iOS BUG: Unbalanced calls to begin/end appearance transitions for <XXXViewController: 0x7fcea3730650>.

自定義TabBarController Push下一級Controller時 會報這樣的錯誤：Unbalanced calls to begin/end appearance transitions for <XXXViewController: 0x7fcea3730650>.

網上的一些回答，都說是動畫引起的，解決方法就是，加一個BOOL型的變數，檢查是否在做動畫。

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

這樣就不會出現剛纔說的那個bug了。 

但是，這並沒有解決我的問題！

所以真正的答案是

自定義了TabBarController 之後必須實現以下

-(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];
}