How would you animate a view that has a constraint?

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Main Source: 🔗 Ace the iOS Interview

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TL/DR

With AutoLayout, you can simply change a view’s constraints and let the system figure out how to resize that view and all of its neighbors.

In interviews, I often see candidates mix AutoLayout constraint changes with manual changes to a view’s frame. This combination of two different layout paradigms really complicates the logic and is often error-prone. I’d recommend you stick with AutoLayout whenever possible.

Now, assuming you have a reference to the constraint you want to manipulate, you can simply animate a change by updating the constraint’s value like so:

imageViewHeightConstraint.constant = 80

UIView.animate(withDuration: 0.5) {
self.view.layoutIfNeeded()
}

Make sure you update the constraint outside of the animation block!

Remember to call layoutIfNeeded() on self.view and not on the view you are attempting to animate. Otherwise, the changes in layout will be applied without animation.

Additionally, Apple recommends calling layoutIfNeeded() once before the animation block to ensure all pending layout operations are completed.

It’s very important that we call layoutIfNeeded() on self.view and not just on the view we are trying to animate. By calling this function on self.view,the animation and layout changes will “trickle down” through all of the other subviews.

Remember, you want to animate changes to the neighboring view’s layouts as well - not just changes to any one particular subview.