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均衡代码职责

文章的标题有点绕口,不过想了半天,想不到更好的标题了。本文的诞生有一部分功劳要归于 iOS应用现状分析 ,标题也是来源于原文中的“能把代码职责均衡的划分到不同的功能类里”。如果你看过我的文章,就会发现我是一个 MVC 主导开发的人。这是因为开发的项目总是算不上大项目,在合理的代码职责分工后项目能保持良好的状态,就没有使用到其他架构开发过项目(如果你的状态跟笔者差不多,就算不适用其他架构模式,你也应该自己学习)

均衡代码职责

OK,简短来说,在很早之前我就有写这么一篇文章的想法,大致是在当初面试很多iOS开发者的时候这样的对话萌生的念头,下面的对话是经过笔者总结的,切勿对号入座:

Q: 你在项目中使用了MVVM的架构结构,能说说为什么采用的是这种结构吗?

A: 这是因为我们的项目在开发中控制器的代码越来越多,超过了一千行,然后觉得这样控制器的职责太多,就采用一个个ViewModel把这些职责分离出来

Q: 能说说你们控制器的职责吗?或者有源码可以参考一下吗?

面试者拿出电脑展示源码

最后的结果就是,笔者不认为面试者需要使用到 MVVM 来改进他们的架构,这里当然是见仁见智了。由于对方代码职责的不合理分工导致了 ViewModel 层几乎没有业务逻辑,从而导致了控制器的失衡,变得笨重。在这种情况下即便他使用了 ViewModel 将控制器的代码分离了出来,充其量只是 将垃圾挪到另一个地方罢了 。我在 MVC架构杂谈 中提到过自身对 MVC 三个模块的职责认识,当你想将 MVC 改进成 MVX 的其他结构时,应当先思考自己的代码职责是不是已经均衡了。

码农小明的项目

在开始之前,还是强烈推荐推荐 《重构-改善既有代码的设计》 这本书,一本好书或者好文章应该让你每次观赏时都能产生不同的感觉。

正常来说,造成你代码笨重的最大凶手是重复的代码,例如曾经笔者看过这样一张界面图以及逻辑代码:

均衡代码职责

@interface XXXViewController

@property (weak, nonatomic) IBOutlet UIButton * rule1;
@property (weak, nonatomic) IBOutlet UIButton * rule2;
@property (weak, nonatomic) IBOutlet UIButton * rule3;
@property (weak, nonatomic) IBOutlet UIButton * rule4;

@end

@implementation XXXViewController

- (IBAction)actionToClickRule1: (id)sender {
    [_rule1 setSelected: YES];
    [_rule2 setSelected: NO];
    [_rule3 setSelected: NO];
    [_rule4 setSelected: NO];
}

- (IBAction)actionToClickRule2: (id)sender {
    [_rule1 setSelected: NO];
    [_rule2 setSelected: YES];
    [_rule3 setSelected: NO];
    [_rule4 setSelected: NO];
}

- (IBAction)actionToClickRule1: (id)sender {
    [_rule1 setSelected: NO];
    [_rule2 setSelected: NO];
    [_rule3 setSelected: YES];
    [_rule4 setSelected: NO];
}

- (IBAction)actionToClickRule1: (id)sender {
    [_rule1 setSelected: NO];
    [_rule2 setSelected: NO];
    [_rule3 setSelected: NO];
    [_rule4 setSelected: YES];
}

@end

别急着嘲笑这样的代码,曾经的我们也写过类似的代码。这就是最直接粗浅的重复代码,所有的重复代码都和上面存在一样的毛病:亢长、无意义、占用了大量的空间。实际上,这些重复的代码总是分散在多个类当中,积少成多让我们的代码变得笨重。因此,在讨论你的项目是否需要改进架构之前,先弄清楚你是否需要消除这些垃圾。

举个例子,小明开发的一款面向B端的应用中允许商户添加优惠活动,包括开始日期和结束日期:

@interface Promotion: NSObject

+ (instancetype)currentPromotion;

@property (readonly, nonatomic) CGFloat discount;
@property (readonly, nonatomic) NSDate * start;
@property (readonly, nonatomic) NSDate * end;

@end

由于商户同一时间只会存在一个优惠活动,小明把活动写成了单例,然后其他模块通过获取活动单例来计算折后价格:

//  module A
Promotion * promotion = [Promotion currentPromotion];
NSDate * now = [NSDate date];
CGFloat discountAmount = _order.amount;
if ([now timeIntervalSinceDate: promotion.start] > 0 && [now timeIntervalSinceDate: promotion.end] < 0) {
    discountAmount *= promotion.discount;
}

//  module B
Promotion * promotion = [Promotion currentPromotion];
NSDate * now = [NSDate date];
if ([now timeIntervalSinceDate: promotion.start] > 0 && [now timeIntervalSinceDate: promotion.end] < 0) {
    [_cycleDisplayView display: @"全场限时%g折", promotion.discount*10];
}

//  module C
...

