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 <XXXViewController: 0x7fcea3730650>.
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];
}