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