Can't Add Constraints to Button Programatically - Swift 2.0 - ios

Question

I have a subView named loginView and within that a few other elements, namely loginButton and loginUsername
I am trying to programatically add a Facebook login button that has constraints relative to loginButton, loginView and loginUsername
Here is my code
self.loginView.addSubview(facebookLoginButton)
let facebookLoginButtonTopConstraint = NSLayoutConstraint(
item: facebookLoginButton,
attribute: NSLayoutAttribute.Top,
relatedBy: NSLayoutRelation.Equal,
toItem: loginButton,
attribute: NSLayoutAttribute.Bottom,
multiplier: 1,
constant: 8
)
let facebookLoginButtonLeadingConstraint = NSLayoutConstraint(
item: facebookLoginButton,
attribute: NSLayoutAttribute.Leading,
relatedBy: NSLayoutRelation.Equal,
toItem: loginView,
attribute: NSLayoutAttribute.Leading,
multiplier: 1,
constant: 8
)
let facebookLoginButtonWidthConstraint = NSLayoutConstraint(
item: facebookLoginButton,
attribute: NSLayoutAttribute.Width,
relatedBy: NSLayoutRelation.Equal,
toItem: loginUsername,
attribute: NSLayoutAttribute.Width,
multiplier: 1,
constant: 0
)
self.loginView.addConstraints([
facebookLoginButtonTopConstraint,
facebookLoginButtonLeadingConstraint
])
self.facebookLoginButton.addConstraint(facebookLoginButtonWidthConstraint)
This code is going in my viewDidLoad method. The error I'm getting is:
The view hierarchy is not prepared for the constraint:
When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled. Break on -[UIView(UIConstraintBasedLayout) _viewHierarchyUnpreparedForConstraint:] to debug.
Any help greatly appreciated, thanks!

Answer 1

The error says all. You need to prepare all views to hierarchy before adding constraints. Check that all four views are added to view hierarchy before adding constraints (didn't you forget addSubvew for some view?). Do not move this code to viewWillLayoutSubviews method, this is wrong hint! Because you don't want to add constraints multiple times. Also, you should add facebookLoginButtonWidthConstraint to view that owns both (loginView?), facebookLoginButton and loginUsername, since this is shared constraint