迪米特法则
- 类图对比

迪米特法则

一个对象应该对尽可能少的对象有接触，也就是只接触那些真正需要接触的对象。

解读

迪米特原则也叫做最少知道原则（Least Know Principle），一个类应该只和它的成员变量，方法的输入，返回参数中的类作交流，而不应该引入其他的类（间接交流）。

类图对比

未实践迪米特法则：

//================== Car.h ==================

@class GasEngine;

@interface Car : NSObject

//构造方法
- (instancetype)initWithEngine:(GasEngine *)engine;

//返回私有成员变量：引擎的实例
- (GasEngine *)usingEngine;

@end




//================== Car.m ==================

#import "Car.h"
#import "GasEngine.h"

@implementation Car
{
    GasEngine *_engine;
}

- (instancetype)initWithEngine:(GasEngine *)engine{
    
    self = [super init];
    
    if (self) {
        _engine = engine;
    }
    return self;
}

- (GasEngine *)usingEngine{
    
    return _engine;
}

@end

//================== GasEngine.h ==================
@interface GasEngine : NSObject

@property (nonatomic, copy) NSString *brandName;

@end

//================== Client.m ==================

#import "GasEngine.h"
#import "Car.h"

- (NSString *)findCarEngineBrandName:(Car *)car{

    GasEngine *engine = [car usingEngine];
    NSString *engineBrandName = engine.brandName;//获取到了引擎的品牌名称
    return engineBrandName;
}

实践了迪米特法则：

//================== Car.h ==================

@class GasEngine;

@interface Car : NSObject

//构造方法
- (instancetype)initWithEngine:(GasEngine *)engine;

//直接返回引擎品牌名称
- (NSString *)usingEngineBrandName;

@end


//================== Car.m ==================

#import "Car.h"
#import "GasEngine.h"

@implementation Car
{
    GasEngine *_engine;
}

- (instancetype)initWithEngine:(GasEngine *)engine{
    
    self = [super init];
    
    if (self) {
        _engine = engine;
    }
    return self;
}


- (NSString *)usingEngineBrandName{
    return _engine.brand;
}

@end

//================== Client.m ==================

#import "Car.h"

- (NSString *)findCarEngineBrandName:(Car *)car{
    
    NSString *engineBrandName = [car usingEngineBrandName]; //直接获取到了引擎的品牌名称
    return engineBrandName;
}

很明显，在实践了迪米特法则的 UML 类图里面，没有了Client对GasEngine的依赖，耦合性降低。

迪米特法则

类图对比

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