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;
}