How do I set width of custom subview to be 1/3rd of parent UIView which does not have width constraint?

Question

I have a custom parent UIView (Say PView) which is defined by Left,Right,Top and Bottom constraints. Now I want to add a custom subview (say SView) to the PView with width equal to MIN of 200 and 1/3rd of PView.

I have the updateConstraints method in my PView class where I update the location constraints for SView. However, I am not sure how can I set its size, because I don't have the widthConstraint of PView?

Answer 1

You can easily achieve this by proportional width feature in storyboard with Autolayout.

Give you PView a proportional width to a ratio of 1:3 to your superView(SView). Parent View Constraint

Child/SubView View Constraint

How to Give Proportional Constraint? You set ParentView and ChildView with equal width then double click on width constraint and set a ratio of 1:3. Width will always be 1/3 of parent view

Coding Approach

[self.yourview addConstraint:[NSLayoutConstraint
                              constraintWithItem:self.yourview
                              attribute:NSLayoutAttributeHeight
                              relatedBy:NSLayoutRelationEqual
                              toItem:self.yourview
                              attribute:NSLayoutAttributeWidth
                              multiplier:(self.yourview.frame.size.height / self.yourview.frame.size.width)
                              constant:0]];

Answer 2

Your parent view's constraint should be like : top,leading,trailing,bottom

Your subview's constraints should be like : top,leading,fixed height

Now select both view and set equal width constraint and then select that constraint and from attribute inspector change it's multiplier to 0.333.

That's it!