Obj-C 實現設計模式 -- Observer

觀察者模式，采用氣象站的例子來說明，本質上跟Java來實現差不多。只不過是針對協議（Delegate）來編程。

簡單說下需求，氣象顯示版向氣象站注冊成功訂閱者（觀察者），氣象站監測到氣溫發生變化，向各個已注冊的氣象顯示版發出通知。

遵守針對接口編程的原則，先來寫Delegate.

第一個，需要被氣象站實現的訂閱和取消訂閱的方法。

#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的是一個實現了ObserverModel的對象。

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

OK，下面進行氣象站的具體編碼。氣象站需要四個屬性，溫度、濕度、氣壓、訂閱者集合。

@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

對應方法的實現，

@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];
}

氣象顯示板的實現，初始化的時候應該把氣象站出入，使得該氣象顯示板決定是否向氣象顯示板注冊訂閱氣象。

@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

同時，顯示板應該實現接口中的屬性更新方法和顯示方法。

- (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，進行整體的實例化，看看氣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];