KVO (Key-Value-Observer, 键值观察)

KVO (Key-Value-Observer, 键值观察)

 

valueForKey 和 valueForKeyPath区别

    valueForKeyPath是valueForKey的升级, valueForKey都能用valueForKeyPath, valueForKey只能一层   

 KVC计算数组和

    NSArray *array = @[@"1", @"2", @"3", @"4", @"5"];
    NSNumber *sum = [array valueForKeyPath:@"@sum.integerValue"];
    NSLog(@"%@", sum);

KVC修改 UITextField的占位字符颜色

// 设置占位文字的颜色为红色
[self setValue:[UIColor redColor] forKeyPath:@"_placeholderLabel.textColor"];

    KVO: Key-Value-Observer, 键值观察

  1.添加观察者

    第一个self代表被观察对象

    第二个self代表观察者

    KeyPath 实例变量路径

    options 可选项, 可以添加多个, 中间用|分开

    context 上下文, 可以用来传值

    [self addObserver:self forKeyPath:@"name" options:(NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld) context:nil];
    //初始化可变数组
    self.array = [@[] mutableCopy];
    //添加观察者
    [self addObserver:self forKeyPath:@"array" options:(NSKeyValueObservingOptionNew) context:nil];

  2. 接收KVO的事件

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context {
    NSLog(@"object = %@, keyPath = %@ change = %@", object, keyPath, change);
}

- (IBAction)kvo_fire:(id)sender {
    self.name = @"568";
}

- (IBAction)array_KVO_fire:(id)sender {
//    [self.array addObject:@"123"];
    [[self mutableArrayValueForKey:@"array"] addObject:@"OK"];
}

- (void)dealloc
{
    //3. 移除观察者
    [self removeObserver:self forKeyPath:@"name"];
    [self removeObserver:self forKeyPath:@"array"];
}