ios - UIViewAnimationOptionBeginFromCurrentState 基本动画的意外行为

Question

我正在尝试在收到按钮点击时执行这个基本的 UIView 动画:

- (IBAction)buttonPress:(id)sender
{
    self.sampleView.alpha = 0.0;
    [UIView animateWithDuration:2.0
                          delay:0.0
                        options:UIViewAnimationOptionCurveEaseInOut | UIViewAnimationOptionBeginFromCurrentState
                     animations:^{self.sampleView.alpha = 1.0;}
                     completion:NULL];
}

在单击按钮之前， View 是可见的并且具有 1.0 的 alpha 值。当我发送按钮操作时，我希望 View 在 2 秒内从 alpha=0.0 淡入到 alpha=1.0，但这并没有发生。当我删除 UIViewAnimationOptionBeginFromCurrentState 时，动画效果很好。

似乎设置了 UIViewAnimationOptionBeginFromCurrentState 选项，未设置 alpha=0.0 并且动画被跳过，因为它认为它已经是 1.0。

我试图理解为什么会发生这种情况，因为 Apple 文档指出如果另一个动画未在进行中，UIViewAnimationOptionBeginFromCurrentState 无效:

"UIViewAnimationOptionBeginFromCurrentState从与已经在飞行中的动画关联的当前设置开始动画。如果此键不存在，则允许在新动画开始之前完成任何飞行中的动画。如果另一个动画不在飞行中，则此键无效。”

Answer 1

事实证明，使用 UIViewAnimationOptionBeginFromCurrentState 并不总能按预期工作。

看这个例子:

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];

    UIButton *theButton = [UIButton new];
    [self.view addSubview:theButton];
    theButton.frame = self.view.frame;
    theButton.backgroundColor = [UIColor redColor];
    theButton.alpha = 0.05;
    [theButton addTarget:self action:@selector(actionPressed:) forControlEvents:UIControlEventTouchUpInside];
}

- (void)actionPressed:(UIButton *)theButton
{
    theButton.alpha = 0.6;
    [UIView animateWithDuration:5
                          delay:0
                        options:UIViewAnimationOptionBeginFromCurrentState
                     animations:^
     {
         // you expect to animate from 0.6 to 1. but it animates from 0.05 to 1
         theButton.alpha = 1;
     }
                     completion:nil];
}

在上面的示例中，您希望 .alpha 从 0.6 到 1 进行动画处理。但是，它从 0.05 到 1 进行动画处理。

为了解决这个问题，您应该将 actionPressed: 更改为以下内容:

- (void)actionPressed:(UIButton *)theButton
{
    [UIView animateWithDuration:0
                     animations:^
     {
         // set new values INSIDE of this block, so that changes are
         // captured in UIViewAnimationOptionBeginFromCurrentState.
         theButton.alpha = 0.6;
     }
                     completion:^(BOOL finished)
     {
         [UIView animateWithDuration:5
                               delay:0
                             options:UIViewAnimationOptionBeginFromCurrentState
                          animations:^
          {
              // now it really animates from 0.6 to 1
              theButton.alpha = 1;
          }
                          completion:nil];
     }];
}

提及 animateWithDuration:0!!!

规则很简单:仅在其他动画 block 之后使用带有 UIViewAnimationOptionBeginFromCurrentState 的动画 block ，以便实际应用您之前的所有更改。

如果您不知道 animateWithDuration:0 block 中到底应该包含什么，那么您可以使用这个技巧:

- (void)actionPressed:(UIButton *)theButton
{
    // make all your changes outside of the animation block
    theButton.alpha = 0.6; 

    // create a fake view and add some animation to it.
    UIView *theFakeView = [UIView new];
    theFakeView.alpha = 1;
    [UIView animateWithDuration:0
                     animations:^
     {
         // we need this line so that all previous changes are ACTUALLY applied
         theFakeView.alpha = 0;
     }
                     completion:^(BOOL finished)
     {
         [UIView animateWithDuration:5
                               delay:0
                             options:UIViewAnimationOptionBeginFromCurrentState
                          animations:^
          {
              // now it really animates from 0.6 to 1
              theButton.alpha = 1;
          }
                          completion:nil];
     }];
}

---

如果你不想记住这个错误的所有细节，那么只需申请 Method Swizzling到 UIView 类。

重要编辑:事实证明，如果调用 performBatchUpdates:completion: 的 ，下面的代码将导致运行时崩溃>UICollectionView 实例。所以，我不建议在这种情况下使用method swizzling!

您的代码可能如下所示:

+ (void)load
{
    static dispatch_once_t theOnceToken;
    dispatch_once(&theOnceToken, ^
                  {
                      Class theClass = object_getClass(self);
                      SEL theOriginalSelector = @selector(animateWithDuration:delay:options:animations:completion:);
                      SEL theSwizzledSelector = @selector(swizzled_animateWithDuration:delay:options:animations:completion:);
                      Method theOriginalMethod = class_getClassMethod(theClass, theOriginalSelector);
                      Method theSwizzledMethod = class_getClassMethod(theClass, theSwizzledSelector);

                      if (!theClass ||!theOriginalSelector || !theSwizzledSelector || !theOriginalMethod || !theSwizzledMethod)
                      {
                          abort();
                      }

                      BOOL didAddMethod = class_addMethod(theClass,
                                                          theOriginalSelector,
                                                          method_getImplementation(theSwizzledMethod),
                                                          method_getTypeEncoding(theSwizzledMethod));

                      if (didAddMethod)
                      {
                          class_replaceMethod(theClass,
                                              theSwizzledSelector,
                                              method_getImplementation(theOriginalMethod),
                                              method_getTypeEncoding(theOriginalMethod));
                      }
                      else
                      {
                          method_exchangeImplementations(theOriginalMethod, theSwizzledMethod);
                      }
                  });
}

+ (void)swizzled_animateWithDuration:(NSTimeInterval)duration
                               delay:(NSTimeInterval)delay
                             options:(UIViewAnimationOptions)options
                          animations:(void (^)(void))animations
                          completion:(void (^)(BOOL))completion
{
    if (options & UIViewAnimationOptionBeginFromCurrentState)
    {
        UIView *theView = [UIView new];
        theView.alpha = 1;
        [UIView animateWithDuration:0
                         animations:^
         {
             theView.alpha = 0;
         }
                         completion:^(BOOL finished)
         {
             [self swizzled_animateWithDuration:duration
                                          delay:delay
                                        options:options
                                     animations:animations
                                     completion:completion];
         }];
    }
    else
    {
        [self swizzled_animateWithDuration:duration
                                     delay:delay
                                   options:options
                                animations:animations
                                completion:completion];
    }
}

如果您将此代码添加到您的自定义 UIView 类别，那么此代码现在可以正常工作:

- (void)actionPressed:(UIButton *)theButton
{
    theButton.alpha = 0.6;
    [UIView animateWithDuration:5
                          delay:0
                        options:UIViewAnimationOptionBeginFromCurrentState
                     animations:^
     {
         // you expect to animate from 0.6 to 1.
         // it will do so ONLY if you add the above code sample to your project.
         theButton.alpha = 1;
     }
                     completion:nil];
}

就我而言，这个错误完全毁了我的动画。见下文:

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