How can I reset the motion effect of a UIView correctly?

Question

I have extended the UIView class in order to implement my own quick method for adding a motion effect in a UIView:

#import "UIView+Extensions.h"

@implementation UIView (Extensions)

// ...

- (void)setMotion:(CGPoint)motion
{
    // Remove all motion effects.
    if (self.motionEffects.count > 0) {

        [self removeMotionEffect:[self.motionEffects objectAtIndex:0]];
        [self removeMotionEffect:[self.motionEffects objectAtIndex:1]];
    }

    // Add motion effects.
    UIInterpolatingMotionEffect *horizontalMotionEffect = [[UIInterpolatingMotionEffect alloc] initWithKeyPath:@"center.x" type:UIInterpolatingMotionEffectTypeTiltAlongHorizontalAxis];
    UIInterpolatingMotionEffect *verticalMotionEffect = [[UIInterpolatingMotionEffect alloc] initWithKeyPath:@"center.y" type:UIInterpolatingMotionEffectTypeTiltAlongVerticalAxis];

    [self addMotionEffect:horizontalMotionEffect];
    [self addMotionEffect:verticalMotionEffect];

    // Set motion effect values.
    horizontalMotionEffect.minimumRelativeValue = @(-motion.x);
    horizontalMotionEffect.maximumRelativeValue = @(motion.x);

    verticalMotionEffect.minimumRelativeValue = @(-motion.y);
    verticalMotionEffect.maximumRelativeValue = @(motion.y);
}

I'm getting a counter-intuitive error.

* Terminating app due to uncaught exception 'NSRangeException', reason: '* -[__NSArrayI objectAtIndex:]: index 1 beyond bounds [0 .. 0]' *** First throw call stack: (0x28d3749f 0x3652ec8b 0x28c4bc35 0x7ab1f 0x79e95 0x7d983 0x7a39f 0x6969f 0x7ddf3 0x2c1f9d0f 0x2c1f9a7d 0x2c1ff953 0x2c1fd3bd 0x2c26760d 0x2c45951b 0x2c45b98b 0x2c466209 0x2c45a217 0x2f4c80d1 0x28cfdd7d 0x28cfd041 0x28cfbb7b 0x28c493c1 0x28c491d3 0x2c25e1bf 0x2c258fa1 0x7f425 0x36aaeaaf) libc++abi.dylib: terminating with uncaught exception of type NSException

Which means that somehow this line of code:

[self removeMotionEffect:[self.motionEffects objectAtIndex:0]];

Removes both effects instead of just the first one.

How is that even possible?

And this is not the worst part....

As you might suggest, changing the code to this:

// Remove all motion effects.
if (self.motionEffects.count > 0) {

    [self removeMotionEffect:[self.motionEffects objectAtIndex:0]]; // Should remove the first, or both but removes the second one.
}

Should fix the problem since the line of code above will remove both. IT DOES NOT. It actually removes the vertical effect, the second one! So, each time you reset the motion the horizontal axis adds-up and goes faster and faster.

This is beyond reasoning.

Answer 1

After the first

[self removeMotionEffect:[self.motionEffects objectAtIndex:0]];

self.motionEffects will contain only 1 object, so you can't use objectAtIndex:1 because you'll get the out of bounds exception...

Use

[self.motionEffects firstObject]

or some loops to do it...

Answer 2

Actually when the object at index zero is removed, the index of other items in the array is changed. For example, index of item at index 1 will be changed to 0 after removing the object at index 0. You can make this code work just by swapping the two lines in the condition if (self.motionEffects.count > 0)

Answer 3

Swift 4

extension UIView {
    func stopParallax() {
        for motion in motionEffects {
            removeMotionEffect(motion)
        }
    }
}