 
                 // 算出した値なので、端数が出ないよう1/600スケールに丸めて返す
                 let time = CMTimeConvertScale(result.time, 600, .RoundHalfAwayFromZero)
-                return (result.index, time)
+                return AssetPosition(index: result.index, time: time)
             }
         }
         return nil

        }
        var currentAsset: AVAsset? = nil
        if let pos = pos {
            if let playerInfo = getAssetInfoOf(pos) {
                currentAsset = _assets[playerInfo.index]
                position = pos
                _currentPresentationTimestamp = playerInfo.time


    :returns: 再生位置(0.0-1.0)。値域外の場合はnilを返す
    */
    private func _getRelativePositionOf(index:Int, time:CMTime) -> Float? {
        let target = AssetPosition(index:index, time:time)

        /*
