Category使用实例，格式化时间
//// NSDateCategory.h// WebGameIphone//// Created by apple on 12-1-9.// Copyright 2012年 __MyCompanyName__. All rights reserved.//#import <Foundation/Foundation.h>@interface NSDate (NSDateCategory)// 根据格式化样式得到时间字符串- (NSString *)stringWithFormat:(NSString*)fmt;// 根据字符串和格式化样式得到时间+ (NSDate *)dateFromString:(NSString*)str withFormat:(NSString*)fmt;// 改变时间字符串格式化样式+ (NSString *)dateStringReplaceFormat:(NSString*)str oldFormat:(NSString*)fmt newFormat:(NSString*)n_fmt;@end@implementation NSDate (NSDateCategory)- (NSString *)stringWithFormat:(NSString *)fmt { static NSDateFormatter *fmtter; if (fmtter == nil) { fmtter = [[NSDateFormatter alloc] init]; } if (fmt == nil || [fmt isEqualToString:@""]) { fmt = @"HH:mm:ss"; } [fmtter setDateFormat:fmt]; return [fmtter stringFromDate:self];}+ (NSDate *)dateFromString:(NSString *)str withFormat:(NSString *)fmt { static NSDateFormatter *fmtter; if (fmtter == nil) { fmtter = [[NSDateFormatter alloc] init]; } if (fmt == nil || [fmt isEqualToString:@""]) { fmt = @"HH:mm:ss"; } [fmtter setDateFormat:fmt]; return [fmtter dateFromString:str];}+ (NSString *)dateStringReplaceFormat:(NSString*)str oldFormat:(NSString*)fmt newFormat:(NSString*)n_fmt { NSDate *n_date = [NSDate dateFromString:str withFormat:fmt]; return [n_date stringWithFormat:n_fmt];}@end