Yesterday, i need to store some offline data in Sqlite, so I convert json(actually it’s NSDictionary) to NSString and stored in Sqlite.

at the first time, i just convert NSDictionary to NSString directly, like:

NSString *str = [NSString stringFromFormat: @"%@", json];

and then what i get seems correct: a string that looks like a NSDictionary.Subsequently, I put’em into DB.What made me annoyed is that when I get the data from DB(a string you know), this piece of string just cannot convert to a NSDictionary as i expected.Which means during the convertion from NSDictionary to NSString, we lost some format or some meta info.

I googled this and got this:

Apple provides a Dictionary -> String method that has no inverse

which completely explain why we failed.

You need format so I give you format. I Give you: NSPropertyListSerialization (Sparticus! :D)

  • Convert NSDictionary to NSString ``` Objective-C NSData *plist = [NSPropertyListSerialization dataWithPropertyList:cache_dictionary // NSDictionary format:NSPropertyListXMLFormat_v1_0 options:kNilOptions error:NULL];

NSString *str = [[NSString alloc] initWithData:plist encoding:NSUTF8StringEncoding];


+ Convert NSString to NSDictionary
``` Objective-C
NSDictionary *article_map = 
						[NSPropertyListSerialization
             propertyListWithData:[article_map_content dataUsingEncoding:NSUTF8StringEncoding]
             options:kNilOptions
             format:NULL
             error:NULL];

There must be some one doubt that why couldnt we use NSKeyedArchiver(NSKeyedUnarchiver), ya absolutely it could do unless you wont store data into Sqlite.

  • Archive 
    Post *aPost = [Post alloc] initWith: postDic];
NSData *data = [NSKeyedArchiver archiveDataWithRootObject: aPost];
  • Unarchive 
    Post *aPost =  [NSKeyedUnarchiver unarchiveObjectWithData: data];

So what are the differences, lets make a conclusion:

  • NSPropertyListSerialization
    • only for NSDictionary
    • could get data formated NSString
    • dont need to do NSCoding work
  • NSKeyedArchiver
    • for Object
    • get data directly
    • need to do NSCoding