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