Question

I have two labels. I want to be able to move both if one is moved. How do I “attach” them together with NSLayoutConstraints? I can do this in IB, but need to do it in code.

also, what are NSLayoutAttributeBaseline, NSLayoutAttributeLeading, and NSLayoutAttributeTrailing?

EDIT:

centering poweredByLabel (aka label02):

[constraints addObject:[NSLayoutConstraint constraintWithItem:poweredByLabel
                                                    attribute:NSLayoutAttributeCenterX
                                                    relatedBy:NSLayoutRelationEqual
                                                       toItem:myImage
                                                    attribute:NSLayoutAttributeCenterX
                                                   multiplier:1.0
                                                     constant:0]];

stack the labels and switch vertically:

[constraints addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[rememberPasswordSwitch]-10-[rememberPasswordLabel]-10-[versionLabel]-[poweredByLabel]-|"
                                                                         options:NSLayoutFormatAlignAllBaseline
                                                                         metrics:nil
                                                                           views:viewsDictionary]];

which produces the error:

* Terminating app due to uncaught exception ‘NSInvalidArgumentException’, reason: ‘Unable to parse constraint
format: Options mask required views to be aligned on a vertical edge,
which is not allowed for layout that is also vertical.
V:[rememberPasswordSwitch]-10-[rememberPasswordLabel]-10-[versionLabel]-[poweredByLabel]-|……………………………………………………………………………………………..^’

w/out the NSLayoutFormatAlignAllBaseline option, it runs fine (they stack but are not all centered horizontally).

Answer 1

If you need to do this in code, first create the NSLayoutConstraint(s), then add the constraint(s) to the labels’ superview.

There are two ways to create constraints in code. constraintsWithVisualFormat is usually much more concise than constraintWithItem.

// Make label1's NSLayoutAttributeTrailing be the 'standard Aqua space' away from label2's NSLayoutAttributeLeading. Also, vertically align their baselines.
NSArray* constraints = [NSLayoutConstraint constraintsWithVisualFormat:@"H:[label1]-[label2]"  options:NSLayoutFormatAlignAllBaseline  metrics:nil  views:NSDictionaryOfVariableBindings(label1, label2) ] ;

Then you add the constraint(s) to the labels’ superview:

[label1.superview  addConstraints:constraints] ;   // Use `label1.superview` or your own reference to the label's superview.

The Cocoa Auto Layout Guide is short and easy to follow. Give it a read, and I’d be happy to answer any questions you still have.

Edit 1

The option NSLayoutFormatAlignAllBaseline creates constraints (in addition to those created by the VisualFormat string) that vertically align the baselines of all specified objects. If your VisualFormat string is creating vertical constraints (it starts with “V:”), you don’t want to use this option. You’d want to use 0 (which means no options), or an option that creates horizontal constraints, like NSLayoutFormatAlignAllCenterX.