iOS觀察者模式學(xué)習(xí)

（1）KVC與KVO簡(jiǎn)介

• KVC

KVC（Key-value coding）是一種間接更改對(duì)象狀態(tài)的方式。

官方文檔描述：

Key-value coding is a mechanism for accessing an object’s properties indirectly, using strings to identify properties, rather than through invocation of an accessor method or accessing them directly through instance variables. In essence, key-value coding defines the patterns and method signatures that your application’s accessor methods implement.

• KVC的作用
  

通過(guò)鍵值的方式訪問(wèn)對(duì)象或者修改對(duì)象的值

比如：NSString *name = [car valueForKey: @"name"];通過(guò)-valueForKey:獲取對(duì)象，它會(huì)先查找以參數(shù)名命名（格式為-key或-isKey）的getter方法，對(duì)于上面的調(diào)用，valueForKey:會(huì)先尋找-name方法。如果沒(méi)有這樣的getter方法，它將會(huì)在對(duì)象內(nèi)尋找名稱格式為_key或key的實(shí)例變量。通過(guò)使用KVC，沒(méi)有相關(guān)getter方法也能獲取對(duì)象，不需要通過(guò)對(duì)象指針來(lái)直接訪問(wèn)實(shí)例變量。

• KVC的優(yōu)缺點(diǎn)
  

1.KVC可以輕松處理集合類。（如：NSArray）

2.KVC沒(méi)有相關(guān)getter方法也能獲取對(duì)象，不需要通過(guò)對(duì)象指針來(lái)直接訪問(wèn)實(shí)例變量

3.簡(jiǎn)化代碼

4.KVC需要解析字符串，速度比較慢

5.編譯器無(wú)法進(jìn)行錯(cuò)誤檢查

• KVO

KVO（Key-value Observing）是基于KVC的，一種監(jiān)聽(tīng)方式，也就是所謂的觀察者模式。

官方文檔描述：

Key-value observing provides a mechanism that allows objects to be notified of changes to specific properties of other objects. It is particularly useful for communication between model and controller layers in an application.

（2）KVC代碼實(shí)現(xiàn)

//1.setValue: forKey
[car setValue: @"奔馳" forKey: @"name"];
[car setValue: [NSNumber numberWithFloat: 80] forKey: @"speed"];
[car setValue: @"黑色" forKey: @"carPaint"];

//1.valueForKey
NSString *name = [currentCar valueForKey: @"name"];
NSNumber *speed = [currentCar valueForKey: @"speed"];
NSString *carPaint = [currentCar valueForKey: @"carPaint"];

//2.setValue:forKeyPath
[car setValue: [NSNumber numberWithFloat: 40] forKeyPath: @"engine.horsepower"];
self.showCarDetail.text = [self showCarDetail: car];

//2.valueForKeyPath
NSNumber *horsepower = [currentCar valueForKeyPath: @"engine.horsepower"];

//3.整體操作:訪問(wèn)數(shù)組，如果使用某個(gè)鍵值來(lái)訪問(wèn)一個(gè)NSArray數(shù)組，它實(shí)際上會(huì)查詢相應(yīng)數(shù)組中的每個(gè)對(duì)象，然后將查詢結(jié)果大包到另一個(gè)數(shù)組并返回，但是不能直接在建路徑中索引這些數(shù)組，如：tires[0].pressure
NSArray *pressures = [currentCar valueForKeyPath: @"tires.pressure"];
NSLog(@"%@", pressures);

