/**
 * 添加一个子控制器
 */
- (void)addSubItemWithViewController:(UIViewController *)viewController;

#define HYTabbarViewHeight 49 //顶部标签条的高度
#define HYColumn 4  //一屏幕宽显示4个标题

//外界传个控制器,添加一个栏目
- (void)addSubItemWithViewController:(UIViewController *)viewController{ 
  
 UIButton * btn = [UIButton buttonWithType:UIButtonTypeCustom];
 [self.tabbar addSubview:btn];
 [self setupBtn:btn withTitle:viewController.title];
 [btn addTarget:self action:@selector(itemSelected:) forControlEvents:UIControlEventTouchUpInside];
 [self.subViewControllers addObject:viewController];
}

//viewDidLoad中添加观察者
[self addObserver:self forKeyPath:@"selectedIndex" options:NSKeyValueObservingOptionOld |NSKeyValueObservingOptionNew context:@"scrollToNextItem"];
 
//让标题按钮居中算法
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context{
   
 if (context == @"scrollToNextItem") {
  self.prevSelectedIndex = [change[@"old"] integerValue];
  if (self.prevSelectedIndex == self.selectedIndex) {
   return;
  }
  //设置按钮选中
  [self itemSelectedIndex:self.selectedIndex];
  UIButton * btn = self.titles[self.selectedIndex];
  
  //让选中按钮居中
  NSInteger min = HYColumn / 2 ;
  if (_selectedIndex = self.titles.count - min) {
   UIButton * tempBtn = self.titles[self.titles.count - min - 1];
   CGFloat btnX = (HYColumn % 2 ) ? tempBtn.center.x : (tempBtn.center.x + btn.frame.size.width * 0.5) ;
   CGFloat offsetX = _tabbar.center.x - btnX;
   [UIView animateWithDuration:0.25 animations:^{
    _tabbar.contentOffset = CGPointMake(- offsetX, 0);
   }];  
  }else if (_selectedIndex > min && _selectedIndex < self.titles.count - min && self.titles.count > HYColumn ) {
   CGFloat btnX = (HYColumn % 2 ) ? btn.center.x : (btn.center.x - btn.frame.size.width * 0.5) ;
   CGFloat offsetX = _tabbar.center.x - btnX;
   [UIView animateWithDuration:0.25 animations:^{
    _tabbar.contentOffset = CGPointMake( - offsetX, 0);
   }];
  }
 } else {
  [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
 }
} 

- (void)viewDidLoad {
 [super viewDidLoad];
 [self.view addSubview:self.tabbarView];
}//懒加载- (HYTabbarView *)tabbarView{ if (!_tabbarView) {
  _tabbarView = ({
   HYTabbarView * tabbar = [[HYTabbarView alloc]initWithFrame:CGRectMake(0,30,[UIScreen mainScreen].bounds.size.width,600)];   
   for (NSInteger i = 0; i< 10; i ++) {    
    UIViewController * vc = [[UIViewController alloc]init];
    vc.title = [NSString stringWithFormat:@"第%ld个",i+1];
    [tabbar addSubItemWithViewController:vc];
   }
   tabbar;
  });
 } return _tabbarView;
}