上一个教程演示了如何自动生成一个pipeline。这次我们打算用一个个element来手动搭建一个pipeline。我们这个教程会演示：
1. 什么是GStreamer的element以及如何建立一个element
2. 如何在element直接建立连接
3. 如何客制化element的行为
4. 如何监视总线上的错误并获得相关的信息
手动建立Hello World
把下面的代码copy到basic-turtorial-2.c文件
[objc] view plain copy print ? #include <gst/gst.h> int main(int argc, charchar *argv[]) { GstElement *pipeline, *source, *sink; GstBus *bus; GstMessage *msg; GstStateChangeReturn ret; /* Initialize GStreamer */ gst_init (&argc, &argv); /* Create the elements */ source = gst_element_factory_make ("videotestsrc", "source"); sink = gst_element_factory_make ("autovideosink", "sink"); /* Create the empty pipeline */ pipeline = gst_pipeline_new ("test-pipeline"); if (!pipeline || !source || !sink) { g_printerr ("Not all elements could be created.\n"); return -1; } /* Build the pipeline */ gst_bin_add_many (GST_BIN (pipeline), source, sink, NULL); if (gst_element_link (source, sink) != TRUE) { g_printerr ("Elements could not be linked.\n"); gst_object_unref (pipeline); return -1; } /* Modify the source's properties */ g_object_set (source, "pattern", 0, NULL); /* Start playing */ ret = gst_element_set_state (pipeline, GST_STATE_PLAYING); if (ret == GST_STATE_CHANGE_FAILURE) { g_printerr ("Unable to set the pipeline to the playing state.\n"); gst_object_unref (pipeline); return -1; } /* Wait until error or EOS */ bus = gst_element_get_bus (pipeline); msg = gst_bus_timed_pop_filtered (bus, GST_CLOCK_TIME_NONE, GST_MESSAGE_ERROR | GST_MESSAGE_EOS); /* Parse message */ if (msg != NULL) { 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); break; case GST_MESSAGE_EOS: g_print ("End-Of-Stream reached.\n"); break; default: /* We should not reach here because we only asked for ERRORs and EOS */ g_printerr ("Unexpected message received.\n"); break; } gst_message_unref (msg); } /* Free resources */ gst_object_unref (bus); gst_element_set_state (pipeline, GST_STATE_NULL); gst_object_unref (pipeline); return 0; } #include <gst/gst.h> int main(int argc, char *argv[]) { GstElement *pipeline, *source, *sink; GstBus *bus; GstMessage *msg; GstStateChangeReturn ret; /* Initialize GStreamer */ gst_init (&argc, &argv); /* Create the elements */ source = gst_element_factory_make ("videotestsrc", "source"); sink = gst_element_factory_make ("autovideosink", "sink"); /* Create the empty pipeline */ pipeline = gst_pipeline_new ("test-pipeline"); if (!pipeline || !source || !sink) { g_printerr ("Not all elements could be created.\n"); return -1; } /* Build the pipeline */ gst_bin_add_many (GST_BIN (pipeline), source, sink, NULL); if (gst_element_link (source, sink) != TRUE) { g_printerr ("Elements could not be linked.\n"); gst_object_unref (pipeline); return -1; } /* Modify the source's properties */ g_object_set (source, "pattern", 0, NULL); /* Start playing */ ret = gst_element_set_state (pipeline, GST_STATE_PLAYING); if (ret == GST_STATE_CHANGE_FAILURE) { g_printerr ("Unable to set the pipeline to the playing state.\n"); gst_object_unref (pipeline); return -1; } /* Wait until error or EOS */ bus = gst_element_get_bus (pipeline); msg = gst_bus_timed_pop_filtered (bus, GST_CLOCK_TIME_NONE, GST_MESSAGE_ERROR | GST_MESSAGE_EOS); /* Parse message */ if (msg != NULL) { 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); break; case GST_MESSAGE_EOS: g_print ("End-Of-Stream reached.\n"); break; default: /* We should not reach here because we only asked for ERRORs and EOS */ g_printerr ("Unexpected message received.\n"); break; } gst_message_unref (msg); } /* Free resources */ gst_object_unref (bus); gst_element_set_state (pipeline, GST_STATE_NULL); gst_object_unref (pipeline); return 0; }
工作流程
GStreamer的基本组成是elements，这些elements把数据从source经过filter传到sink。
建立element
因为上一篇教程以及介绍过了初始化这段内容，所以我们这次略过这一段。
[objc] view plain copy print ? /* Create the elements */ source = gst_element_factory_make ("videotestsrc", "source"); sink = gst_element_factory_make ("autovideosink", "sink"); /* Create the elements */ source = gst_element_factory_make ("videotestsrc", "source"); sink = gst_element_factory_make ("autovideosink", "sink"); 新的element的建立可以使用gst_element_factory_make()。这个API的第一个参数是要创建的element的类型（后面第14讲会介绍一些常见的类型，第10讲会介绍如何获得可用的类型列表），第二个参数是我们想创建的element的名字，这个名字并非是必须的，但在调试中会非常有用，如果你传入的时NULL，那么GStreamer会自动创建一个名字。
在本教程内我们创建了2个elements：videotestsrc和autovideosink.
