iOS设计原则之开闭原则

开闭原则

开闭原则的英文是Open Closed Principle，简写为OCP。它的英文描述是：software entities should be open for extension，but closed for modification。翻译成中文就是：软件实体应该“对扩展开放、对修改关闭”。
添加一个新的功能应该是，在已有代码基础上扩展代码(新增模块、类、方法等)，而非修改已有代码（修改模块、类、方法）。

Api接口监控告警的代码
DMAlertRule 存储告警规则，可以自由设置

@interface DMAlertRuleItem : NSObject

// 最大TPS数
@property (nonatomic, assign) NSInteger mMaxTps;

// 最大ErrorCount数
@property (nonatomic, assign) NSInteger mMaxErrorCount;

// 最大TimeOutTps数
@property (nonatomic, assign) NSInteger mMaxTimeOutTps;

@end

#import "DMAlertRule.h"
#import "DMAlertRuleItem.h"

@interface DMAlertRule ()

@property (nonatomic, strong) NSMutableDictionary *alertRules;

@end

@implementation DMAlertRule

- (instancetype)init {
    if (self = [super init]) {
        // 读取规则
        [self readAlertRules];
    }
    return self;
}

- (void)readAlertRules {
    DMAlertRuleItem *ruleItem = [[DMAlertRuleItem alloc] init];
    ruleItem.mMaxTps = 100;
    ruleItem.mMaxErrorCount = 88;
    ruleItem.mMaxTimeOutTps = 66;
    [self.alertRules setObject:ruleItem forKey:@"openCloseApi"];
}

- (DMAlertRuleItem *)getAlertRuleItemByApiName:(NSString *)apiName {
    DMAlertRuleItem *ruleItem = nil;
    ruleItem = [self.alertRules objectForKey:apiName];
    return ruleItem;
}

#pragma mark - getter and setter
- (NSMutableDictionary *)alertRules {
    if (_alertRules == nil) {
        _alertRules = [[NSMutableDictionary alloc] init];
    }
    return _alertRules;
}

@end

DMOpenClosedNotification 是告警通知类。DMOpenClosedNotificationLevel表示通知的紧急程度

typedef enum : NSUInteger {
    DMOpenClosedNotificationLevelUrgency,   //  紧急
    DMOpenClosedNotificationLevelSevere,    //  严重
    DMOpenClosedNotificationLevelNormal,    //  普通
    DMOpenClosedNotificationLevelTrivial,   //  无关紧要
} DMOpenClosedNotificationLevel;

@interface DMOpenClosedNotification : NSObject

- (void)notify:(DMOpenClosedNotificationLevel)level message:(NSString *)messageStr;

@end

@implementation DMOpenClosedAlert

- (void)check:(NSString *)apiName requestCount:(NSInteger)requestCount errorCount:(NSInteger)errorCount durationOfSeconds:(NSInteger)durationOfSeconds {
    DMAlertRuleItem *ruleItem = [self.alertRule getAlertRuleItemByApiName:apiName];
 
    float tps = (requestCount * 1.0 )/ (durationOfSeconds * 1.0);
    if (tps > (ruleItem.mMaxTps * 1.0)) {
        [self.notification notify:DMOpenClosedNotificationLevelUrgency message:@"tps 超过最大值"];
    }
    if (errorCount > ruleItem.mMaxErrorCount) {
        [self.notification notify:DMOpenClosedNotificationLevelSevere message:@"errorCount 超过最大值"];
    }
}
@end

当接口的TPS超过某个预先设置的最大值时，以及当接口请求出错数大于某个最大允许值时，就会出发告警，通知接口的相关负责人或者团队。
现在要添加一个功能，当每秒钟接口超时个数，超过某个预先设置的最大阈值时，我们也触发告警，发送通知。
改动主要有两处：第一处是修改函数入参，添加一个新的统计数据timeOut，表示超时接口请求数；第二处是函数中添加新的告警逻辑。

// 改动一：添加timeOut参数
- (void)check:(NSString *)apiName requestCount:(NSInteger)requestCount errorCount:(NSInteger)errorCount timeOut:(NSInteger)timeOut durationOfSeconds:(NSInteger)durationOfSeconds {
    DMAlertRuleItem *ruleItem = [self.alertRule getAlertRuleItemByApiName:apiName];
 
    float tps = (requestCount * 1.0 )/ (durationOfSeconds * 1.0);
    if (tps > (ruleItem.mMaxTps * 1.0)) {
        [self.notification notify:DMOpenClosedNotificationLevelUrgency message:@"tps 超过最大值"];
    }
    if (errorCount > ruleItem.mMaxErrorCount) {
        [self.notification notify:DMOpenClosedNotificationLevelSevere message:@"errorCount 超过最大值"];
    }
    // 改动二：添加接口超时处理逻辑
    float timeOutTps = (timeOut * 1.0) / (durationOfSeconds * 1.0);
    if (timeOutTps > ruleItem.mMaxTimeOutTps) {
        [self.notification notify:DMOpenClosedNotificationLevelUrgency message:@"TimeOut tps超过最大值"];
    }
}

