[Gstreamer] 所有类的根 --- GstElement

前言：

GstElement 是 Gstreamer 的最基础类，几乎所有的 Gstreamer 类都继承于此，它包装了GObject 同时提供了通用能力。

GstElement的属性：

None

GstElement 是不可实例化的类，也是没有外在属性的类，无法通过 g_object_set 来对其设置属性。

GstElement 的成员变量：

struct _GstElement
{
  GstObject             object;
 
  /*< public >*/ /* with LOCK */
  GRecMutex             state_lock;
 
  /* element state */
  GCond                 state_cond;
  guint32               state_cookie;
  GstState              target_state;
  GstState              current_state;
  GstState              next_state;
  GstState              pending_state;
  GstStateChangeReturn  last_return;
 
  GstBus               *bus;
 
  /* allocated clock */
  GstClock             *clock;
  GstClockTimeDiff      base_time; /* NULL/READY: 0 - PAUSED: current time - PLAYING: difference to clock */
  GstClockTime          start_time;
 
  /* element pads, these lists can only be iterated while holding
   * the LOCK or checking the cookie after each LOCK. */
  guint16               numpads;
  GList                *pads;
  guint16               numsrcpads;
  GList                *srcpads;
  guint16               numsinkpads;
  GList                *sinkpads;
  guint32               pads_cookie;
 
  /* with object LOCK */
  GList                *contexts;
 
  /*< private >*/
  gpointer _gst_reserved[GST_PADDING-1];
};

base_time：

the time of the clock right before the element is set to PLAYING. Subtracting @base_time from the current clock time in the PLAYING state will yield the running_time against the clock.

记录设置 PLAYING 状态前的那一时刻，近似为当前 ELEMENT 在 PLAYING 状态的开始时间点，用当前时钟值减去这个值无限近似于当前 ELEMENT 已经运行了多久。