UIViewAnimationOptionBeginFromCurrentState unexpected behaviour with basic animation

cocoa-touch, ios, objective-c, uiviewanimation

Solution

It turns out that using `UIViewAnimationOptionBeginFromCurrentState` doesn't always work as expected.

Look at this example:

- (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];
}

In the example above you expect that `.alpha` would animate from 0.6 to 1. However, it animates from 0.05 to 1.

In order to resolve the issue, you should change `actionPressed:` to the following:

- (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];
     }];
}

Mention the `animateWithDuration:0`!!!

The rule is easy: Only use an animation block with `UIViewAnimationOptionBeginFromCurrentState` AFTER some other animation block, so that all your previous changes are actually applied.

In case you don't know what exactly should be included in `animateWithDuration:0` block, then you can use this trick:

- (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];
     }];
}

If you don't want to remember about all details of this bug, then just apply Method Swizzling to UIView class.

IMPORTANT EDIT: It turns out that the code below will cause a crash at runtime, if calling `performBatchUpdates:completion:` of `UICollectionView` instance. So, I do not recommend using `method swizzling` in this case!

Your code may look like this:

+ (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];
    }
}

If you add this code to your custom UIView category, then this code would work fine now:

- (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];
}

In my case, this bug has completely ruined my animations. See below:

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Problem

Original source