Question

我希望在UIView动画中获得scrollView的contentOffset和contentInset，如下所示：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    [self.window makeKeyAndVisible];

    UIScrollView *scrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(100, 100, 120, 100)];
    scrollView.backgroundColor = [UIColor grayColor];
    scrollView.contentSize = scrollView.frame.size;

    UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 120, 30)];
    label.textAlignment = NSTextAlignmentCenter;
    label.text = @"hello world";

    [scrollView addSubview:label];

    UIViewController *vc = [[UIViewController alloc] init];
    [vc.view addSubview:scrollView];

    [scrollView addObserver:self forKeyPath:@"contentOffset" options:NSKeyValueObservingOptionNew context:NULL];
    [scrollView addObserver:self forKeyPath:@"contentInset" options:NSKeyValueObservingOptionNew context:NULL];

    [UIView animateWithDuration:2 delay:0 options:UIViewAnimationOptionAllowUserInteraction animations:^{
        scrollView.contentInset = UIEdgeInsetsMake(50, 0, 0, 0);
        scrollView.contentOffset = CGPointMake(0, -50);
    } completion:nil];

    self.window.rootViewController = vc;

    return YES;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if ([keyPath isEqualToString:@"contentOffset"]) {
        NSLog(@"offset:%@", [change valueForKey:NSKeyValueChangeNewKey]);
    }
    if ([keyPath isEqualToString:@"contentInset"]) {
        NSLog(@"inset:%@", [change valueForKey:NSKeyValueChangeNewKey]);
    }
}

遗憾的是，动画时没有输出，我想念一些东西或KVO在做UIView动画时不起作用吗？

Answer 1

你的结论是正确的，KVO在UIView动画期间不起作用。

这是因为你的scrollview的实际属性在动画过程中没有改变：Core Animation只是动画一个位图，它代表从开始状态到结束状态的scrollview。它不会更新底层对象的属性，因此当动画在飞行中时没有KVO状态发生变化。

不幸的是，如果您尝试通过UIScrollViewDelegate协议方法观察contentOffset和inset，原因相同。

Apple的指南here中可以找到更深入（且相当难以理解）的解释。

在UIView动画期间无法使用KVO获取新值

1 个答案: