Question

我正在尝试在收到按钮点击时执行此基本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];
}

在按钮之前，单击视图是可见的，其alpha值为1.0。当我发送按钮动作时，我希望视图在2秒内从alpha = 0.0淡入alpha = 1.0，但这不会发生。当我删除UIViewAnimationOptionBeginFromCurrentState动画工作正常。

似乎设置了UIViewAnimationOptionBeginFromCurrentState选项，没有设置alpha = 0.0并且跳过动画，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没有进行，UIViewAnimationOptionBeginFromCurrentState没有效果：

“UIViewAnimationOptionBeginFromCurrentState 从与已在飞行中的动画相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正在进行的动画。如果另一个动画没有在飞行中，则此键无效。“

Answer 1

事实证明，使用UIViewAnimationOptionBeginFromCurrentState并不总是按预期工作。

看看这个例子：

- (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的动画效果为0.6到1.但是，动画从0.05到1。

要解决此问题，您应将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：只使用带有UIViewAnimationOptionBeginFromCurrentState动画块的其他动画块，以便实际应用之前的所有更改。

如果您不知道animateWithDuration:0块中应该包含哪些内容，那么您可以使用此技巧：

- (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];
     }];
}

如果您不想记住此错误的所有详细信息，请将Method Swizzling应用于UIView类。

重要编辑：如果调用performBatchUpdates:completion:实例UICollectionView，则以下代码会导致运行时崩溃。因此，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];
}

在我的情况下，这个错误完全破坏了我的动画。见下文：

[ Demo CountPages alpha ] []

Answer 2

为了查看正在发生的事情，动画帧位置而不是alpha是有帮助的。如果您这样做，请尝试每隔一秒左右点按一下按钮，并设置UIViewAnimationOptionBeginFromCurrentState，您就会看到一些动作。

当您将alpha设置为零时，在下一次运行循环之前，这不会更新显示。由于你之后立即开始动画，显示器没有机会更新，当动画框架开始时，它会查看当前显示的alpha（1.0）并查看它必须到达的位置（alpha为1.0）和两者之间的插值没有任何作用。

如果您需要保留UIViewAnimationOptionBeginFromCurrentState选项，因为当此视图上的动画正在进行时可能会点按此按钮，您可以执行以下操作：

- (IBAction)buttonPressed:(id)sender
{
  self.sampleView.alpha = 0.0;

  [self performSelector:@selector(animateAlpha) withObject:nil afterDelay:0.1];
}

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

Answer 3

使用UIViewAnimationOptionBeginFromCurrentState代替performSelector:时，我想在调用UIView之前使用animateWithDuration:＆＃39; s performWithoutAnimation设置初始值。< / p>

UIViewAnimationOptionBeginFromCurrentState具有基本动画的意外行为

3 个答案: