1.观察属性

[self.button addObserver:self forKeyPath:@"selected" options:NSKeyValueObservingOptionNew context:nil];

2.实现方法

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    if ([object isEqual:self.button] && [keyPath isEqualToString:@"selected"]) {
        BOOL newValue = [[change objectForKey:@"new"] boolValue];
        if (newValue) {
            [self.button setTitle:@"选中" forState:UIControlStateNormal];
        } else {
            [self.button setTitle:@"未选中" forState:UIControlStateNormal];
        }
    }
}

3.移除观察者

[self.button removeObserver:self forKeyPath:@"selected"];