在进行网络数据通信的时 候，经常会遇到NSData类型的数据。在该数据是dictionary结构的情况下，系统没有提供现成的转换成NSDictionary的方法，为此可 以通过Category对NSDictionary进行扩展，以支持从NSData到NSDictionary的转换。声明和实现如下：

@interface NSDictionary (Helpers)
+ (NSDictionary *)dictionaryWithContentsOfData:(NSData *)data;
@end

@implementation NSDictionary (Helpers)
+ (NSDictionary *)dictionaryWithContentsOfData:(NSData *)data {
    CFPropertyListRef plist =  CFPropertyListCreateFromXMLData(kCFAllocatorDefault, (CFDataRef)data,
                                                               kCFPropertyListImmutable,
                                                               NULL);
    if(plist == nil) return nil;
    if ([(id)plist isKindOfClass:[NSDictionary class]]) {
        return [(NSDictionary *)plist autorelease];
    }
    else {
        CFRelease(plist);
        return nil;
    }
}
@end