iOS经常需要用到数据类型的转换，下面列举一下常用类型的转换。

1.NSDictionary类型转换为NSData类型：

//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类型转换为NSDictionary类型：

//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类型转换为JSON数据类型：

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

注意JSON与NSDictionary类型的转换一般会用到第三方公开的JSON库，读者可以在网上找到很多。