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