Auto-layout with constraintsWithVisualFormat is not working well - ios

Question

I have Set scrollview and add constraints to scrollview. But it is showing blank screen. Please check below code.
override func loadView()
{
super.loadView()
scrollView = UIScrollView(frame:CGRectZero)
scrollView.backgroundColor = UIColor.whiteColor()
scrollView.sizeToFit()
self.view.addSubview(scrollView)
scrollView.backgroundColor = UIColor.blueColor()
contentView = UIView()
// contentView.setTranslatesAutoresizingMaskIntoConstraints(false)
contentView.backgroundColor = UIColor.redColor()
scrollView.addSubview(contentView)
var viewBindingsDictBoth = [String: AnyObject]()
viewBindingsDictBoth["scrollView"] = scrollView
viewBindingsDictBoth["contentView"] = contentView
viewBindingsDictBoth["mainView"] = self.view
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|-0-[scrollView]-0-|",options: [], metrics: nil, views:viewBindingsDictBoth))
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|-0-[scrollView]-0-|",options: [], metrics: nil, views:viewBindingsDictBoth))
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|[contentView]|",options: [], metrics: nil, views:viewBindingsDictBoth))
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|[contentView]|",options: [], metrics: nil, views:viewBindingsDictBoth))
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:[contentView(==mainView)]",options: [], metrics: nil, views:viewBindingsDictBoth))
self.view.contentMode = UIViewContentMode.Redraw
}
Thanks in Advance..

Answer 1

The most important thing you are missing is
scrollView.translatesAutoresizingMaskIntoConstraints = false
and
contentView.translatesAutoresizingMaskIntoConstraints = false
I think that some of your constraints are not perfect but the biggest problem is that every view is using its autoresizing properties for positioning by default. Those will then collide with your constraints.
From UIView.translatesAutoresizingMaskIntoConstraints documentation:
Note that the autoresizing mask constraints fully specify the view’s size and position; therefore, you cannot add additional constraints to modify this size or position without introducing conflicts. If you want to use Auto Layout to dynamically calculate the size and position of your view, you must set this property to false, and then provide a non ambiguous, nonconflicting set of constraints for the view.
(emphasis mine)

Answer 2

I usually find it very unnecessary to override loadView unless I'm really trying to get the base view (self.view) to be of a different subclass of UIView. loadView is for initializing the value of self.view, which you have forgotten to do.
In your case, for legibility I would recommend moving view initialization to viewDidLoad.
override func viewDidLoad() {
// Do your init here
}

Answer 3

In loadView() method of your viewcontroller you need create a root view. Your root view right now is nil view = UIView()

Answer 4

You have a few mistakes:
override func loadView()
{
super.loadView()
scrollView = UIScrollView(frame:CGRectZero)
scrollView.backgroundColor = UIColor.whiteColor()
scrollView.setTranslatesAutoresizingMaskIntoConstraints(false)
// No need for sizeToFit with autoLayout
//scrollView.sizeToFit()
self.view.addSubview(scrollView)
scrollView.backgroundColor = UIColor.blueColor()
contentView = UIView()
//Always set TranslatesAutoresizingMaskIntoConstraints to false, on every view that users AutoLayout
contentView.setTranslatesAutoresizingMaskIntoConstraints(false)
contentView.backgroundColor = UIColor.redColor()
scrollView.addSubview(contentView)
var viewBindingsDictBoth = [String: AnyObject]()
viewBindingsDictBoth["scrollView"] = scrollView
viewBindingsDictBoth["contentView"] = contentView
viewBindingsDictBoth["mainView"] = self.view
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|-0-[scrollView]-0-|",options: [], metrics: nil, views:viewBindingsDictBoth))
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|-0-[scrollView]-0-|",options: [], metrics: nil, views:viewBindingsDictBoth))
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|[contentView]|",options: [], metrics: nil, views:viewBindingsDictBoth))
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|[contentView]|",options: [], metrics: nil, views:viewBindingsDictBoth))
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:[contentView(==mainView)]",options: [], metrics: nil, views:viewBindingsDictBoth))
// self.view.contentMode = UIViewContentMode.Redraw
}