> For the complete documentation index, see [llms.txt](https://ricardolu.gitbook.io/gstreamer/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ricardolu.gitbook.io/gstreamer/application-development/uridecodebin.md). # uridecodebin `uridecodebin`能够将URI数据解码成raw media，它会自动选择一个能处理uri数据的source element并将其和`decodebin`链接。 **Github:** [**uridecodebin**](https://github.com/gesanqiu/gstreamer-example/tree/main/application_develop/uridecodebin) ### uri ``` bool Create (void) { // ... ​ g_object_set (G_OBJECT (m_source), "uri", m_config.src.c_str(), NULL); ​ // ... } ``` ### signals #### source-setup ``` static void cb_uridecodebin_source_setup ( GstElement* pipeline, GstElement* source, gpointer user_data) { VideoPipeline* vp = reinterpret_cast (user_data); ​ LOG_INFO_MSG ("cb_uridecodebin_source_setup called"); ​ /* Configure rtspsrc if (g_object_class_find_property (G_OBJECT_GET_CLASS (source), "latency")) { LOG_INFO_MSG ("cb_uridecodebin_source_setup set %d latency", vp->m_config.rtsp_latency); g_object_set (G_OBJECT (source), "latency", vp->m_config.rtsp_latency, NULL); } */ ​ /* Configure appsrc GstCaps *m_sCaps; src_Caps = gst_caps_new_simple ("video/x-raw", "format", G_TYPE_STRING, m_config.src_format.c_str(), "width", G_TYPE_INT, m_config.src_width, "height", G_TYPE_INT, m_config.src_height, NULL); g_object_set (G_OBJECT(source), "caps", src_Caps, NULL); g_signal_connect (source, "need-data", G_CALLBACK (start_feed), data); g_signal_connect (source, "enough-data", G_CALLBACK (stop_feed), data); gst_caps_unref (src_Caps); */ } ``` `uridecodebin`会分析`uri`属性的值，然后选择合适的`srouce element`，这个`uri`值必须是完整的绝对路径，由source类型开始。 #### child-added ``` static void cb_decodebin_child_added ( GstChildProxy* child_proxy, GObject* object, gchar* name, gpointer user_data) { LOG_INFO_MSG ("cb_decodebin_child_added called"); ​ VideoPipeline* vp = reinterpret_cast (user_data); ​ LOG_INFO_MSG ("Element '%s' added to decodebin", name); ​ if (g_strrstr (name, "qtdemux") == name) { vp->m_qtdemux = reinterpret_cast (object); } ​ if ((g_strrstr (name, "h264parse") == name)) { vp->m_h264parse = reinterpret_cast (object); } ​ if (g_strrstr (name, "qtivdec") == name) { vp->m_decoder = reinterpret_cast (object); g_object_set (object, "turbo", vp->m_config.turbo, NULL); g_object_set (object, "skip-frames", vp->m_config.skip_frame, NULL); } } ​ static void cb_uridecodebin_child_added ( GstChildProxy* child_proxy, GObject* object, gchar* name, gpointer user_data) { LOG_INFO_MSG ("cb_uridecodebin_child_added called"); ​ LOG_INFO_MSG ("Element '%s' added to uridecodebin", name); VideoPipeline* vp = reinterpret_cast (user_data); ​ if (g_strrstr (name, "decodebin") == name) { g_signal_connect (G_OBJECT (object), "child-added", G_CALLBACK (cb_decodebin_child_added), vp); } } ``` 通过打印log可以看出`uridecodebin`初始化过程中自动添加到pipeline中的所有GstElement： ``` filesrc: qtdemux, multiqueue, h264parse/h265parse, capfilter, aacparse, avdec_aac, qtivdec rtspsrc: rtph264depay/rtph265depay, h264parse/h265parse, capfilter, qtivdec ``` 在`uridecodebin`只会添加`decodebin`一个GstElement，上述的GstElement均由`decodebin`构建，因此除了`uridecodebin`的`child-added`回调，还在其回调中添加了一个`decodebin`的`child-added`回调，用于设置`decodebin`构建的GstElement的属性。 在[build pipeline](https://ricardolu.gitbook.io/gstreamer/application-development/build-pipeline)中提到关于`filesrc`插件的解复用，需要手动进行link，但这个link在实际测试过程中有`decodebin`自动完成，我尝试手动去做link，由于无法获取到`h264parse`的`sink pad`程序抛出了段错误。我打印了这时候`vp->m_h264parse`的指针地址，是一个初始值，说明这时候还未初始化`vp->m_h264parse`。 #### pad-added ``` static void cb_uridecodebin_pad_added ( GstElement* src, GstPad* new_pad, gpointer user_data) { LOG_INFO_MSG ("cb_uridecodebin_pad_added called"); ​ GstPadLinkReturn ret; GstCaps* new_pad_caps = NULL; GstStructure* new_pad_struct = NULL; const gchar* new_pad_type = NULL; GstPad* v_sinkpad = NULL; GstPad* a_sinkpad = NULL; ​ VideoPipeline* vp = reinterpret_cast (user_data); ​ new_pad_caps = gst_pad_get_current_caps (new_pad); new_pad_struct = gst_caps_get_structure (new_pad_caps, 0); new_pad_type = gst_structure_get_name (new_pad_struct); ​ if (g_str_has_prefix (new_pad_type, "video/x-raw")) { LOG_INFO_MSG ("Linking video/x-raw"); /* Attempt the link */ v_sinkpad = gst_element_get_static_pad ( reinterpret_cast (vp->m_videoConv), "sink"); ret = gst_pad_link (new_pad, v_sinkpad); if (GST_PAD_LINK_FAILED (ret)) { LOG_ERROR_MSG ("fail to link video source with waylandsink"); goto exit; } } else if (g_str_has_prefix (new_pad_type, "audio/x-raw")) { LOG_INFO_MSG ("Linking audio/x-raw"); a_sinkpad = gst_element_get_static_pad ( reinterpret_cast (vp->m_audioConv), "sink"); ret = gst_pad_link (new_pad, a_sinkpad); if (GST_PAD_LINK_FAILED (ret)) { LOG_ERROR_MSG ("fail to link audio source and audioconvert"); goto exit; } } ​ exit: /* Unreference the new pad's caps, if we got them */ if (new_pad_caps != NULL) gst_caps_unref (new_pad_caps); ​ /* Unreference the sink pad */ if (v_sinkpad) gst_object_unref (v_sinkpad); if (a_sinkpad) gst_object_unref (a_sinkpad); } ``` 将`uridecodebin`的`src pad`和`waylandsin`的`sink pad`连接起来，由于这时是解码后的数据，因此`caps`为`video/x-raw`或`audio/x-raw`类型。 **在上文中提到关于`qtdemux`解复用器的链接问题，事实上`uridecodebin`内部会自动处理好这部分链接，并且为所有能够解析的数据类型创建`src-pad`，每创建一种类型的`src-pad`，`pad-added`回调就会触发一次，用户需要自行完成`uridecodebin`的这些`src-pad`与后续GstElement的链接。**可以结合[Build Pipeline](https://ricardolu.gitbook.io/gstreamer/application-development/build-pipeline#gst_element_link_pads)中关于pad-link的内容一起理解。 在上述代码中我将`video/x-raw`类型的数据和`videoconvert`插件链接，将`audio/x-raw`与`audioconvert`链接，`videoconvert`和`audioconvert`几乎是万能的格式转换插件能够提高代码的可移植性，使得代码能够在各种平台上正常运行，但切记这两者的转换均是使用CPU完成，因此十分消耗性能，在推流中使用会造成极大的延迟。