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。