Observing KVO Key-Value基本使用原理示例详解

typedef NS_OPTIONS(NSUInteger, NSKeyValueObservingOptions) {
    NSKeyValueObservingOptionNew = 0x01, // 新值
    NSKeyValueObservingOptionOld = 0x02, // 旧值
    // 属性的初始值
    NSKeyValueObservingOptionInitial API_AVAILABLE(macos(10.5), ios(2.0), watchos(2.0), tvos(9.0)) = 0x04,
    // 在变化前发送通知
    // change 字典通过包含键 NSKeyValueChangeNotificationIsPriorKey 和 NSNumber 包装 YES 的值来表示更改前通知。
    NSKeyValueObservingOptionPrior API_AVAILABLE(macos(10.5), ios(2.0), watchos(2.0), tvos(9.0)) = 0x08
};

static void *PersonAccountBalanceContext = &PersonAccountBalanceContext;
static void *PersonAccountInterestRateContext = &PersonAccountInterestRateContext;
- (void)registerAsObserverForAccount:(Account*)account {
    [account addObserver:self
              forKeyPath:@"balance"
                 options:(NSKeyValueObservingOptionNew |
                          NSKeyValueObservingOptionOld)
                 context:PersonAccountBalanceContext];
    [account addObserver:self
              forKeyPath:@"interestRate"
                 options:(NSKeyValueObservingOptionNew |
                          NSKeyValueObservingOptionOld)
                  context:PersonAccountInterestRateContext];
}

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context {
    if (context == PersonAccountBalanceContext) {
        // Do something with the balance…
    } else if (context == PersonAccountInterestRateContext) {
        // Do something with the interest rate…
    } else {
        // Any unrecognized context must belong to super
        [super observeValueForKeyPath:keyPath
                             ofObject:object
                               change:change
                               context:context];
    }
}

- (void)unregisterAsObserverForAccount:(Account*)account {
    [account removeObserver:self
                 forKeyPath:@"balance"
                    context:PersonAccountBalanceContext];
    [account removeObserver:self
                 forKeyPath:@"interestRate"
                    context:PersonAccountInterestRateContext];
}

// 使用访问器方法
[account setName:@"Savings"];
// 使用 setValue:forKey:.
[account setValue:@"Savings" forKey:@"name"];
// 使用 setValue:forKeyPath:
[document setValue:@"Savings" forKeyPath:@"account.name"];
// 使用 mutableArrayValueForKey: 检索关系代理对象。
Transaction *newTransaction = <#Create a new transaction for the account#>;
NSMutableArray *transactions = [account mutableArrayValueForKey:@"transactions"];
[transactions addObject:newTransaction];

+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)theKey {
    BOOL automatic = NO;
    if ([theKey isEqualToString:@"balance"]) {
        automatic = NO;
    }
    else {
        automatic = [super automaticallyNotifiesObserversForKey:theKey];
    }
    return automatic;
}

- (void)setBalance:(double)theBalance {
    [self willChangeValueForKey:@"balance"];
    _balance = theBalance;
    [self didChangeValueForKey:@"balance"];
}

- (void)setBalance:(double)theBalance {
    [self willChangeValueForKey:@"balance"];
    [self willChangeValueForKey:@"itemChanged"];
    _balance = theBalance;
    _itemChanged = _itemChanged+1;
    [self didChangeValueForKey:@"itemChanged"];
    [self didChangeValueForKey:@"balance"];
}

- (void)removeTransactionsAtIndexes:(NSIndexSet *)indexes {
    [self willChange:NSKeyValueChangeRemoval
        valuesAtIndexes:indexes forKey:@"transactions"];
    // Remove the transaction objects at the specified indexes.
    [self didChange:NSKeyValueChangeRemoval
        valuesAtIndexes:indexes forKey:@"transactions"];
}

- (NSString *)fullName {
    return [NSString stringWithFormat:@"%@ %@",firstName, lastName];
}

+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key {
    NSSet *keyPaths = [super keyPathsForValuesAffectingValueForKey:key];
    if ([key isEqualToString:@"fullName"]) {
        NSArray *affectingKeys = @[@"lastName", @"firstName"];
        keyPaths = [keyPaths setByAddingObjectsFromArray:affectingKeys];
    }
    return keyPaths;
}

+ (NSSet *)keyPathsForValuesAffectingFullName {
    return [NSSet setWithObjects:@"lastName", @"firstName", nil];
}

@interface Person : NSObject
@property (nonatomic, assign) NSUInteger age;
@end
@implementation Person
@end
@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    self.person = [Person new];
    // 添加监听
    [self.person addObserver:self 
                  forKeyPath:@"age" 
                     options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld 
                     context:nil];
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    // 改变对象属性值
    self.person.age = 18;
}
- (void)dealloc {
    // 移除监听
    [self.person removeObserver:self forKeyPath:@"age"];
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    // 被监听的属性值改变后的回调
    NSLog(@"%@", change);
}
@end