KVO

https://www.jianshu.com/p/53c68a00d2fc

KVO iOS11之后修改成员变量, 不需要移除监听

KVO 防止崩溃

1. 不实现 -observeValueForKeyPath:ofObject:change:context 方法, 会崩溃

2. 不移除监听, 也会崩溃, (多次添加KVO监听, 移除的时候只移除一次也会崩溃 ios11之前)

3. 分类的属性也能实现KVO

KVO设置依赖 E=A+B+C;  任意一个改变都会触发KVO监听方法

自定义KVO

KVO 聚合运算

NSInteger sum = [[persons valueForKeyPath:@"@sum.age"] integerValue];

NSInteger min = [[persons valueForKeyPath:@"@min.age"] integerValue];

NSInteger max = [[persons valueForKeyPath:@"@max.age"] integerValue];

float avg = [[persons valueForKeyPath:@"@avg.age"] floatValue];

NSInteger count = [[persons valueForKeyPath:@"@count.age"] integerValue];

NSLog(@"sum=%zd", sum);

NSLog(@"min=%zd", min);

NSLog(@"max=%zd", max);

NSLog(@"avg=%g", avg);

NSLog(@"count=%zd", count);

NSArray *arr = @[@1, @2, @3, @4];

NSNumber *avg = [arr valueForKeyPath:@"@avg.floatValue"];

NSNumber *sum = [arr valueForKeyPath:@"@sum.integerValue"];

NSLog(@"avg=%@", avg);

NSLog(@"sum=%@", sum);