Animate to text height (autolayout) - ios

Question

I want to animate from a height of 0 to the height of the text (UILabel). I am using autolayout and I do not know how high the text will be. My approach was to start by setting a height=0 constraint to the text, and animate like this:
//retrieves the height constrain of the clicked item
NSLayoutConstraint *heightContraint = [heightConstraints objectAtIndex:sender.tag];
//activates/deactivates the constraint
heightContraint.active = !heightContraint.active;
//animates
[UIView animateWithDuration:3 animations:^{
[self layoutIfNeeded];
}];
My problem is, that with this approach, the text height does not animate, it changes from a height of 0 to the new height instantly. Only the position/size change of the containing views is animated. How I animate the text height change without knowing the height of the text??

Answer 1

I have found a solution. Instead of setting a constraint to the height, I set a containing view (clipping), which contains the UILabel and the view above. Then, I create 2 constraints that pin the bottom edges to the containing view, one related to the UILabel, and one related to the view above. I just activate/deactivate those constraints like this:
//get the constraints
NSLayoutConstraint *viewAboveTextConstraint =
[viewAboveTextConstraints objectAtIndex:sender.tag];
NSLayoutConstraint *uilabelContraint = [uilabelContraints objectAtIndex:sender.tag];
//flip the active states
viewAboveTextConstraint.active = !viewAboveTextConstraint.active;
uilabelContraint.active = !uilabelContraint.active;
//animate
[UIView animateWithDuration:.3 animations:^{
[self layoutIfNeeded];
}];