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。
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  //  Tire.h

  #import <Foundation/Foundation.h>

  @interface Tire : NSObject

  @end

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  //  Tire.m

  #import "Tire.h"

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

  @end

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  //  Engine.h

  #import <Foundation/Foundation.h>

  @interface Engine : NSObject

  @end

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  //  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：

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      //  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

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      //  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：

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      //  Slant6.h

  #import "Engine.h"

  @interface Slant6 : Engine

  @end

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      //  Slant6.m

  #import "Slant6.h"

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

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      //  AllWeatherRadial.h

  #import "Tire.h"

  @interface AllWeatherRadial : Tire

  @end

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      //  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：

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//  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;
}