OC基础学习5：源文件组织

将程序拆分为多个小文件有助于更快地找到重要的代码，而且其他人在查看项目时也能有个大致的了解。

拆分接口和实现

根据@interface和@implementation，OC的代码通常放在两种文件里：

• 接口部分(.h)：类的@interface指令、公共struct定义、enum常量、#defines和extern全局变量等。
• 实现部分(.m): 类的@implementation指令、全局变量的定义、私有struct等。

另外，复杂的项目可以拥有多个目标，它们源文件的配置各不相同，构建规则也不同。群组关系仅仅是有Xcode负责管理的一项奇妙的功能。

拆分Car程序

• #import 带尖号的是导入系统头文件(只读)，如#import <Foundation/Foundation.h>；双引号是项目本地的代码文件，如#import "Engine.h"。

• 拆分上一篇中的Car程序
  首先拆分继承自NSObject的类：Tire和Engine。

    //  Tire.h
  
    #import <Foundation/Foundation.h>
  
    @interface Tire : NSObject
  
    @end
  

    //  Tire.m
  
    #import "Tire.h"
  
    @implementation Tire
    //#pragma mark -
    - (NSString *)description {
        return (@"I am a Tire.");
    } //description
  
    @end
  
  

    //  Engine.h
  
    #import <Foundation/Foundation.h>
  
    @interface Engine : NSObject
  
    @end
  
  

    //  Engine.m
    #import "Engine.h"
  
    @implementation Engine
    - (NSString *)description {
        return (@"I am a Engine.");
    } //description
    @end
  
  

使用跨文件依赖关系

• @class 是告诉编译器：“这是一个类，只会通过指针来引用它，不需要关注此类的更多信息”。可减少必须导入的头文件的数量，从而缩短编译时间。
  拆分Car类：

    //  Car.h
  
    #import <Foundation/Foundation.h>
  
    @class Engine;
    @class Tire;
  
    @interface Car : NSObject
    - (Engine *) engine;
    - (void) setEngine: (Engine *) newEngine;
    - (Tire *) tireAtIndex: (int) index;
    - (void) setTire: (Tire *) tire atIndex: (int) index;
    - (void)print;
    @end //Car
  

    //  Car.m
  
    #import "Car.h"
    #import "Tire.h"
    #import "Engine.h"
  
    @implementation Car
    {
        Engine *engine;
        Tire *tires[4];
    }
    - (Engine *) engine
    {
        return (engine);
    } //engine
    - (void) setEngine:(Engine *)newEngine
    {
        engine = newEngine;
    } //setEngine
    - (void) setTire:(Tire *)tire atIndex:(int)index
    {
        if (index<0 || index>3) {
            NSLog(@"bad index (%d) in setTire:atIndex", index);
            exit(1);
        }
        tires[index] = tire;
    } // setTire
    - (Tire *) tireAtIndex:(int)index
    {
        if (index<0 || index>3) {
            NSLog(@"bad index (%d) in setTire:atIndex", index);
            exit(1);
        }
        return tires[index];
    } // tireAtIndex
  
    - (id)init
    {
        if (self = [super init]) {  //?
            engine = [Engine new];
            tires[0] = [Tire new];
            tires[1] = [Tire new];
            tires[2] = [Tire new];
            tires[3] = [Tire new];
        }
        return (self);
    }
    - (void)print
    {
        NSLog(@"%@", engine);
        NSLog(@"%@", tires[0]);
        NSLog(@"%@", tires[1]);
        NSLog(@"%@", tires[2]);
        NSLog(@"%@", tires[3]);
    }
  
    @end //Car
  
  

• 编译器需要先知道所有关于超类的信息才能成功地为其子类编译@interface部分。
  拆分Slant6和AllWeatherRadial：

    //  Slant6.h
  
    #import "Engine.h"
  
    @interface Slant6 : Engine
  
    @end
  

    //  Slant6.m
  
    #import "Slant6.h"
  
    @implementation Slant6
    - (NSString *)description {
        return (@"I am a slant-6.VROOM!");
    }
    @end
  
  

    //  AllWeatherRadial.h
  
    #import "Tire.h"
  
    @interface AllWeatherRadial : Tire
  
    @end
  

    //  AllWeatherRadial.m
  
    #import "AllWeatherRadial.h"
  
    @implementation AllWeatherRadial
    -(NSString *)description
    {
        return (@"I am a tire for rain or shine.");
    }
    @end
  

最后是main.m文件：

//  main.m

#import <Foundation/Foundation.h>
#import "Car.h"
#import "Tire.h"
#import "Engine.h"
#import "Slant6.h"
#import "AllWeatherRadial.h"

int main(int argc, const char * argv[]) {
   
    Car *car = [Car new];
    
    Engine *engine = [Slant6 new];
    [car setEngine:engine];
    for (int i=0; i<4; i++) {
        Tire *tire = [AllWeatherRadial new];
        [car setTire:tire atIndex:i];
    }
    
    [car print];
    
    return 0;
}