开闭原则
原则一：开闭原则
一个软件实体如类、模块和函数应该对扩展开放，对修改关闭。
解读：
- 用抽象构建框架，用实现扩展细节。
- 不以改动原有类的方式来实现新需求，而是应该以实现事先抽象出来的接口（或具体类继承抽象类）的方式来实现。
类图对比
未实践开闭原则：
//================== Course.h ==================
@interface Course : NSObject
@property (nonatomic, copy) NSString *courseTitle; //课程名称
@property (nonatomic, copy) NSString *courseIntroduction; //课程介绍
@property (nonatomic, copy) NSString *teacherName; //讲师姓名
@property (nonatomic, copy) NSString *content; //文字内容
//新需求：视频课程
@property (nonatomic, copy) NSString *videoUrl;
//新需求：音频课程
@property (nonatomic, copy) NSString *audioUrl;
//新需求：直播课程
@property (nonatomic, copy) NSString *liveUrl;
@end
实践开闭原则：
//================== Course.h ==================
@interface Course : NSObject
@property (nonatomic, copy) NSString *courseTitle; //课程名称
@property (nonatomic, copy) NSString *courseIntroduction; //课程介绍
@property (nonatomic, copy) NSString *teacherName; //讲师姓名
//================== TextCourse.h ==================
@interface TextCourse : Course
@property (nonatomic, copy) NSString *content; //文字内容
@end
//================== VideoCourse.h ==================
@interface VideoCourse : Course
@property (nonatomic, copy) NSString *videoUrl; //视频地址
@end
//================== AudioCourse.h ==================
@interface AudioCourse : Course
@property (nonatomic, copy) NSString *audioUrl; //音频地址
@end
//================== LiveCourse.h ==================
@interface LiveCourse : Course
@property (nonatomic, copy) NSString *liveUrl; //直播地址
@end
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