依赖倒置原则

依赖倒置原则
- UML 类图对比

依赖抽象，而不是依赖实现。

抽象不应该依赖细节；细节应该依赖抽象。

高层模块不能依赖低层模块，二者都应该依赖抽象。

解读

针对接口编程，而不是针对实现编程。
尽量不要从具体的类派生，而是以继承抽象类或实现接口来实现。
关于高层模块与低层模块的划分可以按照决策能力的高低进行划分。业务层自然就处于上层模块，逻辑层和数据层自然就归类为底层。

UML 类图对比

未实践依赖倒置原则：

//================== FrondEndDeveloper.h ==================

@interface FrondEndDeveloper : NSObject

- (void)writeJavaScriptCode;

@end

//================== FrondEndDeveloper.m ==================

@implementation FrondEndDeveloper

- (void)writeJavaScriptCode{
    NSLog(@"Write JavaScript code");
}

@end

//================== BackEndDeveloper.h ==================

@interface BackEndDeveloper : NSObject

- (void)writeJavaCode;

@end

//================== BackEndDeveloper.m ==================

@implementation BackEndDeveloper

- (void)writeJavaCode{
    NSLog(@"Write Java code");
}
@end

//================== Project.h ==================

@interface Project : NSObject

//构造方法，传入开发者的数组
- (instancetype)initWithDevelopers:(NSArray *)developers;

//开始开发
- (void)startDeveloping;

@end



//================== Project.m ==================

#import "Project.h"
#import "FrondEndDeveloper.h"
#import "BackEndDeveloper.h"

@implementation Project
{
    NSArray *_developers;
}


- (instancetype)initWithDevelopers:(NSArray *)developers{
    
    if (self = [super init]) {
        _developers = developers;
    }
    return self;
}



- (void)startDeveloping{
    
    [_developers enumerateObjectsUsingBlock:^(id  _Nonnull developer, NSUInteger idx, BOOL * _Nonnull stop) {
        
        if ([developer isKindOfClass:[FrondEndDeveloper class]]) {
            
            [developer writeJavaScriptCode];
            
        }else if ([developer isKindOfClass:[BackEndDeveloper class]]){
            
            [developer writeJavaCode];
            
        }else{
            //no such developer
        }
    }];
}

@end

实践了依赖倒置原则：

//================== DeveloperProtocol.h ==================

@protocol DeveloperProtocol <NSObject>

- (void)writeCode;

@end

//================== FrondEndDeveloper.h ==================

@interface FrondEndDeveloper : NSObject<DeveloperProtocol>
@end



//================== FrondEndDeveloper.m ==================

@implementation FrondEndDeveloper

- (void)writeCode{
    NSLog(@"Write JavaScript code");
}
@end

//================== BackEndDeveloper.h ==================

@interface BackEndDeveloper : NSObject<DeveloperProtocol>
@end



//================== BackEndDeveloper.m ==================
@implementation BackEndDeveloper

- (void)writeCode{
    NSLog(@"Write Java code");
}
@end

//================== Project.h ==================

#import "DeveloperProtocol.h"

@interface Project : NSObject

//只需传入遵循DeveloperProtocol的对象数组即可
- (instancetype)initWithDevelopers:(NSArray <id <DeveloperProtocol>>*)developers;

//开始开发
- (void)startDeveloping;

@end


//================== Project.m ==================

#import "FrondEndDeveloper.h"
#import "BackEndDeveloper.h"

@implementation Project
{
    NSArray <id <DeveloperProtocol>>* _developers;
}


- (instancetype)initWithDevelopers:(NSArray <id <DeveloperProtocol>>*)developers{
    
    if (self = [super init]) {
        _developers = developers;
    }
    return self;
    
}


- (void)startDeveloping{
    
    //每次循环，直接向对象发送writeCode方法即可，不需要判断
    [_developers enumerateObjectsUsingBlock:^(id<DeveloperProtocol>  _Nonnull developer, NSUInteger idx, BOOL * _Nonnull stop) {
        
        [developer writeCode];
    }];
    
}

@end

依赖倒置原则

UML 类图对比

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