我制作了一个 UIView 动画，这样当用户触摸切换按钮时它会缩小，当用户再次触摸该按钮时它会扩展回原来的大小。到目前为止一切正常。问题是动画需要一些时间 - 例如3秒。在那段时间里，我仍然希望用户能够与界面进行交互。因此，当用户在动画仍在进行时再次触摸按钮时，动画应该在原处停止并反转。

在 Apple Q&As 中我找到了一种立即暂停所有动画的方法:

https://developer.apple.com/library/ios/#qa/qa2009/qa1673.html

但我看不到从这里反转动画的方法(并省略初始动画的其余部分)。我该如何实现？

- (IBAction)toggleMeter:(id)sender {
    if (self.myView.hidden) {        
        self.myView.hidden = NO;
        [UIView animateWithDuration:3 animations:^{
            self.myView.transform = expandMatrix;
        } completion:nil];
    } else {
        [UIView animateWithDuration:3 animations:^{
            self.myView.transform = shrinkMatrix;
        } completion:^(BOOL finished) {
            self.myView.hidden = YES;
        }];
    }
}

最佳答案

除了下面的方法(我们从表示层获取当前状态，停止动画，从保存的表示层重置当前状态，并启动新动画)，还有一个更简单的解决方案。

如果执行基于 block 的动画，如果您想在 8.0 之前的 iOS 版本中停止动画并启动新动画，您可以简单地使用 UIViewAnimationOptionBeginFromCurrentState 选项。 (在 iOS 8 中有效，默认行为不仅是从当前状态开始，而且以反射(reflect)当前位置和当前速度的方式进行，因此基本上不必担心这个问题. 有关详细信息，请参阅 WWDC 2014 视频 Building Interruptible and Responsive Interactions。)

[UIView animateWithDuration:3.0
                      delay:0.0
                    options:UIViewAnimationOptionBeginFromCurrentState | UIViewAnimationOptionAllowUserInteraction
                 animations:^{
                     // specify the new `frame`, `transform`, etc. here
                 }
                 completion:NULL];

您可以通过停止当前动画并从当前动画停止的地方开始新动画来实现这一点。你可以用 Quartz 2D 来做到这一点:

1. Add QuartzCore.framework to your project如果你还没有。 (在现代版本的 Xcode 中，通常不需要显式执行此操作，因为它会自动链接到项目。)

2. 如果您还没有导入必要的 header (同样，现代版本的 Xcode 不需要):

   #import <QuartzCore/QuartzCore.h>
   

3. 让您的代码停止现有动画:

   [self.subview.layer removeAllAnimations];
   

4. 获取对当前表示层的引用(即此时的 View 状态):

   CALayer *currentLayer = self.subview.layer.presentationLayer;
   

5. 根据 presentationLayer 中的当前值重置 transform(或 frame 或其他):

   self.subview.layer.transform = currentLayer.transform;
   

6. 现在从 transform(或 frame 或其他)到新值的动画:

   [UIView animateWithDuration:1.0
                         delay:0.0
                       options:UIViewAnimationOptionAllowUserInteraction
                    animations:^{
                        self.subview.layer.transform = newTransform;
                    }
                    completion:NULL];
   

将所有这些放在一起，这是一个将我的变换比例从 2.0x 切换到识别和返回的例程:

- (IBAction)didTouchUpInsideAnimateButton:(id)sender
{
    CALayer *currentLayer = self.subview.layer.presentationLayer;

    [self.subview.layer removeAllAnimations];

    self.subview.layer.transform = currentLayer.transform;

    CATransform3D newTransform;

    self.large = !self.large;

    if (self.large)
        newTransform = CATransform3DMakeScale(2.0, 2.0, 1.0);
    else
        newTransform = CATransform3DIdentity;

    [UIView animateWithDuration:1.0
                          delay:0.0
                        options:UIViewAnimationOptionAllowUserInteraction
                     animations:^{
                         self.subview.layer.transform = newTransform;
                     }
                     completion:NULL];
}

或者，如果您想将 frame 大小从 100x100 切换到 200x200 并返回:

- (IBAction)didTouchUpInsideAnimateButton:(id)sender
{
    CALayer *currentLayer = self.subview.layer.presentationLayer;

    [self.subview.layer removeAllAnimations];

    CGRect newFrame = currentLayer.frame;

    self.subview.frame = currentLayer.frame;

    self.large = !self.large;

    if (self.large)
        newFrame.size = CGSizeMake(200.0, 200.0);
    else
        newFrame.size = CGSizeMake(100.0, 100.0);

    [UIView animateWithDuration:1.0
                          delay:0.0
                        options:UIViewAnimationOptionAllowUserInteraction
                     animations:^{
                         self.subview.frame = newFrame;
                     }
                     completion:NULL];
}

顺便说一句，虽然对于真正快速的动画来说通常并不重要，但对于像您这样的慢速动画，您可能希望将反转动画的持续时间设置为与您的进度相同当前动画(例如，如果你从 0.5 秒进入一个 3.0 秒的动画，当你反转时，你可能不想用 3.0 秒来反转你到目前为止已经完成的那一小部分动画，而只是 0.5秒)。因此，这可能看起来像:

- (IBAction)didTouchUpInsideAnimateButton:(id)sender
{
    CFTimeInterval duration = kAnimationDuration;             // default the duration to some constant
    CFTimeInterval currentMediaTime = CACurrentMediaTime();   // get the current media time
    static CFTimeInterval lastAnimationStart = 0.0;           // media time of last animation (zero the first time)

    // if we previously animated, then calculate how far along in the previous animation we were
    // and we'll use that for the duration of the reversing animation; if larger than
    // kAnimationDuration that means the prior animation was done, so we'll just use
    // kAnimationDuration for the length of this animation

    if (lastAnimationStart)
        duration = MIN(kAnimationDuration, (currentMediaTime - lastAnimationStart));

    // save our media time for future reference (i.e. future invocations of this routine)

    lastAnimationStart = currentMediaTime;

    // if you want the animations to stay relative the same speed if reversing an ongoing
    // reversal, you can backdate the lastAnimationStart to what the lastAnimationStart
    // would have been if it was a full animation; if you don't do this, if you repeatedly
    // reverse a reversal that is still in progress, they'll incrementally speed up.

    if (duration < kAnimationDuration)
        lastAnimationStart -= (kAnimationDuration - duration);

    // grab the state of the layer as it is right now

    CALayer *currentLayer = self.subview.layer.presentationLayer;

    // cancel any animations in progress

    [self.subview.layer removeAllAnimations];

    // set the transform to be as it is now, possibly in the middle of an animation

    self.subview.layer.transform = currentLayer.transform;

    // toggle our flag as to whether we're looking at large view or not

    self.large = !self.large;

    // set the transform based upon the state of the `large` boolean

    CATransform3D newTransform;

    if (self.large)
        newTransform = CATransform3DMakeScale(2.0, 2.0, 1.0);
    else
        newTransform = CATransform3DIdentity;

    // now animate to our new setting

    [UIView animateWithDuration:duration
                          delay:0.0
                        options:UIViewAnimationOptionAllowUserInteraction
                     animations:^{
                         self.subview.layer.transform = newTransform;
                     }
                     completion:NULL];
}

关于ios - 如何停止和反转 UIView 动画？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/16303010/