對(duì)于iOS 開發(fā)者來說，KVO(key-value-observing)的使用大家已經(jīng)不再陌生，而且使用起來也是非常方便。

KVO的簡單使用：

KVOObject *object = [[KVOObject alloc] init];
 [object addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];
object.name = @"123";

這樣我們就已經(jīng)為object的name屬性添加了監(jiān)聽，只要object的name屬性發(fā)生改變，我們就可以通過KVO的回調(diào)方法獲取其新值。

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
     NSLog(@"新值為 %@", [change objectForKey:NSKeyValueChangeNewKey]);
}

2017-12-18 16:44:36.096356+0800 KVO[5973:274888] 新值為 123

KVO的基本使用就是這樣，那么KVO是用什么原理實(shí)現(xiàn)的呢？如何自己實(shí)現(xiàn)一個(gè)帶block的KVO呢？
KVO原理：例如在為Object類添加監(jiān)聽時(shí)，蘋果動(dòng)態(tài)的為我們添加了一個(gè)類，類的名字是NSKVONotifying_Object，并且NSKVONotifying_Object是Object的子類，然后把指向Object的類指向了NSKVONotifying_Object，然后在子類中重寫setter方法。

直接上代碼：

-(void)sp_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context callBack:(void (^)(id _Nullable))block{
    NSString *className = [@"SP_" stringByAppendingString:NSStringFromClass([self class])];
    Class newClass = objc_allocateClassPair([self class], className.UTF8String, 0);//動(dòng)態(tài)生成一個(gè)類，類名在原類基礎(chǔ)上加一個(gè)前綴SP_
    objc_registerClassPair(newClass);//注冊(cè)該類
    object_setClass(self, newClass);//把指針指向子類
    class_addMethod(newClass, @selector(setName:), (IMP)classSetName, "v@:@");//重寫set方法
    objc_setAssociatedObject(self, &blockKey, block, OBJC_ASSOCIATION_RETAIN_NONATOMIC);//關(guān)聯(lián)block對(duì)象
    
}

void classSetName(id self,SEL _cmd, NSString * newName){
    struct objc_super superClass = {
        .receiver = self,
        .super_class = class_getSuperclass(object_getClass(self))
    };
    
    // 調(diào)用父類中setter方法
    objc_msgSendSuper(&superClass,_cmd,newName);
    void(^block)(id paramter) = objc_getAssociatedObject(self, &blockKey);
    if (block) {
        block(newName);
    }
}

viewController中調(diào)用：

@interface ViewController ()
@property (nonatomic, strong) KVOObject * object;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    KVOObject *object = [KVOObject new];
    [object sp_addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil callBack:^(id  _Nullable paramter) {
        NSLog(@"block回調(diào): %@\nobject name屬性值: %@",paramter,object.name);
    }];
    _object = object;
}


-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    static int a = 0;
    a++;
    _object.name = [NSString stringWithFormat:@"%d",a];
}

KVO[6404:296022] block回調(diào): 1
object name屬性值: 1
2017-12-18 17:05:34.432499+0800 KVO[6404:296022] block回調(diào): 2
object name屬性值: 2