Trailing constraint not applying when programmatically adding UIConstraints

Question

I'm trying to add a subview to a view with constraints through code. I had some success but the trailing constraint seems to be completely ignored for whatever reason.

My code:

leading_const = 16.f;
trailing_const = 16.f;
top_const = 12.f;
bottom_const = 12.f;


insertView.translatesAutoresizingMaskIntoConstraints = NO;
[view addSubview:insertView];
NSLayoutConstraint *leading = [NSLayoutConstraint constraintWithItem:insertView
                                                           attribute:NSLayoutAttributeLeading
                                                           relatedBy:NSLayoutRelationEqual
                                                              toItem:view
                                                           attribute:NSLayoutAttributeLeading
                                                          multiplier:1.f
                                                            constant:leading_const];
NSLayoutConstraint *trailing = [NSLayoutConstraint constraintWithItem:insertView
                                                            attribute:NSLayoutAttributeTrailing
                                                            relatedBy:NSLayoutRelationEqual
                                                               toItem:view
                                                            attribute:NSLayoutAttributeTrailing
                                                           multiplier:1.f
                                                             constant:trailing_const];
NSLayoutConstraint *top = [NSLayoutConstraint constraintWithItem:insertView
                                                       attribute:NSLayoutAttributeTop
                                                       relatedBy:NSLayoutRelationEqual
                                                          toItem:view
                                                       attribute:NSLayoutAttributeTop
                                                      multiplier:1.f
                                                        constant:top_const];
NSLayoutConstraint *height = [NSLayoutConstraint constraintWithItem:insertView
                                                                   attribute:NSLayoutAttributeHeight
                                                                   relatedBy:NSLayoutRelationEqual
                                                                      toItem:nil
                                                                   attribute:NSLayoutAttributeHeight
                                                                  multiplier:1.f
                                                                    constant:130.f];
[superView addConstraints:@[leading, trailing, top, height]];

Result:

Appreciate any guidance!

You can call NSLayoutConstraint.activate(@[leading, trailing, top, height]) — Piotr Bernad, Feb 03 '17 at 10:57
trailing constraint should be negative value, then only it works.Try it — Pavankumar, Feb 03 '17 at 11:27
@Pavankumar that was it!! Interesting why it's a negative value in code but it would be a positive value in the interface builder? — Kevin, Feb 06 '17 at 01:43

score 12 · Accepted Answer · edited Sep 25 '18 at 14:26

12

Your constraint is being applied but as you have set it to 16 it is going 16 points past the trailing edge of the view. You therefore should use a negative value for the constant instead.

edited Sep 25 '18 at 14:26

Simple Maurya

105
6

answered Feb 03 '17 at 11:49

Hodson

3,438
1
23
51

This solved it! Interesting why it's a negative value in code but it would be a positive value in the interface builder? – Kevin Feb 06 '17 at 01:43
3

It depends on how you are relating the views with the constraint. In your case you was relating the trailing edge of `insertView` to the trailing edge of `view` so therefore you have to bring it 16 points back from the edge of `view`. If you was to relate your view the other way, `view` --> `insertView` then you would use a positive number to move it 16 points forward from the trailing edge of the `insertView`. I hope that makes sense. – Hodson Feb 06 '17 at 08:37
Thanks for the explanation! Makes total sense. – Kevin Feb 06 '17 at 10:12
@Hodson Thanks for the explanation. It solved my issue. – Ramakrishna Sep 05 '17 at 05:09

Trailing constraint not applying when programmatically adding UIConstraints

1 Answers1