问@synthesize vs @dynamic，有什么区别？

#import <Foundation/Foundation.h>

@interface Book : NSObject

{

   NSMutableDictionary *data;

}

@property (retain) NSString *title;

@property (retain) NSString *author;

@end

@implementation Book

@dynamic title, author;

- (id)init

{

    if ((self = [super init])) {

        data = [[NSMutableDictionary alloc] init];

        [data setObject:@"Tom Sawyer" forKey:@"title"];

        [data setObject:@"Mark Twain" forKey:@"author"];

    }

    return self;

}

- (void)dealloc

{

    [data release];

    [super dealloc];

}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)selecto

{

    NSString *sel = NSStringFromSelector(selector);

    if ([sel rangeOfString:@"set"].location == 0) {

        return [NSMethodSignature signatureWithObjCTypes:"v@:@"];

    } else {

        return [NSMethodSignature signatureWithObjCTypes:"@@:"];

    }

 }

- (void)forwardInvocation:(NSInvocation *)invocation

{

    NSString *key = NSStringFromSelector([invocation selector]);

    if ([key rangeOfString:@"set"].location == 0) {

        key = [[key substringWithRange:NSMakeRange(3, [key length]-4)] lowercaseString];

        NSString *obj;

        [invocation getArgument:&obj atIndex:2];

        [data setObject:obj forKey:key];

    } else {

        NSString *obj = [data objectForKey:key];

        [invocation setReturnValue:&obj];

    }

}

@end

int main(int argc, char **argv)

{

    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    Book *book = [[Book alloc] init];

    printf("%s is written by %s\n", [book.title UTF8String], [book.author UTF8String]);

    book.title = @"1984";

    book.author = @"George Orwell";

    printf("%s is written by %s\n", [book.title UTF8String], [book.author UTF8String]);

   [book release];

   [pool release];

   return 0;

}