这里是将downTaskEntity.downloadDetails中的文件进行拼接，打开第一个文件准备写入filehandleA，定义FILE_SIZE为600KB，如果单个文件小于FILE_SIZE，直接在filehandleA结尾拼接，如果大于FILE_SIZE，将该文件按照FILE_SIZE大小拆分并拼接到filehandleA结尾处。这样做的原因主要是直接写入大文件会有性能问题。

 for (idx, obj) in downTaskEntity.downloadDetails.enumerated(){

           filepath = obj.filePath

           if idx == 0 {
               filehandleA = FileHandle.init(forWritingAtPath: filepath)
               
           } else {
               fileSize = MDDownloadManager.getSize(filePath: filepath)
               if fileSize < FILE_SIZE {
                   let data = NSData.init(contentsOfFile: filepath)
                   if data != nil {
                       filehandleA?.seekToEndOfFile()
                       filehandleA?.write(data! as Data)
                   }
               } else {
                   filehandleB = FileHandle.init(forReadingAtPath: filepath)

                   let sizes = (fileSize + FILE_SIZE / 2) / FILE_SIZE

                   for index in 0...sizes {
                       filehandleA?.seekToEndOfFile()
                       filehandleB?.seek(toFileOffset: UInt64(index*FILE_SIZE))
                       if index == sizes {
                           let data = filehandleB?.readDataToEndOfFile()
                           filehandleA?.write(data!)
                           filehandleB?.closeFile()
                           self.deleteFileAtPath(path: filepath)
                       } else {
                           let data = filehandleB?.readData(ofLength: FILE_SIZE)
                           filehandleA?.write(data!)
                       }
                   }
               }
           }
       }
       filehandleA?.closeFile()
       filehandleB?.closeFile()

欢迎大神批评指正哦！