什么是Category

• 在OC中，扩展一个类的方式有两种：继承与分类
• 可以在不修改原来类的基础上，为这个类扩充一些方法
• 一个庞大的类可以分模块开发

Category的格式

• 通过Category给某类添加方法，分为声明和实现俩部分
• 创建Category时，必须给Category的名称加上专用前缀
• 创建Category的方法时，必须给方法名称加上专用前缀
• 分类声明

@interface Person (Play)
- (void)pGotoTheCinema;
- (void)pGotoSportGame;
@end

• 分类实现

#import "Person+Play.h"
@implementation Person (Play)
- (void)pGotoTheCinema{
}
- (void)pGotoSportGame{
}
@end

Category的类型

• Class-continuation类型的Category
  • 它必须定义在其所接续的那个类的实现文件中
  • 此分类可以声明属性，且此分类没有特定的是现实文件,其中方法都定义在主实现文件中
  • 一般存放不需要对外公开的属性(例子中的age)

  #import <Foundation/Foundation.h>
  @interface Person : NSObject
  @property (nonatomic,copy,readonly) NSString *firstName;
  @property (nonatomic,copy,readonly) NSString *lastName;
  @property (nonatomic,strong,readonly) NSArray *friends;
  - (instancetype)initWithFristName:(NSString *)firstName withLastName:(NSString *)lastName;
  @end
  --------------------
  #import "Person.h"
  @interface Person()
  @property (nonatomic,assign) NSInteger age;
  @property (nonatomic,strong,readwrite) NSArray *friends;
  @end
  @implementation Person
  - (instancetype)initWithFristName:(NSString *)firstName withLastName:(NSString *)lastName{
      self = [super init];
      if (self) {
          _firstName = firstName;
          _lastName = lastName;
      }
      return self;
  }
  @end
  

  • 如果某属性在主接口中声明为“只读”，而类内部要用setter方法修改此属性，那么就在Class-continuation分类中将其扩展为“readwrite”（例子中的friends）

  #import <Foundation/Foundation.h>
  @interface Person : NSObject
  @property (nonatomic,copy,readonly) NSString *firstName;
  @property (nonatomic,copy,readonly) NSString *lastName;
  @property (nonatomic,strong,readonly) NSArray *friends;
  
  - (instancetype)initWithFristName:(NSString *)firstName withLastName:(NSString *)lastName;
  @end
  
  -------------------------------
  #import "Person.h"
  @interface Person()
  @property (nonatomic,strong,readwrite) NSArray *friends;
  @end
  
  @implementation Person
  - (instancetype)initWithFristName:(NSString *)firstName withLastName:(NSString *)lastName{
     self = [super init];
     if (self) {
         _firstName = firstName;
         _lastName = lastName;
     }
     return self;
  }
  @end 
  

  • 若想使类遵循的协议不为人知道，则可以在Class-continuation分类中声明

  #import <Foundation/Foundation.h>
  @class User;
  @interface APP : NSObject
  @property (nonatomic,strong) User *user;
  @end
  
  -----------------------------
  #import "APP.h"
  #import "User.h"
  @interface APP()<UserDelegate>
  
  @end
  @implementation APP
  -(void)doSomething{
      NSLog(@"dosomething");
  }
  @end
  

• 普通类型的Category

#import "Person.h"
@interface Person (Play)
- (void)pGotoTheCinema;
- (void)pGotoSportGame;
@end
--------------
#import "Person+Play.h"
@implementation Person (Play)
- (void)pGotoTheCinema{
}
- (void)pGotoSportGame{
}
@end

Category的运用

• 在开发中，类的实现文件特别大，难于管理与维护，因此经常使用分类机制把类的实现代码划分成易于管理的小块，以便单独检视

#import <Foundation/Foundation.h>
@interface Person : NSObject
@property (nonatomic,copy) NSString *firstName;
@property (nonatomic,copy) NSString *lastName;
@property (nonatomic,strong) NSArray *friendArray;

- (instancetype)initWithFristName:(NSString *)firstName withLastName:(NSString *)lastName;
- (void)addPerson:(Person *)person;
- (void)removePerson:(Person *)person;
- (BOOL)isFriendWithPerson:(Person *)person;
- (void)performDayWork;
- (void)takeVacationFromWork;
- (void)gotoTheCinema;
- (void)gotoSportGame;
@end

• 实现文件里，所有的方法都写在一个类，内容太多，所以我们可根据其不同功能分成多个分类

#import <Foundation/Foundation.h>
@interface Person : NSObject
@property (nonatomic,copy) NSString *firstName;
@property (nonatomic,copy) NSString *lastName;
@property (nonatomic,strong) NSArray *friends;
- (instancetype)initWithFristName:(NSString *)firstName withLastName:(NSString *)lastName;
@end
-------------------------
#import "Person.h"
@interface Person (Work)
- (void)wPerformDayWork;
- (void)wTakeVacationFromWork;
@end
-------------------------
#import "Person.h"
@interface Person (Play)
- (void)pGotoTheCinema;
- (void)pGotoSportGame;
@end
-------------------------
#import "Person.h"
@interface Person (Friendship)
- (void)fAddPerson:(Person *)person;
- (void)fRemovePerson:(Person *)person;
- (BOOL)fIsFriendWithPerson:(Person *)person;
@end

Category的注意事项

• Category只能添加方法，不能添加属性。因为Category中的@property，只会生成setter/getter的方法声明，不会生成实现及私有的成员变量（在.m文件(Class-continuation)中的分类可以声明属性，同时也可以生成setter、getter方法）

 #import "Person.h"
 @interface Person (Play)
 @property (nonatomic,copy) NSString *playGameName;
 - (void)pGotoTheCinema;
 - (void)pGotoSportGame;
 @end
 -------------
 - (void)viewDidLoad {
   [super viewDidLoad];
   Person *p = [[Person alloc]initWithFristName:@"firstName" withLastName:@"lastName"];
   p.playGameName = @"football";
}

Xcode会有警告

Xcode会有警告.png

运行程序会崩溃

运行程序会崩溃.png

• 如果分类中有和原类中同名的方法，程序只会调用分类里的方法，如果多个分类中都有和原类中同名的方法，程序只会由编译器决定，编译器最后一个执行的方法来响应
  调用优先级(Category->本类->父类)

#import <Foundation/Foundation.h>
@interface Person : NSObject
@property (nonatomic,copy,readonly) NSString *firstName;
@property (nonatomic,copy,readonly) NSString *lastName;
@property (nonatomic,strong,readonly) NSArray *friends;
- (instancetype)initWithFristName:(NSString *)firstName withLastName:(NSString *)lastName;
- (void)run;
@end
------------------------------------
#import "Person.h"
@interface Person (Play)
@property (nonatomic,copy) NSString *playGameName;
- (void)pGotoTheCinema;
- (void)pGotoSportGame;
- (void)run;
@end
-------------------------------------
#import "Person.h"
@interface Person (Work)
- (void)wPerformDayWork;
- (void)wTakeVacationFromWork;
- (void)run;
@end
-------------------------------------
Person *p = [[Person alloc]initWithFristName:@"firstName" withLastName:@"lastName"];
[p run];

log.png