OC - KVO

什么是KVO：

KVO,即：Key-Value Observing，它提供一种机制，当指定的对象的属性被修改后，则对象就会接受到通知。简单的说就是每次指定的被观察的对象的属性被修改后，KVO就会自动通知相应的观察者了。

使用步骤

1. 注册，指定被观察者的属性，

2. 实现回调方法

3. 移除观察（必须移除观察否则后果严重）

具体运用

1. 控制器

- (void)viewDidLoad {
    [super viewDidLoad];
    //添加测试按钮点击发送消息
    UIButton *btn = [UIButton buttonWithType:UIButtonTypeContactAdd];
    [self.view addSubview:btn];
    btn.frame = CGRectMake(100, 100, 20, 20);
    [btn addTarget:self action:@selector(btnClick) forControlEvents:UIControlEventTouchUpInside];
}

- (void)btnClick{
    NSLog(@"点击了按钮");
    self.tabBarItem.badgeValue = @"111";//改变角标的值
}

2. 添加观察对象

[item addObserver:self forKeyPath:@"badgeValue" options:0 context:nil];

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context
{
    [self setTitle:self.item.title forState:UIControlStateNormal];
    [self setTitle:self.item.title forState:UIControlStateSelected];
    [self setImage:self.item.image forState:UIControlStateNormal];
    [self setImage:self.item.selectedImage forState:UIControlStateSelected];
    if(self.item.badgeValue)
    {
        self.badgeButton.hidden = NO;
        [self.badgeButton setTitle:self.item.badgeValue forState:UIControlStateNormal];
        
        CGFloat badgeY = 2;
        CGFloat badgeH = self.badgeButton.currentBackgroundImage.size.height;
        CGFloat badgeW = self.badgeButton.currentBackgroundImage.size.width;
        CGFloat badgeX = self.frame.size.width - badgeW - 10;
        if(self.item.badgeValue.length>1)
        {
            CGSize badgeSize = [self.item.badgeValue sizeWithFont:self.badgeButton.titleLabel.font];
            badgeW = badgeSize.width + 10;
        }
        if(badgeW>self.badgeButton.currentBackgroundImage.size.width)
        {
            badgeX = self.frame.size.width - badgeW;
        }
        self.badgeButton.frame = CGRectMake(badgeX, badgeY, badgeW, badgeH);
    }
    else{
        self.badgeButton.hidden = YES;
    }
}

3. 移除观察

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