在上述情況下, 我們可以使用Cocoa提供給我們的KVO(Key-value observing)來實現(xiàn):

Key-value observing is a mechanism that allows objects to be notified of changes to specified properties of other objects.

KVO也體現(xiàn)了在iOS開發(fā)中常使用的一種設(shè)計模式 - 觀察者設(shè)計模式.

KVO 的使用

步驟

1. 添加監(jiān)聽: addObserver: forKeyPath: options: context:
2. 實現(xiàn)監(jiān)聽方法: observeValueForKeyPath: ofObject: change: context:
3. 移除監(jiān)聽: removeObserver: forKeyPath:

示例

1. 在ViewController創(chuàng)建一個屬性

@property (nonatomic, copy) NSString *name;

2. 添加key-value-observer

[self addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];

3. 實現(xiàn)監(jiān)聽值(此處為name)變化時的監(jiān)聽方法:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    
    if ([keyPath isEqualToString:@"name"]) {
        NSLog(@"name = %@", self.name);
    }
}

4. 在ViewController的dealloc中移除

- (void)dealloc{
    [self removeObserver:self forKeyPath:@"name"];
}

• 注: 移除observer視實際情況而定, 也可以在viewDidDisappear:或者處理完監(jiān)聽, 在observeValueForKeyPath: ofObject: change: context:最后.

測試

1. 在viewController中添加一個title為更改name 的按鈕, 為其添加一個事件, 用來修改name, 如下:

static NSInteger idx;
///修改name
- (IBAction)modifyNameAction:(id)sender {
    
    NSArray *nameArr = @[@"張三", @"李四", @"王五", @"趙六", @"Jim七", @"David八", @"Kevin九", @"Danny十"];
    self.name = nameArr[idx];
    idx++;
    if (idx > 9) {
        idx = 0;
    }
    
}

• 注: 此處為了使代碼緊湊, 未優(yōu)化nameArr

2. 點擊按鈕, 更改name屬性, 可以看到KVO的監(jiān)聽方法被觸發(fā):
   
   KVO觸發(fā).gif

以上即為KVO的基本使用, 也是系統(tǒng)的自動調(diào)用. KVO自動調(diào)用的原理為:

1. 系統(tǒng)會重寫被監(jiān)聽屬性的setter方法, 如上述的setName:, 所以, 必須監(jiān)聽屬性, 有setter方法
2. 系統(tǒng)會依次調(diào)用:

• 1)- willChangeValueForKey:
• 2)setter方法
• 3)- didChangeValueForKey:
• 4)通知觀察者.
  這也解釋了NSKeyValueObservingOptionOld(舊值)及NSKeyValueObservingOptionNew(新值)的來源.

驗證:

重寫setter, willChangeValueForKey:, didChangeValueForKey:

- (void)setName:(NSString *)name{
    NSLog(@"22---setter");
    _name = name;
}

- (void)willChangeValueForKey:(NSString *)key{
    [super willChangeValueForKey:key];
    NSLog(@"11---will key = %@", key);
}

- (void)didChangeValueForKey:(NSString *)key{
    [super didChangeValueForKey:key];
    NSLog(@"33--- did key = %@", key);
    
}

觀察打印如下:

方法調(diào)用順序.gif

• 有自動調(diào)用, 就有手動調(diào)用, 手動調(diào)用我們將在后面講述.

監(jiān)聽一個屬性, 實現(xiàn)監(jiān)聽多個屬性

我們使用間接屬性來舉例

1. 定義一個Child類, 它有4個屬性: birthday, year, month, day:

///生日
@property (nonatomic, copy) NSString *birthday;
///生日的年
@property (nonatomic, assign) NSInteger year;
///生日的月
@property (nonatomic, assign) NSInteger month;
///生日的日
@property (nonatomic, assign) NSInteger day;

2. Child.m 中, 初始化上述屬性:

- (instancetype)init{
    if (self = [super init]) {
        self.birthday = @"2000-01-01";
        self.year = 2000;
        self.month = 1;
        self.day = 1;
    }
    return self;
}

3. 定義一個Worker類, 它有一個Child屬性:

@property (nonatomic, strong) Child *child;

4. viewController 類中添加一個worker屬性:

@property (nonatomic, strong) Worker *worker;

5. 監(jiān)聽worker 的child 中birthday 的改變:

[self.worker addObserver:self forKeyPath:@"child.birthday" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];

6. 添加更改間接屬性的事件:

//更改間接屬性值事件
- (IBAction)modifyObjectAction:(id)sender {
    self.worker.child.birthday = @"2001-12-31";
}

這樣在監(jiān)聽方法中, 我們便能得到worker.child.birthday 更改前后的值:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
   
    if ([keyPath isEqualToString:@"child.birthday"]) {
        NSLog(@“change info = %@", change);
    }
}

