CMTime

public struct CMTime {

    // value/timescale = seconds

    public var value: CMTimeValue 
    public var timescale: CMTimeScale 
    public var flags: CMTimeFlags 
    public var epoch: CMTimeEpoch 

    public init()

    public init(value: CMTimeValue, timescale: CMTimeScale, flags: CMTimeFlags, epoch: CMTimeEpoch)
}
/**
CMTime定义是一个C语言的结构体，
    CMTime是以分数的形式表示时间，
    value表示分子，
    timescale表示分母，
    flags是位掩码，
    epoch 表示时间的指定状态
**/


typealias CMTimeValue = Int64
typealias CMTimeScale = Int32
typealias CMTimeEpoch = Int64

// value/timescale = seconds
CMTime(seconds: <Double>, preferredTimescale: <CMTimeScale>)
CMTime(value: <CMTimeValue>, timescale: <CMTimeScale>)
CMTime(value: <CMTimeValue>, timescale: <CMTimeScale>, flags: <CMTimeFlags>, epoch: <CMTimeEpoch>)