小明在开发完成后优化代码时发现了多个模块存在这样的重复代码,于是他写了一个 NSDate 的扩展来简化了这段代码,顺便还添加了一个安全监测:

@implementation NSDate (convenience)

- (BOOL)betweenFront: (NSDate *)front andBehind: (NSDate *)behind {
    if (!front || !behind) { return NO; }
    return ([self timeIntervalSinceDate: front] > 0 && [self timeIntervalSinceDate: behind] < 0);
}

@end

//  module A
Promotion * promotion = [Promotion currentPromotion];
NSDate * now = [NSDate date];
CGFloat discountAmount = _order.amount;
if ([now betweenFront: promotion.start andBehind: promotion.end]) {
    discountAmount *= promotion.discount;
}

//  module B
Promotion * promotion = [Promotion currentPromotion];
NSDate * now = [NSDate date];
if ([now betweenFront: promotion.start andBehind: promotion.end]) {
    [_cycleDisplayView display: @"全场限时%g折", promotion.discount*10];
}

过了一段时间,产品找到小明说:小明啊,商户反映说只有一个优惠活动是不够的,他们需要存在多个不同的活动。小明一想,那么就取消 Promotion 的单例属性,增加一个管理单例:

@interface PromotionManager: NSObject

@property (readonly, nonatomic) NSArray<Promotion *> * promotions

+ (instancetype)sharedManager;
- (void)requestPromotionsWithComplete: (void(^)(PromotionManager * manager))complete;

@end

//  module A
- (void)viewDidLoad {
    PromotionManager * manager = [PromotionManager sharedManager];
    if (manager.promotions) {
        [manager requestPromotionsWithComplete: ^(PromotionManager * manager) {
            _promotions = manager.promotions;
            [self calculateOrder];
        }
    } else {
        _promotions = manager.promotions;
        [self calculateOrder];
    }
}

- (void)calculateOrder {
    CGFloat orderAmount = _order.amount;
    for (Promotion * promotion in _promotions) {
        if ([[NSDate date] betweenFront: promotion.start andBehind: promotion.end]) {
            orderAmount *= promotion.discount;
        }
    }
}

随着日子一天天过去,产品提出的需求也越来越多。有一天,产品说应该让商户可以自由开关优惠活动,于是 Promotion 多了一个 isActived 是否激活的属性。其他模块的判断除了判断时间还多了判断是否启动了活动。再后来,还添加了一个 synchronize 属性判断是否可以与其他活动同时计算判断。最近产品告诉小明活动现在不仅局限于折扣,还新增了固定优惠,以及满额优惠,于是代码变成了下面这样:

@interface Promotion: NSObject

@property (assign, nonatomic) BOOL isActived;
@property (assign, nonatomic) BOOL synchronize;
@property (assign, nonatomic) CGFloat discount;
@property (assign, nonatomic) CGFloat discountCondition;
@property (assign, nonatomic) DiscountType discountType;
@property (assign, nonatomic) PromotionType promotionType;

@property (readonly, nonatomic) NSDate * start;
@property (readonly, nonatomic) NSDate * end;

@end

//  module A
- (void)viewDidLoad {
    PromotionManager * manager = [PromotionManager sharedManager];
    if (manager.promotions) {
        [manager requestPromotionsWithComplete: ^(PromotionManager * manager) {
            _promotions = manager.promotions;
            [self calculateOrder];
        }
    } else {
        _promotions = manager.promotions;
        [self calculateOrder];
    }
}

- (void)calculateOrder {
    CGFloat orderAmount = _order.amount;
    NSMutableArray * fullPromotions = @[].mutableCopy;
    NSMutableArray * discountPromotions = @[].mutableCopy;
    for (Promotion p in _promotions) {
        if (p.isActived && [[NSDate date] betweenFront: p.start andBehind: p.end]) {
            if (p.promotionType == PromotionTypeFullPromotion) {
                [fullPromotions addObject: p];
            } else if (p.promotionType == PromotionTypeDiscount) {
                [discountPromotions addObject: p];
            }
        }
    }

