         // 注意点：
         // - このビデオトラックにはコンポジション上のビデオトラックを指定すること
         // - IOSurfaceで作成しなくても再生できるが、念のため付けておく
-        let videoTracks = composition.tracksWithMediaType(AVMediaTypeVideo)
-        var output = AVAssetReaderVideoCompositionOutput(videoTracks: videoTracks,
+        let compoVideoTracks = composition.tracksWithMediaType(AVMediaTypeVideo)
+        var output = AVAssetReaderVideoCompositionOutput(videoTracks: compoVideoTracks,
             videoSettings: [kCVPixelBufferPixelFormatTypeKey : kCVPixelFormatType_32BGRA,
                 kCVPixelBufferIOSurfacePropertiesKey : [:]])
         output.videoComposition = videoComposition

    カメラロールから古い順で10個のビデオを取り出し、リーダーをセットアップする
    */
    func setupAssets() {
        weak var weakSelf = self
        let collections = PHAssetCollection.fetchAssetCollectionsWithType(.SmartAlbum, subtype:.SmartAlbumVideos, options: nil)
        collections.enumerateObjectsUsingBlock { (collection, index, stop)  in

            let assets = PHAsset.fetchAssetsInAssetCollection(collection as PHAssetCollection, options: nil)


                _ = PHImageManager.defaultManager().requestAVAssetForVideo(asset as PHAsset,
                    options:nil, resultHandler: { (avasset:AVAsset!, audioMix:AVAudioMix!, info) -> Void in
                        dispatch_async(dispatch_get_main_queue()) {
                            if self.readers.count < 10 {
                                let reader = self.buildAssetReader(avasset)
                                self.readers.append(reader!)
                                reader!.startReading()
                                NSLog("[\(index)] start reading")
                            } else {
