Swift: programmatically adding a field in a position relative to the bottom of the view

Question

I want to do this programmatically without Interface Builder. I want to add a UITextField to my iOS app a constant distance from the bottom of the app view. I've looked at a lot of existing questions and answers. None are in Swift and this topic seems relatively undocumented.

Answer 1

There are several somewhat related answers out there, but here are all the pieces it took to get this working:

1) I needed to create a constraint like

let bottomConstraint : NSLayoutConstraint = NSLayoutConstraint.constraintsWithVisualFormat("V:[inputBox]-50-|", options: NSLayoutFormatOptions(0), metrics: nil, views: ["inputBox": inputBox])[0] as! NSLayoutConstraint

This uses Swift's domain specific language "visual format language" (VFL): V:[inputBox]-50-|, where the 'V' means vertical, 50 is a number of pixels and the '|' refers to the bottom of the parent view.

2) The UITextField had to be added to the view before adding the constraint to the view so '|' above could find the parent view.

3) Once I used one NSLayoutConstraint I had to use them for all aspects of sizing and positioning for this element. Hence I had to add these two lines to describe its width and height:

let heightConstraint : NSLayoutConstraint = NSLayoutConstraint.constraintsWithVisualFormat("V:[inputBox(==70)]", options: NSLayoutFormatOptions(0), metrics: nil, views: ["inputBox": inputBox])[0] as! NSLayoutConstraint
let widthConstraint : NSLayoutConstraint = NSLayoutConstraint.constraintsWithVisualFormat("H:[inputBox(==superview)]", options: NSLayoutFormatOptions(0), metrics: nil, views: ["inputBox": inputBox, "superview":self.view])[0] as! NSLayoutConstraint

4) To get my UITextField to interact correctly with VFL I had to add this line:

inputBox.setTranslatesAutoresizingMaskIntoConstraints(false)

Putting it all together:

let inputBox = UITextField(frame: rect)
inputBox.backgroundColor = UIColor.yellowColor()
inputBox.setTranslatesAutoresizingMaskIntoConstraints(false)
self.view.addSubview(inputBox)

let bottomConstraint : NSLayoutConstraint = NSLayoutConstraint.constraintsWithVisualFormat("V:[inputBox]-50-|", options: NSLayoutFormatOptions(0), metrics: nil, views: ["inputBox": inputBox])[0] as! NSLayoutConstraint
let heightConstraint : NSLayoutConstraint = NSLayoutConstraint.constraintsWithVisualFormat("V:[inputBox(==70)]", options: NSLayoutFormatOptions(0), metrics: nil, views: ["inputBox": inputBox])[0] as! NSLayoutConstraint
let widthConstraint : NSLayoutConstraint = NSLayoutConstraint.constraintsWithVisualFormat("H:[inputBox(==superview)]", options: NSLayoutFormatOptions(0), metrics: nil, views: ["inputBox": inputBox, "superview":self.view])[0] as! NSLayoutConstraint

view.addConstraint(bottomConstraint)
view.addConstraint(heightConstraint)
view.addConstraint(widthConstraint)