyze_streams函数，它基本上从流中恢复标签，并将它们写入GUI中的文本小部件中。
error_cb、eos_cb和state_changed_cb实际上不值得解释，因为它们的作用与之 前所有教程中的相同，但它们现在有自己的功能。
就是它了!本教程中的代码量可能令人生畏，好在所需的概念并不太
总结
本教程演示了:
如何使用gst_video_overlay_set_window_handle()将视频输出到特定的窗口句柄。
如何通过使用g_timeout_add_seconds()注册一个超时回调来定期刷新GUI。
如何通过gst_element_post_message()总线通过应用程序消息将信息传递给主线程。
如何通过使用gst_bus_add_signal_watch()让总线发出信号并使用信号细节区分所有消息类型来只通知感兴趣的消息。
（此篇文章翻译自官网，旨在分享技术理念，如有不当敬请指正）