KVO的用法

注冊

[object addObserver:observer forKeyPath:@"text" options:NSKeyValueObservingOptionNew context:nil];

實現回調方法

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if([keyPath isEqualToString:@"text"])
    {
        NSLog(@"text:@%@", change[NSKeyValueChangeNewKey]);
    }
}

釋放的時候取消注冊

[object removeObserver:self forKeyPath:@"text"];

這裡有幾個問題

注冊的時候參數過多釋放的時候必須取消注冊只有一個回調,當注冊的觀察者過多的時候,會使得代碼變得雜亂

KVO的封裝

下面我們將針對這幾個問題進行封裝

定義一個觀察者類

@interface XYObserver : NSObject
@end


@interface XYObserver ()

@property (nonatomic, assign) XYObserverType type;      // 觀察者的類型

@property (nonatomic, weak) id target;                  // 被觀察的對象的值改變時後的響應方法所在的對象
@property (nonatomic, assign) SEL selector;             // 被觀察的對象的值改變時後的響應方法
@property (nonatomic, copy) XYObserver_block_sourceObject_new_old block;        // 值改變時執行的block

@property (nonatomic, assign) id  sourceObject;         // 被觀察的對象
@property (nonatomic, strong) NSString *keyPath;        // 被觀察的對象的keyPath

-(instancetype) initWithSourceObject:(id)sourceObject keyPath:(NSString*)keyPath target:(id)target selector:(SEL)selector type:(XYObserverType)type;

-(instancetype) initWithSourceObject:(id)sourceObject keyPath:(NSString*)keyPath block:(XYObserver_block_sourceObject_new_old)block;

@end

添加NSObject關於觀察者的類別

@interface NSObject (XYObserver)

@property (nonatomic, readonly, strong) NSMutableDictionary *observers;

/**
 * api parameters 說明
 *
 * sourceObject 被觀察的對象

 * keyPath 被觀察的屬性keypath

 * target 默認是self

 * selector @selector(propertyNew:)
            @selector(propertyNew:old:)
            @selector(propertyIn:new:)
            @selector(propertyIn:new:old:)

 * type 根據selector自動賦值

 * block selector, block二選一
 */
-(void) observeWithObject:(id)sourceObject property:(NSString*)property;
-(void) observeWithObject:(id)sourceObject property:(NSString*)property block:(XYObserver_block_sourceObject_new_old)block;

-(void) removeObserverWithObject:(id)sourceObject property:(NSString *)property;
-(void) removeObserverWithObject:(id)sourceObject;
-(void) removeAllObserver;

@end

在這裡我們查詢的實現的方法

-(void) observeWithObject:(id)object property:(NSString*)property{
 SEL aSel = NSSelectorFromString([NSString stringWithFormat:@"%@New:", property]);
    if ([self respondsToSelector:aSel]) {
        [self observeWithObject:object
                        keyPath:property
                         target:self
                       selector:aSel
                        type:XYObserverType_new];
        return;
    }
    .
    .
    .
}

用block的話就直接保存

-(void) observeWithObject:(id)object property:(NSString*)property block:(XYObserver_block_sourceObject_new_old)block{
    [self observeWithObject:object keyPath:property block:block];
}

處理實現方法

-(void) observeValueForKeyPath:(NSString*)keyPath ofObject:(id)object change:(NSDictionary*)change context:(void*)context
{
    __weak __typeof(self) weakSelf = self;
    if (_block) {
        _block(weakSelf, change[NSKeyValueChangeNewKey], change[NSKeyValueChangeOldKey]);
        return;
    }

    if (_type == XYObserverType_new) {
        action(_target, _selector, change[NSKeyValueChangeNewKey]);
    }else if (_type == XYObserverType_new_old) {
        action(_target, _selector, change[NSKeyValueChangeNewKey], change[NSKeyValueChangeOldKey]);
    }else if (_type == XYObserverType_self_new) {
        action(_target, _selector, self, change[NSKeyValueChangeNewKey]);
    }else if (_type == XYObserverType_self_new_old) {
        action(_target, _selector, self, change[NSKeyValueChangeNewKey], change[NSKeyValueChangeOldKey]);
    }
}

把所有的觀察者添加到一個字典裡

-(void) observeWithObject:(id)object keyPath:(NSString*)keyPath target:(id)target selector:(SEL)selector type:(XYObserverType)type{
    XYObserver *ob = [[XYObserver alloc] initWithSourceObject:object keyPath:keyPath target:target selector:selector type:type];

    NSString *key = [NSString stringWithFormat:@"%@_%@", object, keyPath];
    [self.observers setObject:ob forKey:key];
}

-(void) observeWithObject:(id)object property:(NSString*)property block:(XYObserver_block_sourceObject_new_old)block{
    [self observeWithObject:object keyPath:property block:block];
}

-(id) observers{
    id object = objc_getAssociatedObject(self, NSObject_observers);

    if (nil == object) {
        NSMutableDictionary *dic = [NSMutableDictionary dictionaryWithCapacity:8];
        objc_setAssociatedObject(self, NSObject_observers, dic, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
        return dic;
    }

    return object;
}

當對象釋放的時候會清空字典裡的觀察者對象,在觀察者對象的dealloc方法裡面取消注冊觀察者

-(void) dealloc
{
    if (_sourceObject) { [_sourceObject removeObserver:self forKeyPath:_keyPath]; }
}

為了方便書寫,定義幾個宏

#define ON_KVO_1_( __property )     -(void) __property##New:(id)newValue
#define ON_KVO_2_( __property )     -(void) __property##New:(id)newValue old:(id)oldValue
#define ON_KVO_3_( __property )     -(void) __property##In:(id)sourceObject new:(id)newValue
#define ON_KVO_4_( __property )     -(void) __property##In:(id)sourceObject new:(id)newValue old:(id)oldValue

使用的demo

[self observeWithObject:self property:@"testKVO"];

ON_KVO_4_(testKVO){
     NSLogD(@"obj:%@ new:%@ old:%@", sourceObject, newValue, oldValue);
}

[self observeWithObject:self property:@"testKVO2" block:^(id sourceObject, id newValue, id oldValue) {
        NSLogD(@"obj:%@ new:%@ old:%@", sourceObject, newValue, oldValue);
    }];

這個封裝的優點是在使用KVO的時候不需要記住太多東西.

代碼可以在https://github.com/uxyheaven/XYQuickDevelop下載到