監(jiān)聽間接屬性`child.birthday`.gif

• 上例中, 對于Child來說, 其屬性birthday是由year, month, day影響的, 即當(dāng)year, month, day其一改變時, 關(guān)心birthday的外界也需要收到監(jiān)聽. 這種情況下, 當(dāng)Child的year, month 或 day改變時, 應(yīng)當(dāng)告訴birthday的監(jiān)聽者.
• 這里就需要實現(xiàn) KVO 的這個方法:
  + (NSSet<NSString *> *)keyPathsForValuesAffectingKey
• 在我們重寫這個方法, 系統(tǒng)自動補全提示時, 會將Key替換成我們的屬性名稱. 如此處重寫的方法為:

///當(dāng)kvo當(dāng)前對象的birthday屬性時,如果year,month,day的值發(fā)生變化,都會觸發(fā)這個KVO
+ (NSSet<NSString *> *)keyPathsForValuesAffectingBirthday{
    return [NSSet setWithObjects:NSStringFromSelector(@selector(year)),
            NSStringFromSelector(@selector(month)),
            NSStringFromSelector(@selector(day)),
            nil];
}

這樣, 只要KVO監(jiān)聽了birthday , 當(dāng)year, month, day 改變時, 也會觸發(fā)監(jiān)聽方法.

• 注: 這種操作, 我們在change中得到的還是birthday的值.

監(jiān)聽數(shù)組

實際上, 能使用KVO來監(jiān)聽的屬性, 必須符合Key-Value Coding, 而數(shù)組并不符合.
所以, 直接監(jiān)聽數(shù)組屬性, 用數(shù)組默認(rèn)的API來操作數(shù)組時, 是不會觸發(fā)監(jiān)聽方法的.
實現(xiàn):

1. 被監(jiān)聽的對象需要實現(xiàn)下面方法
2. 且操作數(shù)組屬性時, 也要使用下面對應(yīng)的方法:

- objectInMyArrayAtIndex:

- insertObject:inMyArrayAtIndex:

- removeObjectFromMyArrayAtIndex:

- replaceObjectInMyArrayAtIndex:withObject:

同KVO的其它方法一樣, 重寫這些方法時, 系統(tǒng)也會有補全提示, 而上述中的MyArray會替換成實際的屬性名稱.

1. 依然在上述例子中, 我們?yōu)?code>worker添加一個cities屬性:

@property (nonatomic, strong) NSMutableArray *cities;

2. 在Worker.m中初始化:

- (instancetype)init{
    if (self = [super init]) {
        self.cities = [NSMutableArray array];
    }
    return self;
}

3. 實現(xiàn)KVO數(shù)組相關(guān)的方法:

- (id)objectInCitiesAtIndex:(NSUInteger)index{
    return [self.cities objectAtIndex:index];
}

- (void)insertObject:(NSString *)object inCitiesAtIndex:(NSUInteger)index{
    [self.cities insertObject:object atIndex:index];
}

- (void)removeObjectFromCitiesAtIndex:(NSUInteger)index{
    [self.cities removeObjectAtIndex:index];
}

- (void)replaceObjectInCitiesAtIndex:(NSUInteger)index withObject:(id)object{
    [self.cities replaceObjectAtIndex:index withObject:object];
}

- (void)addCitiesObject:(NSString *)object{
    [self.cities addObject:object];
}

4. 并在Worker.h文件中公開上述方法:

- (id)objectInCitiesAtIndex:(NSUInteger)index;

- (void)insertObject:(NSString *)object inCitiesAtIndex:(NSUInteger)index;

- (void)removeObjectFromCitiesAtIndex:(NSUInteger)index;

- (void)replaceObjectInCitiesAtIndex:(NSUInteger)index withObject:(id)object;

- (void)addCitiesObject:(NSString *)object;

5. 在viewController中監(jiān)聽:

    [self.worker addObserver:self forKeyPath:NSStringFromSelector(@selector(cities)) options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:nil];

6. 依次執(zhí)行下面方法:

    [self.worker insertObject:@"nanjing" inCitiesAtIndex:0];
    [self.worker insertObject:@"suzhou" inCitiesAtIndex:1];
    [self.worker replaceObjectInCitiesAtIndex:1 withObject:@"wuxi"];
    [self.worker removeObjectFromCitiesAtIndex:self.worker.cities.count-1];

過濾掉無用信息后, 對應(yīng)打印結(jié)果如下:

//1. [self.worker insertObject:@"nanjing" inCitiesAtIndex:0];
 change info = {
    kind = 2;
    new =     (
        nanjing
    );
}
 //2. [self.worker insertObject:@"suzhou" inCitiesAtIndex:1];
change info = {
    kind = 2;
    new =     (
        suzhou
    );
}
 //3. [self.worker replaceObjectInCitiesAtIndex:1 withObject:@"wuxi"];

change info = {
    kind = 4;
    new =     (
        wuxi
    );
    old =     (
        suzhou
    );
}
  //4. [self.worker removeObjectFromCitiesAtIndex:self.worker.cities.count-1];

change info = {
    kind = 3;
    old =     (
        wuxi
    );
}

因為字典change中存儲的是變化的數(shù)組元素的值, 而不是整個數(shù)組的值, 所以對應(yīng)步驟解析如下:

