iOS經常需要用到資料型別的轉換，下面列舉一下常用型別的轉換。

1.NSDictionary型別轉換為NSData型別：

//NSDictionary -> NSData:
NSDictionary *params = [NSDictionary dictionaryWithObjectsAndKeys:
                                @"balance", @"key",
                               @"remaining balance", @"label",
                                @"45", @"value",
                                @"USD", @"currencyCode",nil];
         
NSMutableData *data = [[NSMutableData alloc] init];
NSKeyedArchiver *archiver = [[NSKeyedArchiver alloc] initForWritingWithMutableData:data];
[archiver encodeObject:params forKey:@"Some Key Value"];
[archiver finishEncoding];
 

2.NSData型別轉換為NSDictionary型別：

//NSData -> NSDictionary
NSData *data = [[NSMutableData alloc] initWithContentsOfFile:[self dataFilePath]];
NSKeyedUnarchiver *unarchiver = [[NSKeyedUnarchiver alloc] initForReadingWithData:data];
NSDictionary *myDictionary = [[unarchiver decodeObjectForKey:@"Some Key Value"] retain];
[unarchiver finishDecoding];
[unarchiver release];
[data release];
 

3.NSDictionary型別轉換為JSON資料型別：

//NSDictionary -> JSON:
NSString *jsonStr=[dict JSONRepresentation];