    Promotion * syncPromotion = nil;
    Promotion * singlePromotion = nil;
    for (Promotion * p in fullPromotions) {
        if (p.synchronize) {
            if (p.discountCondition != 0) {
                if (p.discountCondition > syncPromotion.discountCondition) {
                    syncPromotion = p;
                }
            } else {
                if (p.discount > syncPromotion.discount) {
                    syncPromotion = p;
                }
            }
        } else {
            if (p.discountCondition != 0) {
                if (p.discountCondition > singlePromotion.discountCondition) {
                    singlePromotion = p;
                }
            } else {
                if (p.discount > singlePromotion.discount) {
                    singlePromotion = p;
                }
            }
        }
    }
    //  find discount promotions
    ......
}

这时候模块获取优惠活动信息的代价已经变得十分的昂贵,一堆亢长的代码,重复度高。这时候小明的同事对他说,我们改进一下架构吧,通过 ViewModel 把这部分的代码从控制器分离出去。其实这时候 ViewModel 的做法跟上面小明直接扩展 NSDate 的目的是一样的,在这个时候 ViewModel 几乎无作为,基本所有逻辑都在控制器中不断地撑胖它。小明认真思考,完完全全将代码阅览后,告诉同事现在最大的原因在于代码职责混乱,并不能很好的分离到 VC 的模块中,解决的方式应该是从逻辑分工下手。

首先,小明发现 Promotion 本身除了存储活动信息,没有进行任何的逻辑操作。而控制器中判断活动是否有效以及折扣金额计算的业务理可以由 Promotion 来完成:

@interface Promotion: NSObject

- (BOOL)isEffective;
- (BOOL)isWorking;
- (CGFloat)discountAmount: (CGFloat)amount;

@end

@implementation Promotion

- (BOOL)isEffective {
    return [[NSDate date] betweenFront: _start andBehind: _end];
}

- (BOOL)isWorking {
    return ( [self isEffective] && _isActived );
}

- (CGFloat)discountAmount: (CGFloat)amount {
    if ([self isWorking]) {
        if (_promotionType == PromotionTypeDiscount) {
            return [self calculateDiscount: amount];
        } else {
            if (amount < _discountCondition) { return amount; }
            return [self calculateDiscount: amount];
        }
    }
    return amount;
}

#pragma mark - Private
- (CGFloat)calculateDiscount: (CGFloat)amount {
    if (_discountType == DiscountTypeCoupon) {
        return amount - _discount;
    } else {
        return amount * _discount;
    }
}

@end

除此之外,小明发现先前封装的活动管理类 PromotionManager 本身涉及了网络请求和数据管理两个业务,因此需要将其中一个业务分离出来。于是网络请求封装成 PromotionRequest ,另一方面原有的数据管理只有获取数据的功能,因此增加增删改以及对活动进行初步筛选的功能:

#pragma mark -  PromotionManager.h
@class PromotionManager;
typeof void(^PromotionRequestComplete)(PromotionManager * manager);

@interface PromotionRequest: NSObject

+ (void)requestPromotionsWithComplete: (PromotionRequestComplete)complete;
+ (void)insertPromotion: (Promotion *)promotion withComplete: (PromotionRequestComplete)complete;
+ (void)updatePromotion: (Promotion *)promotion withComplete: (PromotionRequestComplete)complete;
+ (void)deletePromotion: (Promotion *)promotion withComplete: (PromotionRequestComplete)complete;

@end


@interface PromotionManager: NSObject

+ (instancetype)sharedManager;

- (NSArray<Promotion *> *)workingPromotions;
- (NSArray<Promotion *> *)effectivePromotions;
- (NSArray<Promotion *> *)fullPromotions;
- (NSArray<Promotion *> *)discountPromotions;

- (void)insertPromotion: (Promotion *)promotion;
- (void)updatePromotion: (Promotion *)promotion;
- (void)deletePromotion: (Promotion *)promotion;

@end



#pragma mark -  PromotionManager.m
@interface PromotionManager ()

@property (nonatomic, strong) NSArray<Promotion *> * promotions;

@end

@implementation PromotionManager

+ (instancetype)sharedManager { ... }

- (NSArray<Promotion *> *)fullPromotions {
    return [self filterPromotionsWithType: PromotionTypeFullPromote];
}

