Set constraint so that firstItem's top equals secondItem's height - ios

Question

I'd like to set a top constraint so that it has a relationship which is equal with another item's height.
I prepare it in interface builder so that item 1's top is equal to item 2's top.
Then, in code (because I don't think it can be done in Interface Builder), I try to set the constraint's secondAttribute property.
This seems logical to me based on a basic understanding of how constraints are composed (of two items, an attribute for each, a relationship type, and a constant), but it does not work:
#IBOutlet var fillTopToContainer: NSLayoutConstraint!
// ...
override func viewDidLoad() {
fillTopToContainer.secondAttribute = NSLayoutAttribute.Height
}
Swift compiler error:
Cannot assign to the result of this expression.
I have fiddled with the constant to make sure that topDistEqualsHeight contains the constraint I expect, and it does. The other values in the constraint are correct for my needs, I only want to change the attribute.
Is this a known limitation, a syntax issue, or a big piece of missing knowledge?
Update
I've also tried this, which throws a runtime error:
var pushTopDown = NSLayoutConstraint(item: self.view,
attribute: NSLayoutAttribute.Height,
relatedBy: NSLayoutRelation.Equal,
toItem: fillRect,
attribute: NSLayoutAttribute.Top,
multiplier: 1,
constant: -10)
self.view.addConstraint(pushTopDown)
This is the layout I'm trying to achieve. It's a scrollview which is exactly two screens tall, the bottom half has a fill color.
#P-double suggested that the fillRect match it's top to the bottom position of a full height object, which would work except you can't set the top of a grandchild relative to its grandparent in IB.
View Hierarchy:
frame (fills the screen, root view)
scrollView (fills the frame. Content size is determined by constraints of inside views)
fillRect (height==frame, bottom==scrollView.bottom, top==?)

Answer 1

Constraint's do not work in the way you are trying to use them, most notably constraints properties are all immutable apart from the constant property.
This pairing of constraints does not work, because one relates to an origin point (y-positon), and one relates to a size dimension. It's not clear what you are trying to achieve, but there will be other ways in which you can achieve your desired layout. If you want the second view to sit below the first (in the y-plane, it doesn't necessarily have to align centre-x positions), why not pin the bottom of the first to the top of the second? If you'd like to post some more details, I'll do my best to help.
EDIT
To achieve you desired layout, you should pin the top of the fillRect to the top of the scrollView, and give the constraints constant the value of the frame height. Such as this
var pushTopDown = NSLayoutConstraint(item: scrollView,
attribute: NSLayoutAttribute.Top,
relatedBy: NSLayoutRelation.Equal,
toItem: fillRect,
attribute: NSLayoutAttribute.Top,
multiplier: 1,
constant: self.view.height)
scrollView.addConstraint(pushTopDown)
Also notice that the constraint is added to the scroll view, not the view controllers view. You'll also want to the make the width of fillRect equal to the scrollViews frame width.
As #Rob points out, you'll need to make sure you haven't already added constraint for the top. Interface builder will complain though if the view is not fully constrained. The trick is to add a top constraint in interface builder, but to mark it as a design time constraint. To do this, select the constraint you want to replace in code, the open the attributes inspector on the right, and tick the 'Remove at build time' option. (See picture) This allows the xib/storyboard to compile without error, but doesn't actually add the constraint to the view.

Answer 2

Bottom line, if you try to define a constraint such that the "top" attribute of an item to be equal to the "height" attribute of another, you will receive an error that says:
Invalid pairing of layout attributes
Bottom line, you cannot define constraints between "top" and "height" attributes.
If you want to avoid using spacer views, the other technique to try when vertically spacing views is to set the .CenterYWithinMargins attribute with respect to the superview's .CenterYWithinMargins, applying the appropriate multiple. You can equally space views with judicious use of different multiple values for each item's respective .CenterY attribute.
A couple of observations regarding a few of your attempts: Notably, you cannot mutate secondAttribute of an existing constraint. Only the constant property may be modified after the constraint creation. Also, in your update to your question, you illustrate the attempt to create a new constraint, and you'll obviously want to make sure make sure you remove the old constraint (or define it with a lower priority) before you create a new one.
To illustrate the concept, here is a scroll view with a tinted view that is off screen all created programmatically (it's the most concise way to describe the constraints, methinks):
let scrollView = UIScrollView()
scrollView.setTranslatesAutoresizingMaskIntoConstraints(false)
view.addSubview(scrollView)
let tintedView = UIView()
tintedView.setTranslatesAutoresizingMaskIntoConstraints(false)
tintedView.backgroundColor = UIColor.orangeColor()
scrollView.addSubview(tintedView)
let views = ["scrollView" : scrollView, "tintedView" : tintedView]
// vfl for `frame` of scrollView
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|[scrollView]|", options: nil, metrics: nil, views: views))
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|[scrollView]|", options: nil, metrics: nil, views: views))
// vfl for `contentSize` of scrollView
scrollView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|[tintedView]|", options: nil, metrics: nil, views: views))
scrollView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:[tintedView]|", options: nil, metrics: nil, views: views))
// vfl for `frame` of tintedView
//
// (note, this can be integrated into the above constraints, reducing
// the extraneous VFL, but I implemented them as separate VFL to
// clearly differentiate between settings of the scrollView `contentSize`
// and the tintedView `frame`)
scrollView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:[tintedView(==scrollView)]", options: nil, metrics: nil, views: views))
scrollView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:[tintedView(==scrollView)]", options: nil, metrics: nil, views: views))
// offset tinted view
scrollView.addConstraint(NSLayoutConstraint(item: tintedView, attribute: .CenterY, relatedBy: .Equal, toItem: scrollView, attribute: .CenterY, multiplier: 3.0, constant: 0.0))