后备存储

• In contrast , streams provide a way to write and read bytes to and from a backing store that can be one of several storage mediums
  相反，流提供一种向后备存储写入字节和从后备存储读取字节的方式，后备存储可以为多种存储媒介之一。
• To write text to a custom backing store using the same apis you would use for a string or a stream , or to add support for text formatting
  的实例将文本写入自定义后备存储区（所使用的api与用于字符串或流的api相同） ，或者增加对文本格式化的支持。