为NSString添加降序排序的category

@interface NSString (Compare)- (void)sayHi;//添加两个字符串比较的方法- (NSComparisonResult)compareAgainst:(NSString *)anString;@end#import "NSString+Compare.h"@implementation NSString (Compare)- (void)sayHi {    NSLog(@"hello, category");}//添加两个字符串比较的方法- (NSComparisonResult)compareAgainst:(NSString *)anString {    return -[self compare:anString];}@endNSArray *arr = @[@"dd", @"aa", @"cc", @"bb", @"ff"];NSArray *desArr = [arr sortedArrayUsingSelector:@selector(compareAgainst:)];        NSLog(@"%@", desArr);