Q:

Animate a view containing subviews with autolayout

This is the setup:

A UIView created on Interface Builder, linked to an IBOutlet variable (_vAbout)
A constraint for this view that we want to animate, linked to an IBOutlet variable (_ctrBottomAboutView)

I am using this code to animate:
_ctrBottomAboutView.constant = -100;
[UIView animateWithDuration:0.5 animations:^{
    [_vAbout layoutIfNeeded];
}

My problem is: whenever the view has any subviews in it, the animation doesn't work. However, if the view has no children, the animation works correctly.
Do you have any idea of a solution? I have tried everything: adding and removing constraints instead of modificating the constant value, adding constraints to the subviews on Interface Builder...

A:

After some experiments starting from the ground with an empty project, this is what I've found:
Given A the view we want to animate and B its superview

It's very important to keep in mind that the view that receives the layoutIfNeeded message is the view that owns the constraint. 
In the case of NSLayoutAttributeWidth and NSLayoutAttributeHeight the owner of the constraint is actually A, but in all the other cases, the view that owns the constraint is B

HOWEVER

If A does not have any subviews, we can call [A layoutIfNeeded] at any time on our code and it will work
If A has one or more subviews but we start the animation on viewDidLoad we can call [A layoutIfNeeded] and it will work

