Obj-C 实现设计模式 -- Observer

观察者模式，采用气象站的例子来说明，本质上跟Java来实现差不多。只不过是针对协议（Delegate）来编程。

简单说下需求，气象显示版向气象站注册成功订阅者（观察者），气象站监测到气温发生变化，向各个已注册的气象显示版发出通知。

遵守针对接口编程的原则，先来写Delegate.

第一个，需要被气象站实现的订阅和取消订阅的方法。

#import "ObserverModel.h"@protocol SubjectProtocol <NSObject>- (void) registerObserver:(id<ObserverModel>) obs;
- (void) removeObserver:(id<ObserverModel>) obs;
- (void) notifyObservers;@end

上面被register和remove的是一个实现了ObserverModel的对象。

@protocol ObserverModel <NSObject>
- (void) updateWithTemp:(float) temp withHumidity:(float)humidity withPressure:(float)pressure;
@end@protocol DisplayDelegate <NSObject>
- (void) display;
@end

OK，下面进行气象站的具体编码。气象站需要四个属性，温度、湿度、气压、订阅者集合。

@interface WeatherData : NSObject <SubjectProtocol>{NSMutableArray * _objArray;float  _tempurature;float  _humidity;float  _pressure;  
}
@property (nonatomic, retain) NSMutableArray * objArray;
@property (nonatomic, assign) float tempurature;
@property (nonatomic, assign) float humidity;
@property (nonatomic, assign) float pressure;- (id) initWithArray;
- (void) registerObserver:(id<ObserverModel>)obs;
- (void) removeObserver:(id<ObserverModel>)obs;
- (void) notifyObservers;
- (void) measurementChanged;
- (void) setMeasurementsWithTemp:(float)temp withHumidity:(float)humidity withPressure:(float)pressure;
@end

对应方法的实现，

@synthesize objArray = _objArray;
@synthesize humidity = _humidity;
@synthesize tempurature = _tempurature;
@synthesize pressure = _pressure;- (id) initWithArray{self = [super init];if (self) {_objArray = [[NSMutableArray alloc] initWithCapacity:10];}return self;
}- (void) registerObserver:(id<ObserverModel>)obs{[_objArray addObject:obs];
}- (void) removeObserver:(id<ObserverModel>)obs{int i = [_objArray indexOfObject:obs];if (i>=0) {[_objArray removeObjectAtIndex:i];}
}- (void) notifyObservers{for (int i = 0; i < [_objArray count]; ++i) {id<ObserverModel> obj = (id<ObserverModel>)[_objArray objectAtIndex:i];[obj updateWithTemp:_tempurature withHumidity:_humidity withPressure:_pressure];}
}- (void) measurementChanged{[self notifyObservers];
}- (void) setMeasurementsWithTemp:(float)temp withHumidity:(float)humidity withPressure:(float)pressure{_tempurature = temp;_humidity = humidity;_pressure = pressure;[self measurementChanged];
}

气象显示板的实现，初始化的时候应该把气象站出入，使得该气象显示板决定是否向气象显示板注册订阅气象。

@interface CurrentConditionsDisplay : NSObject <ObserverModel,DisplayDelegate>{float _temperature;float _humidity;id<SubjectProtocol> _weatherData;
}
- (void) CurrentConditionsDisplayWithObj:(id<SubjectProtocol> ) weatherData;
- (void) updateWithTemp:(float)temp withHumidity:(float)humidity withPressure:(float)pressure;
@end

同时，显示板应该实现接口中的属性更新方法和显示方法。

- (void) CurrentConditionsDisplayWithObj:(id<SubjectProtocol> ) weatherData{_weatherData = weatherData;[weatherData registerObserver:self];
}
- (void) updateWithTemp:(float)temp withHumidity:(float)humidity withPressure:(float)pressure{_temperature = temp;_humidity = humidity;[self display];
}- (void) display{NSLog(@"Temperature is %f and Humidity is %f",_temperature,_humidity);
}

最后，进行整体的实例化，看看气象站是不是能正常工作了。

WeatherData * weatherData = [[WeatherData alloc] initWithArray];
CurrentConditionsDisplay * cCD = [[CurrentConditionsDisplay alloc] init];
[cCD CurrentConditionsDisplayWithObj:weatherData];
[weatherData setMeasurementsWithTemp:20.0 withHumidity:22.0 withPressure:24.0];
[weatherData setMeasurementsWithTemp:30.0 withHumidity:32.0 withPressure:34.0];