//4.快速運(yùn)算@count,@sum,@avg,@min,@max,@distinctUnionOfObjects
NSNumber *count = [currentCar valueForKeyPath: @"tires.@count"];    //對(duì)左邊鍵值返回?cái)?shù)組操作，獲取數(shù)組數(shù)量
NSNumber *sum = [currentCar valueForKeyPath: @"tires.@sum.pressure"];    //對(duì)左邊鍵值返回?cái)?shù)組操作，獲取每個(gè)數(shù)組里面的pressure值，求和
NSNumber *avg = [currentCar valueForKeyPath: @"tires.@avg.pressure"];    //對(duì)左邊鍵值返回?cái)?shù)組操作，獲取每個(gè)數(shù)組里面的pressure值，求平均值
NSNumber *min = [currentCar valueForKeyPath: @"tires.@min.pressure"];    //對(duì)左邊鍵值返回?cái)?shù)組操作，獲取每個(gè)數(shù)組里面的pressure值，找出最小值
NSNumber *max = [currentCar valueForKeyPath: @"tires.@max.pressure"];    //對(duì)左邊鍵值返回?cái)?shù)組操作，獲取每個(gè)數(shù)組里面的pressure值，找出最大值
NSArray *tireType = [currentCar valueForKeyPath: @"tires.@distinctUnionOfObjects.pressure"];    //對(duì)左邊鍵值返回?cái)?shù)組操作，獲取每個(gè)數(shù)組里面的pressure值，去掉重復(fù)的值，返回一個(gè)包含所有不重復(fù)值的數(shù)組
NSLog(@"TiresCount: %@ Sum: %@, Avg: %@, Min: %@, Max: %@, TireType: %@", count, sum, avg, min, max, tireType);

//5.批處理
//字典里面不能為nil，如果返回值有nil，則KVC會(huì)自己處理，將返回[NSNull null]表示nil
//<null>和(null)的區(qū)別：前者是[NSNull null]對(duì)象，而后者是正真的nil。
NSArray *keys = [NSArray arrayWithObjects: @"make", @"modelYear", @"numberOfDoors", nil];
NSDictionary *carValues = [currentCar dictionaryWithValuesForKeys: keys];
NSLog(@"%@", carValues);
    
//6.nil處理
//可以自己運(yùn)行一下，重寫和沒(méi)有重寫setNilValueForKey:方法的代碼
[currentCar setValue: nil forKey: @"speed"];
NSLog(@"carSpeed: %@", [currentCar valueForKey: @"speed"]);
    
//7.處理未定義的值
[currentCar setValue: [NSNumber numberWithFloat: 1500] forKey: @"mileage"];
[currentCar setValue: [NSNull null] forKey: @"price"];
[currentCar setValue: nil forKey: @"capacity"];
NSLog(@"Mileage: %@ Price: %@, Capacity: %@", [currentCar valueForKey: @"mileage"], [currentCar valueForKey: @"price"], [currentCar valueForKey: @"capacity"]);

//8.KVC有自動(dòng)裝箱的功能，當(dāng)使用valueForKey時(shí)，它自動(dòng)將標(biāo)量值（int,float,struct...）放入NSNumber或NSValue中；當(dāng)使用setValueForKey，它自動(dòng)將標(biāo)量值從這些對(duì)象取出。

（3）KVO代碼實(shí)現(xiàn)

//應(yīng)該在監(jiān)聽(tīng)者里添加監(jiān)聽(tīng)，而不是在被監(jiān)聽(tīng)者里添加監(jiān)聽(tīng)
//在被監(jiān)聽(tīng)者添加監(jiān)聽(tīng)會(huì)造成循環(huán)引用的問(wèn)題
car = [[Car alloc] init];
[car addObserver: self forKeyPath: @"carPaint" options: NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context: nil];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context {
    if ([keyPath isEqualToString: @"carPaint"]) {
        NSLog(@"%@", [change objectForKey: @"new"]);    //修改之后的值
        self.showCarDetail.text = [NSString stringWithFormat: @"車的顏色是%@", [change objectForKey: @"new"]];
    }
}

- (void)dealloc {
    //監(jiān)聽(tīng)者應(yīng)該是self，被監(jiān)聽(tīng)者是car
    [car removeObserver: self forKeyPath: @"carPaint"];
}

(4)Demo展示：

Observer.gif

Demo下載