摘要：
1.声明类别@interface NSString (NumberConvenience)-(NSNumber *) lengthAsNumber;@end该声明表示，类别的名称是NumberConvenience，而且该类别将向NSString类中添加方法。2.实现类别@implementation NSString (NumberConvenience)-(NSNumber *) lengthAsNumber{ unsigned int length = [self length]; return ([NSNumber numberWithUnsignedInt: length]);}例1 阅读全文