vieotestsrc是一个source element（生产数据），会创建一个video模式。这个element常用在调试中，很少用于实际的应用。
autovideosink是一个sink element（消费数据），会在一个窗口显示收到的图像。在不同的操作系统中，会存在多个的video sink，autovideosink会自动选择一个最合适的，所以你不需要关心更多的细节，代码也会有更好的移植性。
建立pipeline
[objc] view plain copy print ? /* Create the empty pipeline */ pipeline = gst_pipeline_new ("test-pipeline"); /* Create the empty pipeline */ pipeline = gst_pipeline_new ("test-pipeline"); 因为要统一处理时钟和一些信息，GStreamer中的所有elements都必须在使用之前包含到pipeline中。我们用gst_pipeline_new()来创建pipeline。
[objc] view plain copy print ? /* Build the pipeline */ gst_bin_add_many (GST_BIN (pipeline), source, sink, NULL); if (gst_element_link (source, sink) != TRUE) { g_printerr ("Elements could not be linked.\n"); gst_object_unref (pipeline); return -1; } /* Build the pipeline */ gst_bin_add_many (GST_BIN (pipeline), source, sink, NULL); if (gst_element_link (source, sink) != TRUE) { g_printerr ("Elements could not be linked.\n"); gst_object_unref (pipeline); return -1; } 一个pipeline就是一个特定类型的可以包含其他element的bin，而且所以可以用在bin上的方法也都可以用在pipeline上。在这个例子中，我们调用了gst_bin_add_many()方法在pipeline中加入element。这个方法会接受一系列的element作为输入参数，最后由NULL来终止。增加单个element的方法是gst_bin_add()。
这个时候，这些刚增加的elements还没有互相连接起来。我们用gst_element_link()方法来把element连接起来，这个方法的第一个参数是源，第二个参数是目标，这个顺序不能搞错，因为这确定了数据的流向。记住只有在同一个bin里面的element才能连接起来，所以一定要把element在连接之前加入到pipeline中。
属性
[objc] view plain copy print ? /* Modify the source's properties */ g_object_set (source, "pattern", 0, NULL); /* Modify the source's properties */ g_object_set (source, "pattern", 0, NULL); 绝大部分的GStreamer elements有可以定制化的属性：只读的属性会显示element的内部状态，可写的属性会影响element的行为。我们用g_object_get()方法来获得属性，用g_object_set()方法来设置属性。
g_object_set()方法接受一个用NULL结束的属性名称/属性值的组成的对，所以可以一次同时修改多项属性。
上面的代码修改了videotestsrc的“pattern”属性，这个属性控制了视频的输出，大家可以试试不同的值看一下效果。
关于一个element的名字和取值范围，使用gst-inspect工具可以查询到。
错误检查
在这一点上，本系列的教程内容都比较相似，和第一讲没什么不同，只是我们监测更多的错误罢了。
[objc] view plain copy print ? /* Start playing */ ret = gst_element_set_state (pipeline, GST_STATE_PLAYING); if (ret == GST_STATE_CHANGE_FAILURE) { g_printerr ("Unable to set the pipeline to the playing state.\n"); gst_object_unref (pipeline); return -1; } /* Start playing */ ret = gst_element_set_state (pipeline, GST_STATE_PLAYING); if (ret == GST_STATE_CHANGE_FAILURE) { g_printerr ("Unable to set the pipeline to the playing state.\n"); gst_object_unref (pipeline); return -1; } 我们调用gst_element_set_state()方法，但这次我们检查它的返回值。状态转换是一个很微妙的过程，在下一篇教程中我们会有更多的一些细节。
[objc] view plain copy print ? /* Wait until error or EOS */ bus = gst_element_get_bus (pipeline); msg = gst_bus_timed_pop_filtered (bus, GST_CLOCK_TIME_NONE, GST_MESSAGE_ERROR | GST_MESSAGE_EOS); /* Parse message */ if (msg != NULL) { 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); break; case GST_MESSAGE_EOS: g_print ("End-Of-Stream reached.\n"); break; default: /* We should not reach here because we only asked for ERRORs and EOS */ g_printerr ("Unexpected message received.\n"); break; } gst_message_unref (msg); } /* Wait until error or EOS */ bus = gst_element_get_bus (pipeline); msg = gst_bus_timed_pop_filtered (bus, GST_CLOCK_TIME_NONE, GST_MESSAGE_ERROR | GST_MESSAGE_EOS); /* Parse message */ if (msg != NULL) { 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); break; case GST_MESSAGE_EOS: g_print ("End-Of-Stream reached.\n"); break; default: /* We should not reach here because we only asked for ERRORs and EOS */ g_printerr ("Unexpected message received.\n"); break; } gst_message_unref (msg); } gst_bus_timed_pop_filted()会一直等到运行结束，然后返回一个GstMessage参数。我们让gst_bus_timed_pop_filtered()方法仅在收到错误或者播放结束的消息时才返回。所以我们需要检查是哪个消息并打印出来。
GstMessage是一个非常通用的结构，它可以传递很多信息。幸好GStreamer提供了一系列的解析函数，在本教程里面，我们一旦知道message里面包含一个错误（通过使用GST_MESSAGE_TYPE宏），我们可以使用gst_message_parse_error()方法，这个方法会返回一个GLib的GError结构。
GStreamer总线
这里稍微介绍一下GStreamer总线。GStreamer总线本身也是一个对象，是创建来传递elements生成的GstMessage的对象。消息可以在总线上用gst_bus_timed_pop_filtered()方法抓出来，你的应用需要随时注意出错的信息和播放相关的其他问题。
其他的代码是释放内存的，和上一篇教程是一样的。
