ios Objective-C 5.0 复合

复合和继承一样是建立两个类之间的关系的一种方式。
编程中的复合就像音乐中的作曲，将多个组件组合在一起，配合使用，从而得到完整的作品。

基本类型int、float、enum、struct这些基本类型只能是对象的一部分，只有对象间的组合才能叫做复合。

如：一辆汽车有一个发动机和4个轮子组成

@inteface  <Foundation/Foundation.h>

@interface Tire:NSObject
- (NSString *) description
@end   //Tire 类申明


@implementation Tire
- (NSString *) description{

      return (@"I am a tire. I last a while.");
}
@end 


//------------------------------Engine-----------------------
@interface Engine:NSObject
- (NSString *) description
@end   //Engine类申明


@implementation Engine
- (NSString *) description{

      return (@"I am a Engine.");
}
@end

• 在Cocoa中get有着特殊的含义，所以我们在申明属性的get和set方法时，建议最好不要使用get命名，这样会让开发者混淆，该方法的作用。

//------------------------------Car-----------------------
@interface Car:NSObject
{
        Engine  *engine;
        Tire  *tire[4];
}
-(void)    setEngine:(Engine *)   newEngine;
-(Engine *)    engine;
-(Tire *)    tireAtIndex:(int) index;
-(void)     setTire:(Tire  *) tire   atIndex:(int)  index;
-(void)     print;
-(NSString *)     description;
@end   //Car类申明

@implementation Car
- (NSString *) description{

      return (@"I am a Engine.");
}

-(Engine *) engine
{
      return (engine);
}

-(void) setEngine:(Engine  *) newEngine
{
        engine = newEngine;
}

-(void)  setTire:(Tire *) tire atIndex:(int) index
{
    if(index <0  || index >3){

        NSLog(@"bad index (%d) in setTire:atIndex: ",index);
        exit(1);
    }
    tire[index]  = tire;
}  //setTire:atIndex;

-(Tire *) tireAtIndex: (int ) Index{
      if(index <0  || index >3){

        NSLog(@"bad index (%d) in setTire:atIndex: ",index);
        exit(1);
      }

      return (tire[index]);
    
} //TireAtIndex;

@end

• 此处的代码使用的是防御式编程思想，防御式编程能够在开发的早期发现错误，比如数组的越界问题。
• 在Object-C中所有对象间的交互都是通过指针实现的，所以get方法返回的都是指针。

    //------- Engine 的使用 -----------

      Car   *car = [Car new];
            
      Engine  *engine = [Engine new];
      [car  setEngine:engine];
      NSLog(@"the car's engine is  %@",[car  engine]);


      //---------Tire 的使用---------------
      Tire  *tire = [Tire new];
      [car  setTire:tire  atIndex:2];
      NSLog(@"tire  number  two is %@",[car  tireAtIndex:2]);


      //or
      for(int i =0 ; i <4 ; i++ ){
          Tire  *tire = [Tire  new];
          [car setTire: tire atIndex: i];
      }

      [car   print];
      return (0);

2.如何使用继承和复合的形式组装Car

//----   new brand  Engine ----
@interface Slant6:Engine
@end

@implemention Slant6
-(NSString  *) descrption{

    return (@"I  am a  slant - 6.  VROOOM !");
}
@end  //Slant6

//----   new brand  Tire ----
@interface AllWeatherRadial:Tire
@end

@implemention AllWeatherRadial:Tire
-(NSString  *) descrption{

    return (@"I  am a  tire for rain or shine .");
}
@end  //AllWeatherRadial


//-----  main  ------


 Car   *car = [Car new];
 for(int i = 0;i<4;i++){
      Tire   *tire  = [AllWeatherRadia  new];
      [car  setTire:tire atIndex:i]
  }
  
  Engine  *engine = [Slant6  new];
  [car setEngine:engine];
  [car   print];
  return (0);