复合: 对象间的组合(类中中包括类)

composition在音乐中翻译为作曲:将多个组件组合在一起,配合使用,从而得到完整的作品。在OC中,复合是通过包含作为实例变量的对象指针实现的。

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@interface Car : NSObject
{
    Engine *engine;
    Tire *tires[4];
}
* (void)print;
@end //Car

自定义NSLog()

NSLog()使用%@格式说明符表示输出对象,也就是NSLog()会给这个对象发送了description消息,然后对象的description方法生成一个NSString并将其返回。在类中提供description方法就可以自定义NSLog()如何输出对象。

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@interface Tire : NSObject
@end //Tire 轮胎
@implementation Tire
- (NSString *)description {
    return (@"I am a tire.");
}
@end

int main(int argc, const char * argv[]) {
    
    Tire *tire = [Tire new];
    NSLog(@"%@", tire);    // 输出 "I am a tire."
     
    return 0;
}

存取方法

  • 存取(accessor) 方法是用来读取或改变某个对象属性的方法。分为 getter方法 和 setter方法。
  • 应该尽量使用对象提供的存取方法,不要直接改变对象里面的值。 存取方法总是成对出现的。 命名方式:setter方法在要更改属性的加上前缀set;getter方法直接是属性名称。
    @interface Car : NSObject
    {
        Engine *engine;
        Tire *tires[4];
    }
    - (Engine *) engine;
    - (void) setEngine: (Engine *) newEngine;
    - (Tire *) tireAtIndex: (int) index;
    - (void) setTire: (Tire *) tire atIndex: (int) index;
    - (void)print;
    @end //Car
  • 在OC中所有对象间的交互都是通过指针实现的。

复合还是继承

继承的关系:“Is a”(是一个)。如三角形是一个形状,Slant6是一个发动机…… 复合的关系:“has a”(有一个)。如形状有个填充颜色,汽车有个发动机和四个轮胎……

详细代码

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#import <Foundation/Foundation.h>

@interface Tire : NSObject
@end //Tire 轮胎
@implementation Tire
- (NSString *)description {
    return (@"I am a tire.");
}
@end

@interface Engine : NSObject
@end //Engine
@implementation Engine
- (NSString *)description {
    return (@"I am a Engine.");
} //description
@end

@interface Slant6 : Engine
@end //slant-6型号的发动机
@implementation Slant6

- (NSString *)description {
    return (@"I am a slant-6型号的发动机.VROOM!");
}

@end

@interface AllWeatherRadial : Tire
@end // 新型轮胎
@implementation AllWeatherRadial

-(NSString *)description
{
    return (@"I am a tire for rain or shine.");
}

@end


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


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;
}