                 [unowned self] in
                 let lock = ScopedLock(self)
 
-                println("move to next")
                 self._prepareNextAssetReaders()
 
             }

    */
    func advanceToNextAsset() {
        if !_readers.isEmpty {
            // window外の古いアセットを削除
            if self.autoRemoveOutdatedAssets {
                if let assetPos = self._getAssetPositionOf(0.0) {
                    println("\(0..<assetPos.index) is outdated.")

                if let assetreader = AssetReaderFragment(asset:asset) {
                    self._readers.append(assetreader)
                } else {
                    NSLog("0) Failed to instantiate a reader of [\((asset as AVURLAsset).URL.lastPathComponent!)]")
                }
            }
        }
