第二条：在类的头文件中尽量少引入其他头文件

用 Objective-C语言编写“类”(class)的标准方式为:以类名做文件名,分别创建两个文件,头文件后缀用h,实现文件后缀用,m。创建好一个类之后,其代码看上去如下所示:

//EOCPerson. h
#import <Foundation/ Foundation.h>

@interface EOCPerson :NSObject
@property (nonatomic, copy) NSString *firstName;
@property (nonatomic, copy) NSString *lastName;
@end

∥ EOCPerson.m
#import "EOCPerson.h"

implementation EOCPerson
//Implementation of method
@end

增加EOCEmployer的新类

//EOCPerson. h
#import <Foundation/ Foundation.h>
//常见的方法是加入
#import "EOCEmployer.h"

@interface EOCPerson :NSObject
@property (nonatomic, copy) NSString *firstName;
@property (nonatomic, copy) NSString *lastName;
@property (nonatomic, strong) EOCEmployer *employer;

@end

更优雅的写法-使用"向前声明"，声明该类

//EOCPerson. h
#import <Foundation/ Foundation.h>

@class EOCEmployer;

@interface EOCPerson :NSObject
@property (nonatomic, copy) NSString *firstName;
@property (nonatomic, copy) NSString *lastName;
@property (nonatomic, strong) EOCEmployer *employer;

@end

∥ EOCPerson.m
#import "EOCPerson.h"
//在此加入EOCEmployer引用
#import "EOCEmployer.h"

implementation EOCPerson
//Implementation of method
@end

@class一般情况下写在.h文件中，告诉编译器这是一个类，不会进行拷贝类。在.m中使用的时候在进行#Improt ,会提升编译效率。切记，要使用类，在.m中别忘记引用。