沙盒文件的创建（简单举例）

//创建文件
nsstring *documentsdirectory = [nshomedirectory() stringbyappendingpathcomponent:@"documents"];
    nsfilemanager *filemanager = [nsfilemanager defaultmanager];
    nsstring *filepath = [documentsdirectory stringbyappendingpathcomponent:@"xxx.txt"];
    nslog(@"-----%@",filepath);
    if(![filemanager fileexistsatpath:filepath]) //如果文件xxx不存在
    {
        nslog(@"xxx.txt is not exist");
        if( [filemanager createfileatpath:filepath contents:nil attributes:nil]) //创建文件         
            {
             //
            }
      }
      else {
                 //
      }

//数据写入文件
  nsstring *buf=@"数据";
                [buf writetofile:filepath atomically:yes encoding:nsutf8stringencoding error:nil];

//从文件读数据
nsstring * buf = [nsstring stringwithcontentsoffile:filepath encoding:nsutf8stringencoding error:nil];