• 1.添加.所以只有新值,沒有舊值
• 2.同上
• 3.替換.新值替換舊值, 所以既有舊值,也有新值
• 4.刪除.只是刪除舊值, 沒有新值加入,所以只有舊值
  • 注:添加元素時,只能insertObject:AtIndex, 沒有直接addObject:

關(guān)閉系統(tǒng)自動調(diào)用KVO, 改為手動調(diào)用

在很多情況下, 我們都應(yīng)該關(guān)閉自動調(diào)用, 改為手動調(diào)用. 因為每次調(diào)用setter, 都會調(diào)用監(jiān)聽方法, 即使舊值與新值相同.

如我們要關(guān)閉屬性name的自動調(diào)用

1. 重寫觸發(fā)手動或自動調(diào)用的類方法, 并返回NO. 如

+ (BOOL)automaticallyNotifiesObserversOfName{
    return NO;
}

或者

+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key{
    if ([key isEqualToString:@"name"]) {
        return NO;
    }
    //name之外的屬性,還是由系統(tǒng)自動調(diào)用
    return YES;
}

是的, 正如你所料, 系統(tǒng)是默認(rèn)返回YES

2. 在name改變, 需要觸發(fā)監(jiān)聽方法observeValueForKeyPath: ofObject: change: context:時, 手動調(diào)用-willChangeValueForKey:及- didChangeValueForKey:

實現(xiàn)

1. 把我們的名字?jǐn)?shù)組的李四變成張三, 這樣我們就有兩個張三了:
   

   兩個`張三.png

2. 重寫setter方法:

- (void)setName:(NSString *)name{
    
    if (![_name isEqualToString:name]) {
        
        [self willChangeValueForKey:@"name"];
        NSLog(@"22---setter");
        _name = [name copy];
        
        [self didChangeValueForKey:@"name"];
    }
}

3. 打印如下:

   
   
   手動調(diào)用`KVO`.gif

利用上述KVO手動調(diào)用的原理, 我們可以監(jiān)聽成員變量. 步驟:

1.添加一個成員變量:

{
    int _age;
}

2.監(jiān)聽:

    [self addObserver:self forKeyPath:@"_age" options:NSKeyValueObservingOptionNew context:nil];

3.實現(xiàn)監(jiān)聽方法:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    
    if ([keyPath isEqualToString:@"name"]) {
        
        NSLog(@"kvo name = %@", self.name);
        
    }else if ([keyPath isEqualToString:@"_age"]){
        NSLog(@"age = %zd", _age);
    }
    
}

4.添加一個修改_age的事件

///修改age
- (IBAction)modifyAgeAction:(id)sender {
    
    [self willChangeValueForKey:@"_age"];
    
    _age++;
    
    [self didChangeValueForKey:@"_age"];
}

5.打印如下:

利用手動調(diào)用`KVO`,實現(xiàn)監(jiān)聽.gif

context參數(shù)

最后我們再來看下addObserver: forKeyPath:options:context:的context參數(shù).它是監(jiān)聽的唯一標(biāo)識,它會被代入監(jiān)聽方法中:observeValueForKeyPath: ofObject: change: context:
通常情況下, 我們不需要 context 參數(shù)來區(qū)別我們的監(jiān)聽, 但是在下面的小概率事件時:

• 繼承
• 父類使用了KVO

就需要用到了.

• 如上述的viewController繼承自BaseViewController
• BaseViewController也使用到了KVO.
  此時在viewController中的方法observeValueForKeyPath: ofObject: change: context:就覆蓋了父類的實現(xiàn).
  解決方法是:
• 定義一個唯一的context, 如:

static void *ViewControllerContext = &ViewControllerContext;

• 監(jiān)聽時,傳入context:

    [self.worker addObserver:self forKeyPath:NSStringFromSelector(@selector(cities)) options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:ViewControllerContext];

• 在監(jiān)聽方法中,根據(jù)context判斷:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
   
    if (context == ViewControllerContext) {
        if ([keyPath isEqualToString:NSStringFromSelector(@selector(cities))]) {
            NSLog(@"change info = %@", change);
        }
       
    }else{
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

• 如果使用了手動KVO, 也要注意調(diào)用super

+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key{
    if ([key isEqualToString:@"name"]) {
        return NO;
    }
    return [super automaticallyNotifiesObserversForKey:key];
}

以上就是我對KVO的總結(jié), 如發(fā)現(xiàn)有欠妥之處, 請隨時指出, 幫助我進(jìn)步, 謝謝.