- (NSArray<Promotion *> *)discountPromotions {
    return [self filterPromotionsWithType: PromotionDiscountPromote];
}

- (NSArray<Promotion *> *)workingPromotions {
    return _promotions.filter(^BOOL(Promotion * p) {
        return (p.isWorking);
    });
}

- (NSArray<Promotion *> *)effectivePromotions {
    return _promotions.filter(^BOOL(Promotion * p) {
        return (p.isEffective);
    });
}

- (NSArray<Promotion *> *)filterPromotionsWithType: (PromotionType)type {
    return [self workingPromotions].filter(^BOOL(Promotion * p) {
        return (p.promotionType == type);
    });
}

- (void)insertPromotion: (Promotion *)promotion { 
    if ([_promotions containsObject: promotion]) {
        [PromotionRequest updatePromotion: promotion withComplete: nil];
    } else {
        [PromotionRequest insertPromotion: promotion withComplete: nil];
    }
 }

- (void)updatePromotion: (Promotion *)promotion { 
    if ([_promotions containsObject: promotion]) {
        [PromotionRequest updatePromotion: promotion withComplete: nil];
    }
 }

- (void)deletePromotion: (Promotion *)promotion { 
    if ([_promotions containsObject: promotion]) {
        [PromotionRequest deletePromotion: promotion withComplete: nil];
    }
}

- (void)obtainPromotionsFromJSON: (id)JSON { ... }

@end

最后,小明发现其他模块在寻找最优惠活动的逻辑代码非常的多,另外由于存在满额优惠和普通优惠两种活动,进一步加大了代码量。因此小明新建了一个计算类 PromotionCalculator 用来完成查找最优活动和计算最优价格的接口:

@interface PromotionCalculator: NSObject

+ (CGFloat)calculateAmount: (CGFloat)amount;
+ (Promotion *)bestFullPromotion: (CGFloat)amount;
+ (Promotion *)bestDiscountPromotion: (CGFloat)amount;

@end

@implementation PromotionCalculator

+ (CGFloat)calculateAmount: (CGFloat)amount {
    Promotion * bestFullPromotion = [self bestFullPromotion: amount];
    Promotion * bestDiscountPromotion = [self bestDiscountPromotion: amount];
    if (bestFullPromotion.synchronize && bestDiscountPromotion.synchronize) {
        return [bestFullPromotion discountAmount: [bestDiscountPromotion discountAmount: amount]];
    } else {
        return MAX([bestDiscountPromotion discountAmount: amount], [bestFullPromotion discountAmount: amount]);
    }
}

+ (Promotion *)bestFullPromotion: (CGFloat)amount {
    PromotionManager * manager = [PromotionManager sharedManager];
    return [self bestPromotionInPromotions: [manager fullPromotions] amount: amount];
}

+ (Promotion *)bestDiscountPromotion: (CGFloat)amount {
    PromotionManager * manager = [PromotionManager sharedManager];
    return [self bestPromotionInPromotions: [manager discountPromotions] amount: amount];
}  

+ (Promotion *)bestPromotionInPromotions: (NSArray *)promotions amount: (CGFloat)amount {
    CGFloat discount = amount;
    Promotion * best = nil;
    for (Promotion * promotion in promotions) {
        CGFloat tmp = [promotion discountAmount: amount];
        if (tmp < discount) {
            discount = tmp;
            best = promotion;
        }
    }
    return best;
}

@end

当这些代码逻辑被小明分散到各处之后,小明惊讶的发现其他模块在进行计算时剩下几行代码而已:

- (void)viewDidLoad {
    [PromotionRequest requestPromotionsWithComplete: ^(PromotionManager * manager) {
        _discountAmount = [PromotionCalculator calculateAmount: _order.amount];
    }];
}

这时候代码职责的结构图,小明成功的均衡了不同组件之间的代码职责,避免了改变项目原架构带来的风险以及不必要的工作:

均衡代码职责

尾语

这是第二篇讲 MVC 的文章,仍然要告诉大家的是 MVC 确确实实存在着缺陷,这个缺陷会在项目变得很大的时候暴露出来(笔者没有开发过大型项目的弱鸡),如果你的项目结构分层做的足够完善的话,那么该改进更换架构的时候就不要犹豫。但千万要记住,如果仅仅是因为重复了太多的无用代码,又或者是逻辑全部塞到控制器中,那么更换架构无非是将垃圾再次分散罢了。

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