How to position a child view relative to containin

Question

I want to be able to position my child view 25% the size of the super view from the top.

NSLayoutConstraint *topPositionConstraint = [NSLayoutConstraint  constraintWithItem:_containerView 
                                                                          attribute:NSLayoutAttributeTop 
                                                                          relatedBy:NSLayoutRelationEqual 
                                                                             toItem:_childView 
                                                                          attribute:NSLayoutAttributeHeight 
                                                                         multiplier:0.25f 
                                                                           constant:0.0f];

However, right now I'm getting the following exception:

'NSInvalidArgumentException', reason: '*** +[NSLayoutConstraint constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:]: Invalid pairing of layout attributes'

Why does the error occur and how can I achieve what I want?

Answer 1

You can't use top and height in the same constraint. Although it makes sense to say it the system doesn't like it.

What you could do instead is something like...

NSLayoutConstraint *topPositionConstraint =
    [NSLayoutConstraint  constraintWithItem:_childView
                                  attribute:NSLayoutAttributeTop 
                                  relatedBy:NSLayoutRelationEqual 
                                     toItem:_containerView
                                  attribute:NSLayoutAttributeTop 
                                 multiplier:1.0 
                                   constant:_containerView.frame.size.height * 0.25];

Answer 2

Instead of using the frame as in the accepted answer, you can move the percentage to the multiplier if you use the bottom instead of height. I'm using this technique for percentage based positioning of child views. It is also nice because the frame of your container view may not be set when you're creating the constraint:

NSLayoutConstraint *topPositionConstraint =
    [NSLayoutConstraint  constraintWithItem:_childView
                              attribute:NSLayoutAttributeTop 
                              relatedBy:NSLayoutRelationEqual 
                                 toItem:_containerView
                              attribute:NSLayoutAttributeBottom 
                             multiplier:0.25 
                               constant:0];