GStreamer中AppSrc的简单使用（续）

文章《GStreamer中AppSrc的简单使用》介绍了AppSrc的初步使用，但对于各种复杂的媒体格式，需要自动解析媒体内容，自动生成Pipeline，这需要借助Decodebin来完成。Decodebin是Playbin的后端组件，它支持对输入进来的数据类型进行自动探测并设置解码器。对于任意一路解码后的媒体流（audio或video），它都会emit一个”pad-added”信号，可以基于这个信号增加相应的sink组件。 
针对AppSrc，上篇文章的代码可演化为（仅Audio）：

 19 #include <stdio.h>
 20 #include <stdlib.h>
 21 #include <gst/gst.h>
 22 #include <gst/app/gstappsrc.h>
 23
 24 static FILE * fp;
 25 static GstPad * audioPad;
 26
 27 void _needData( GstAppSrc * src, guint arg1, gpointer user_data )
 28 {
 29     GstBuffer * _buf;
 30     GstFlowReturn _ret;
 31     int _size;
 32
 33     _buf    =   gst_buffer_new_and_alloc( arg1 );
 34
 35     GST_BUFFER_OFFSET( _buf )   =   ftell( fp );
 36
 37     _size   =   fread( GST_BUFFER_DATA( _buf ), 1, arg1, fp );
 38
 39     GST_BUFFER_SIZE( _buf ) =   _size;
 40
 41     if( feof( fp ))
 42     {
 43         g_message( "rewind to head" );
 44         fseek( fp, 0, SEEK_SET );
 45     }
 46
 47     g_signal_emit_by_name( src, "push-buffer", _buf, &_ret );
 48
 49     gst_buffer_unref( _buf );
 50 }
 51
 52 void _enoughData( GstElement * src, gpointer user_data )
 53 {
 54     g_message( "%s", __func__ );
 55 }
 57 /* 58 * === FUNCTION ====================================================================== 59 * Name: _decNewPad 60 * Description: 61 * ===================================================================================== 62 */
 63 static void _decNewPad( GstElement * decodebin, GstPad * pad, gpointer user_data )
 64 {
 65     g_message( "%s", __func__ );
 66     gst_pad_link( pad, audioPad );
 67 }       /* ----- end of static function _decNewPad ----- */
 68
 69 /* 70 * === FUNCTION ====================================================================== 71 * Name: main 72 * Description: 73 * ===================================================================================== 74 */
 75 int main ( int argc, char *argv[] )
 76 {
 77     GstElement * _pipeline, * _src, * _codec, * _sink;
 78
 79     fp  =   fopen( argv[1], "r" );
 80
 81     g_assert( fp != NULL );
 82
 83     gst_init( &argc, &argv );
 84
 85     g_assert( _src  =   gst_element_factory_make( "appsrc", "source" ));
 86
 87     g_assert( _codec    =   gst_element_factory_make( "decodebin", "codec" ));
 88     g_signal_connect( _codec, "pad-added", G_CALLBACK( _decNewPad ), NULL );
 89
 90     g_assert( _sink =   gst_element_factory_make( "alsasink", "sink" ));
 91     g_assert( audioPad  =   gst_element_get_static_pad( _sink, "sink" ));
 92
 93     _pipeline   =   gst_pipeline_new( "pipeline" );
 94
 95     gst_bin_add_many( GST_BIN( _pipeline ), _src, _codec, _sink, NULL );
 96
 97     gst_element_link_many( _src, _codec, NULL );
 98
 99     g_signal_connect ( _src, "need-data", G_CALLBACK( _needData ), NULL );

100     g_signal_connect ( _src, "enough-data", G_CALLBACK( _enoughData ), NULL );
101
102     gst_element_set_state( _pipeline, GST_STATE_PLAYING );
103
104     pause();
105
106     return 0;
107 }               /* ---------- end of function main ---------- */