自定义TabBarController Push下一级Controller时 会报这样的错误：Unbalanced calls to begin/end appearance transitions for <UIVIewController>.

网上的一些回答，都说是动画引起的，解决方法就是，加一个BOOL型的变量，检查是否在做动画。

    if (transiting) {
        return;
    }
    transiting = YES;
    [self transitionFromViewController:_currentVC toViewController:newVC duration:0.5 options:UIViewAnimationOptionTransitionCrossDissolve animations:^{
        
    } completion:^(BOOL finished) {
        
        transiting = NO;
    }];

这样就不会出现刚才说的那个bug了。 

但是，这并没有解决我的问题！

所以真正的答案是

自定义了TabBarController 之后必须实现以下

-(void)viewWillAppear:(BOOL)animated
{
    [self.selectedViewController beginAppearanceTransition: YES animated: animated];
}

-(void) viewDidAppear:(BOOL)animated
{
    [self.selectedViewController endAppearanceTransition];
}

-(void) viewWillDisappear:(BOOL)animated
{
    [self.selectedViewController beginAppearanceTransition: NO animated: animated];
}

-(void) viewDidDisappear:(BOOL)animated
{
    [self.selectedViewController endAppearanceTransition];
}