循环表达式
loopIn(type = "cycle", numKeyframes = 0)
loopOut(type = "cycle", numKeyframes = 0)
loopInDuration(type = "cycle", duration = 0)
loopOutDuration(type = "cycle", duration = 0)

循环类型
cycle 循环区间内动画帧
pingpong 来回循环区间内动画帧
offset 延续开始的循环状态
Continue 持续循环结束时的状态