transitionFromViewController 返回NO

- (void)transitionFromViewController:(UIViewController *)fromViewController toViewController:(UIViewController *)toViewController duration:(NSTimeInterval)duration options:(UIViewAnimationOptions)options animations:(void (^ __nullable)(void))animations completion:(void (^ __nullable)(BOOL finished))completion NS_AVAILABLE_IOS(5_0);

fromViewController：当前显示的子试图控制器，将被替换为非显示状态 

toViewController：将要显示的子视图控制器 

duration：交换动画持续的时间，单位秒 

options：动画的方式 

animations：动画Block 

completion：完成后执行的Block

- (void)viewCotrollerChange:(UISegmentedControl *)sender {
    if (sender.selectedSegmentIndex == _selectedIndex) {
        return;
    }
    _selectedIndex = sender.selectedSegmentIndex;
    if (sender.selectedSegmentIndex == 0) {
        [self changeToNormalImageView];
    }else if (sender.selectedSegmentIndex == 1) {
        [self changeToVRImageView];
    }else if (sender.selectedSegmentIndex == 2) {
        [self changeToVideView];
    }
}


- (void)changeToNormalImageView {
    [self addChildViewController:self.normalImageVC];
    UIViewController *oldViewController = _currentViewController;
    [self transitionFromViewController:_currentViewController toViewController:self.normalImageVC duration:0.5 options:UIViewAnimationOptionTransitionNone animations:^{
    } completion:^(BOOL finished) {
        if (finished) {
            [_normalImageVC didMoveToParentViewController:self];
            [_currentViewController willMoveToParentViewController:nil];
            [_currentViewController removeFromParentViewController];
            _currentViewController = self.normalImageVC;
        }else {
            _currentViewController = oldViewController;
        }
    }];
}

在写的过程中出现过一个很奇怪的问题，切换到normalImageView的时候，transitionFromViewController的completion的回调 finished一直为NO；导致子视图控制器的切换失败。

经过多次调试后发现，normalImageVC的viewWillAppear里有个刷新，每次切换到该子视图控制器的时候都会刷新，导致切换失败。增加duration的动画时间效果不明显，因为用户可以疯狂的点击Segment进行切换。 最终通过修改normalImageVC的viewWillAppear方法，不让他刷新，解决了这个问题，这里做一个简单的记录。