問題解決：自動隱藏UITabBar上面的自定義按鈕

在UITabBar中間添加按鈕，像下圖：

實現如下：

// Create a custom UIButton and add it to the center of our tab bar
-(void) addCenterButtonWithImage:(UIImage*)buttonImage highlightImage:(UIImage*)highlightImage
{
  UIButton* button = [UIButton buttonWithType:UIButtonTypeCustom];
  button.autoresizingMask = UIViewAutoresizingFlexibleRightMargin | UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleBottomMargin | UIViewAutoresizingFlexibleTopMargin;
  button.frame = CGRectMake(0.0, 0.0, buttonImage.size.width, buttonImage.size.height);
  [button setBackgroundImage:buttonImage forState:UIControlStateNormal];
  [button setBackgroundImage:highlightImage forState:UIControlStateHighlighted];

  CGFloat heightDifference = buttonImage.size.height - self.tabBar.frame.size.height;
  if (heightDifference < 0)
    button.center = self.tabBar.center;
  else
  {
    CGPoint center = self.tabBar.center;
    center.y = center.y - heightDifference/2.0;
    button.center = center;
  }
  
  [self.view addSubview:button];
}

如果我們要導航到下一層並且要隱藏底部的UITabBar，代碼如下：

    UIViewController *vc = [[UIViewController alloc] init];
    vc.hidesBottomBarWhenPushed = YES;
    [self.navigationController pushViewController:vc animated:YES];

通過將hidesBottomBarWhenPushed屬性設置爲YES，壓棧的時候可以把UITabBar隱藏，但是中間的按鈕並沒有被隱藏。這裏有兩種解決方法：

一、提供方法手工隱藏/顯示中間按鈕，顯示的時候觸發動畫

- (void)hideTabBar:(BOOL)hidden {
    UIButton *btn = (UIButton *)[self.tabBarController.view viewWithTag:kCENTER_BUTTON_TAG];
    [[btn layer] removeAnimationForKey:kANIMATION_KEY];
    if (btn.hidden) {
        CATransition *animation = [CATransition animation];
        animation.delegate = self;
        animation.duration = 0.35;// 0.618
        animation.timingFunction = UIViewAnimationCurveEaseInOut;
        animation.type = kCATransitionFade;
        [[btn layer] addAnimation:animation forKey:kANIMATION_KEY];
    }
    btn.hidden = hidden;
}

二、自動隱藏

1、實現UINavigationControllerDelegate協議，爲UITabBarController的viewControllers註冊協議

- (void)navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated {
    if (viewController.hidesBottomBarWhenPushed) {
        [self hideTabBar:viewController.hidesBottomBarWhenPushed];
    }
}

[self.tabBarController.viewControllers enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        UINavigationController *navVC = obj;
        navVC.delegate = self;
    }];

2、註冊UITabBar的hidden的kvo

[self.tabBarController addObserver:self forKeyPath:@"tabBar.hidden" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if ([keyPath isEqualToString:kTABBAR_HIDDEN_KEYPATH]) {
        [self hideTabBar:self.tabBar.hidden];
    }
}

思路：

1、因爲設置hidesBottomBarWhenPushed爲YES，導致UITabBar的hidden屬性變化，所以一開始希望通過註冊UITabBar的hidden的kvo來實現自動隱藏/顯示中間按鈕，但是在UINavigationController壓棧完成後才觸發kvo事件，導致進入下一層裏面才調用隱藏中間按鈕的方法；

2、接着就實現UINavigationControllerDelegate協議，爲UITabBarController的viewControllers註冊協議，在UINavigationController壓棧的時候觸發方法navigationController:willShowViewController:animated，判斷hidesBottomBarWhenPushed屬性設置如果設置爲YES，則直接隱藏中間按鈕；

3、結果是只要在UINavigationController壓棧的時候設置viewController的hidesBottomBarWhenPushed屬性爲YES，壓棧的時候就可以直接隱藏中間按鈕，出棧的時候觸發中間按鈕的顯示動畫。

在UITabBar中間添加按鈕的實現：https://github.com/boctor/idev-recipes

例子代碼下載地址：http://vdisk.weibo.com/s/s_B6F