ios kvo 的用法

[self
addObserver:self
forKeyPath:@"Ab"
options:NSKeyValueObservingOptionNew
context:nil];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString
*,id> *)change context:(void *)context{

NSLog(@"%@",keyPath);
if ([keyPath
isEqualToString:@"Ab"]) {
NSLog(@"change====%@",change);

NSLog(@"new==%@",[change
valueForKey:@"new"]);

}
}