UIView animateWithDuration线性吗？

Question

I want to animate my multiple UIImageViews to move from point A to point B linearly.

i'm using options:UIViewAnimationOptionCurveLinear - Apple docs says: " A linear animation curve causes an animation to occur evenly over its duration. ".

Here's the code that i'm using:

[UIView animateWithDuration:1.5
                              delay:0.0
                            options:UIViewAnimationOptionCurveLinear
                         animations:^{

                             //start animation of random grasses with const speed and const y
                             for (int i=0;i<45;i++){
                                 if ([self.view viewWithTag:i+100]){

                                     CGRect frameOfGrass = [self.view viewWithTag:i+100].frame;
                                     frameOfGrass.origin.y = -100;
                                     [self.view viewWithTag:i+100].frame = frameOfGrass;
                                 }}
        }
                         completion:^(BOOL finished){
                           //
  }];

NOTE : Y position of every imageView is random number from 600-700.

But the result looks more like UIViewAnimationOptionCurveEaseOut - "An ease-out curve causes the animation to begin quickly, and then slow as it completes." Because all the images slows down at the and.

Here's screenshots of app running:

Any idea why this is happening?

Answer 1

The travel distance is not the same for all the grass images. Remember v=d/t. In your case, all the grass images will travel at different speeds because they need to reach y.origin = -100 at the same time.

Try this:

frameOfGrass.origin.y = frameOfGrass.origin.y - 600;

This should make all the grass images travel the same distance over the same time.