面试题：Objective-C策略模式在复杂业务场景中的优化

Question

在一个电商APP中，有多种促销策略，如满减、折扣、赠品等。每个促销策略可能会涉及到复杂的计算逻辑和与后端数据的交互。请用Objective-C设计一个基于策略模式的架构来管理这些促销策略，并说明如何优化代码以提高可维护性和扩展性，尤其是在新的促销策略不断加入的情况下。

Answer 1

1. 策略模式设计

1. 定义抽象策略类

#import <Foundation/Foundation.h>

// 抽象促销策略类
@interface PromotionStrategy : NSObject

// 计算促销结果
- (CGFloat)calculatePromotionWithOriginalPrice:(CGFloat)originalPrice;

@end

2. 具体策略类实现
   • 满减策略

#import "PromotionStrategy.h"

@interface FullReductionStrategy : PromotionStrategy

@property (nonatomic, assign) CGFloat fullAmount;
@property (nonatomic, assign) CGFloat reductionAmount;

- (instancetype)initWithFullAmount:(CGFloat)fullAmount reductionAmount:(CGFloat)reductionAmount;

@end

@implementation FullReductionStrategy

- (instancetype)initWithFullAmount:(CGFloat)fullAmount reductionAmount:(CGFloat)reductionAmount {
    self = [super init];
    if (self) {
        _fullAmount = fullAmount;
        _reductionAmount = reductionAmount;
    }
    return self;
}

- (CGFloat)calculatePromotionWithOriginalPrice:(CGFloat)originalPrice {
    if (originalPrice >= self.fullAmount) {
        return originalPrice - self.reductionAmount;
    }
    return originalPrice;
}

@end

- **折扣策略**

#import "PromotionStrategy.h"

@interface DiscountStrategy : PromotionStrategy

@property (nonatomic, assign) CGFloat discount;

- (instancetype)initWithDiscount:(CGFloat)discount;

@end

@implementation DiscountStrategy

- (instancetype)initWithDiscount:(CGFloat)discount {
    self = [super init];
    if (self) {
        _discount = discount;
    }
    return self;
}

- (CGFloat)calculatePromotionWithOriginalPrice:(CGFloat)originalPrice {
    return originalPrice * self.discount;
}

@end

- **赠品策略（这里简单示例，实际可能涉及更复杂逻辑和后端交互）**

#import "PromotionStrategy.h"

@interface GiftStrategy : PromotionStrategy

// 这里可以添加赠品相关属性，如赠品ID等
@property (nonatomic, strong) NSString *giftID;

- (instancetype)initWithGiftID:(NSString *)giftID;

@end

@implementation GiftStrategy

- (instancetype)initWithGiftID:(NSString *)giftID {
    self = [super init];
    if (self) {
        _giftID = giftID;
    }
    return self;
}

- (CGFloat)calculatePromotionWithOriginalPrice:(CGFloat)originalPrice {
    // 赠品策略可能不直接影响价格，这里返回原价
    return originalPrice;
}

@end

3. 上下文类

#import <Foundation/Foundation.h>
#import "PromotionStrategy.h"

// 促销策略上下文类
@interface PromotionContext : NSObject

@property (nonatomic, strong) PromotionStrategy *strategy;

- (instancetype)initWithStrategy:(PromotionStrategy *)strategy;

- (CGFloat)executePromotionWithOriginalPrice:(CGFloat)originalPrice;

@end

@implementation PromotionContext

- (instancetype)initWithStrategy:(PromotionStrategy *)strategy {
    self = [super init];
    if (self) {
        _strategy = strategy;
    }
    return self;
}

- (CGFloat)executePromotionWithOriginalPrice:(CGFloat)originalPrice {
    return [self.strategy calculatePromotionWithOriginalPrice:originalPrice];
}

@end

4. 使用示例

#import <UIKit/UIKit.h>
#import "PromotionContext.h"
#import "FullReductionStrategy.h"
#import "DiscountStrategy.h"
#import "GiftStrategy.h"

@interface ViewController : UIViewController

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // 满减策略
    FullReductionStrategy *fullReduction = [[FullReductionStrategy alloc] initWithFullAmount:200 reductionAmount:50];
    PromotionContext *fullReductionContext = [[PromotionContext alloc] initWithStrategy:fullReduction];
    CGFloat fullReductionResult = [fullReductionContext executePromotionWithOriginalPrice:300];
    NSLog(@"满减结果: %f", fullReductionResult);
    
    // 折扣策略
    DiscountStrategy *discount = [[DiscountStrategy alloc] initWithDiscount:0.8];
    PromotionContext *discountContext = [[PromotionContext alloc] initWithStrategy:discount];
    CGFloat discountResult = [discountContext executePromotionWithOriginalPrice:300];
    NSLog(@"折扣结果: %f", discountResult);
    
    // 赠品策略
    GiftStrategy *gift = [[GiftStrategy alloc] initWithGiftID:@"gift001"];
    PromotionContext *giftContext = [[PromotionContext alloc] initWithStrategy:gift];
    CGFloat giftResult = [giftContext executePromotionWithOriginalPrice:300];
    NSLog(@"赠品结果: %f", giftResult);
}

@end

2. 代码优化以提高可维护性和扩展性

1. 单一职责原则：每个具体策略类只负责一种促销策略的计算逻辑，如满减策略类只处理满减相关计算，这样代码职责清晰，易于维护。
2. 依赖注入：通过PromotionContext类的initWithStrategy:方法，将具体的促销策略对象注入到上下文中，使得上下文与具体策略解耦，便于替换不同的策略。
3. 模块化管理：将不同的促销策略类放在不同的文件中，便于管理和查找。同时，对于复杂的后端交互逻辑，可以封装在具体策略类的私有方法中，避免污染外部接口。
4. 扩展性优化：当有新的促销策略加入时，只需要创建一个新的具体策略类继承自PromotionStrategy，实现calculatePromotionWithOriginalPrice:方法即可。然后在需要使用该策略的地方，通过PromotionContext注入新的策略对象，不需要修改原有策略类的代码，符合开闭原则，提高了代码的扩展性。