Creating Auto Layout constraints to topLayoutGuide and bottomLayoutGuide in code

Question

Apple's documentation on creating Auto Layout constraints between a view and one of the layout guides only shows an example using VFL.

Is there any way to create these constraints programmatically without VFL (using NSLayoutConstraint's other API or similar)?

(Note: I'm specifically asking about doing this in code, not in Interface Builder. And I don't want the calculated length of the guide set as a static constant on a constraint, I want a constraint where changes to the layout guide length would automatically cause the constrained view to adjust position.)

Answer 1

For a UIButton that you want to place 20 points below the UIViewController.topLayoutGuide you create the NSLayoutConstraint like so:

[NSLayoutConstraint constraintWithItem:self.button                              attribute:NSLayoutAttributeTop                              relatedBy:NSLayoutRelationEqual                                 toItem:self.topLayoutGuide                              attribute:NSLayoutAttributeBottom                             multiplier:1.0                               constant:20.0]; 

With iOS 9 you can also create the NSLayoutConstraint this way:

[self.button.topAnchor constraintEqualToAnchor:self.topLayoutGuide.bottomAnchor                                       constant:20.0];