上面的代码是基于“修改”的方式来实现新功能的。如果我们遵循开闭原则，也就是“对扩展开放，对修改关闭”。如何通过“扩展”的方式，实现同样的功能？如下重构
第一部分：将参数封装成DMOpenClosedAPiStateInfo类

@interface DMOpenClosedAPiStateInfo : NSObject

@property (nonatomic, strong) NSString *api;

@property (nonatomic, assign) NSInteger requestCount;

@property (nonatomic, assign) NSInteger errorCount;

@property (nonatomic, assign) NSInteger durationOfSeconds;

@end

第二部分：引入Handler概念，将if判断逻辑分散在各个Handler中

@protocol DMOpenClosedAlertHandler <NSObject>

- (void)checkApiStateInfo:(DMOpenClosedAPiStateInfo *)apiStateInfo;

@end

声明DMOpenClosedAlertHandler协议

@interface DMTPSAlertHandler () <DMOpenClosedAlertHandler>

@end

@implementation DMTPSAlertHandler

- (void)checkApiStateInfo:(DMOpenClosedAPiStateInfo *)apiStateInfo {
    DMAlertRuleItem *ruleItem = [self.alertRule getAlertRuleItemByApiName:apiStateInfo.api];
 
    float tps = (apiStateInfo.requestCount * 1.0 )/ (apiStateInfo.durationOfSeconds * 1.0);
    if (tps > (ruleItem.mMaxTps * 1.0)) {
        [self.notification notify:DMOpenClosedNotificationLevelUrgency message:@"tps 超过最大值"];
    }
}

@end

DMTPSAlertHandler 遵守DMOpenClosedAlertHandler协议，实现tps判断逻辑

@interface DMErrorCountAlertHandler () <DMOpenClosedAlertHandler>

@end

@implementation DMErrorCountAlertHandler

- (void)checkApiStateInfo:(DMOpenClosedAPiStateInfo *)apiStateInfo {
    DMAlertRuleItem *ruleItem = [self.alertRule getAlertRuleItemByApiName:apiStateInfo.api];
    if (apiStateInfo.errorCount > ruleItem.mMaxErrorCount) {
        [self.notification notify:DMOpenClosedNotificationLevelSevere message:@"errorCount 超过最大值"];
    }
}

@end

DMErrorCountAlertHandler遵守DMOpenClosedAlertHandler协议，实现errorCount判断逻辑

@interface DMOpenClosedAlert ()

@property (nonatomic, strong) NSMutableArray *alertHandlers;

@end

@implementation DMOpenClosedAlert

- (void)check:(DMOpenClosedAPiStateInfo *)apiStateInfo {
    for (id<DMOpenClosedAlertHandler> alertHandler in self.alertHandlers) {
        [alertHandler checkApiStateInfo:apiStateInfo];
    }
}

- (void)addAlertHandler:(id<DMOpenClosedAlertHandler>)alertHandler {
    [self.alertHandlers addObject:alertHandler];
}

- (NSMutableArray *)alertHandlers {
    if (_alertHandlers == nil) {
        _alertHandlers = [[NSMutableArray alloc] init];
    }
    return _alertHandlers;
}

@end

DMOpenClosedAlert 负责存储hander在数组中，check时，遍历handler即可。

现在再来看下，如何实现每秒钟接口超时个数，超过某个预先设置的最大阈值时就触发告警这个新功能。
在DMOpenClosedAPiStateInfo中添加timeOutCount新字段

@interface DMOpenClosedAPiStateInfo : NSObject

@property (nonatomic, strong) NSString *api;

@property (nonatomic, assign) NSInteger requestCount;

@property (nonatomic, assign) NSInteger errorCount;

// 改动：添加新字段
@property (nonatomic, assign) NSInteger timeOutCount;

@property (nonatomic, assign) NSInteger durationOfSeconds;

@end

添加新的遵守DMOpenClosedAlertHandler协议的类DMTimeOutAlertHandler

@interface DMTimeOutAlertHandler () <DMOpenClosedAlertHandler>

@end
@implementation DMTimeOutAlertHandler

- (void)checkApiStateInfo:(DMOpenClosedAPiStateInfo *)apiStateInfo {
    DMAlertRuleItem *ruleItem = [self.alertRule getAlertRuleItemByApiName:apiStateInfo.api];
    float timeOutTps = (apiStateInfo.timeOutCount * 1.0) / (apiStateInfo.durationOfSeconds * 1.0);
    if (timeOutTps > ruleItem.mMaxTimeOutTps) {
        [self.notification notify:DMOpenClosedNotificationLevelUrgency message:@"TimeOut tps超过最大值"];
    }
}

@end

DMTimeOutAlertHandler 遵守DMOpenClosedAlertHandler协议，实现timeOut tps的判断逻辑

重构之后的代码更加灵活和易扩展。如果我们想要添加新的告警逻辑，只需要基于扩展的方式创建新的遵守DMOpenClosedAlertHandler协议的Handler类即可，不需要改动原来的check方法的逻辑。