如何在 iOS8 的自定义过渡动画中“传递”持久标签？答案

【问题标题】：How can I "pass" a persistent label in a custom transition animation in iOS8?如何在 iOS8 的自定义过渡动画中“传递”持久标签？
【发布时间】：2015-03-02 23:03:04
【问题描述】：

我在视图控制器之间有一个自定义过渡动画，我希望 UILabel 在 fromViewController 和 toViewController 上都（或出现）相同。

我尝试了以下方法：

toViewController.nameLabel = fromViewController.nameLabel;

在以下代码的上下文中，但得到以下错误：

[UINavigationController nameLabel]：无法识别的选择器发送到实例

我做错了什么？

- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext {
    // Grab the from and to view controllers from the context
    HC_ExercisePageVC *fromViewController = (HC_ExercisePageVC *)[transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    HC_TimerVC *toViewController = (HC_TimerVC *)[transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];

    if (self.presenting) {

        fromViewController.view.userInteractionEnabled = NO;

        [transitionContext.containerView addSubview:toViewController.view];

        CGRect startFrame = endFrame;
        startFrame.origin.y += 75;
        toViewController.movingViews.frame = startFrame;

        toViewController.nameLabel = fromViewController.nameLabel;

        [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^{
            toViewController.movingViews.frame = endFrame;
        } completion:^(BOOL finished) {
            [transitionContext completeTransition:YES];
        }];
    }
    else {

更新：根据@Gavin 的建议，我将代码替换为：

// Grab the from and to view controllers from the context
HC_ExercisePageVC *fromViewController = (HC_ExercisePageVC *)[transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
UINavigationController *toViewControllerNavigation = (id)[transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
HC_TimerVC * toViewController = (HC_TimerVC *)toViewControllerNavigation.viewControllers.firstObject;

但是当我这样做时，我得到了错误：

-[HC_TimerVC viewControllers]：无法识别的选择器发送到实例

我总是纠结于如何处理导航控制器...

【问题讨论】：

标签： uiviewcontroller ios8 transitions

【解决方案1】：

看起来您的 toViewController 包含在 UINavigationController 中，因此它会将其作为目标返回。

所以你需要从导航控制器中获取HC_TimerVC：

- (void)animateTransition:(id <UIViewControllerContextTransitioning>)transitionContext {
    // Grab the from and to view controllers from the context
    HC_ExercisePageVC *fromViewController = (HC_ExercisePageVC *)[transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UINavigationController *toViewControllerNavigation = (id)[transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    HC_TimerVC * toViewController = toViewControllerNavigation.viewControllers.firstObject;
    ....

【讨论】：