Hot to add a View to a Scrollview programmatically using layout constraints - ios

Question

I'm triying to instert programatically a view inside an UIScrollView, but it doesn't appear here is my code:
mainScrollView.translatesAutoresizingMaskIntoConstraints = false
self.view.addSubview(mainScrollView)
//Add Trailing
let trailingConstraint = NSLayoutConstraint(item: mainScrollView, attribute: .trailing, relatedBy: .equal, toItem: self.view, attribute: .trailing, multiplier: 1, constant: 0)
self.view.addConstraint(trailingConstraint)
//Add Leading
let leadingConstraint = NSLayoutConstraint(item: mainScrollView, attribute: .leading, relatedBy: .equal, toItem: self.view, attribute: .leading, multiplier: 1, constant: 0)
self.view.addConstraint(leadingConstraint)
//Add Top
let topConstraint = NSLayoutConstraint(item: mainScrollView, attribute: .top, relatedBy: .equal, toItem: self.view, attribute: .top, multiplier: 1, constant: 0)
self.view.addConstraint(topConstraint)
//Add Bottom
let bottomConstraint = NSLayoutConstraint(item: mainScrollView, attribute: .bottom, relatedBy: .equal, toItem: self.view, attribute: .bottom, multiplier: 1, constant: 0)
self.view.addConstraint(bottomConstraint)
let contentView = UIView()
contentView.translatesAutoresizingMaskIntoConstraints = false
contentView.backgroundColor = .blue
mainScrollView.addSubview(contentView)
//Add Trailing
let trailingConstraintContent = NSLayoutConstraint(item: contentView, attribute: .trailing, relatedBy: .equal, toItem: mainScrollView, attribute: .trailing, multiplier: 1, constant: 0)
mainScrollView.addConstraint(trailingConstraintContent)
//Add Leading
let leadingConstraintContent = NSLayoutConstraint(item: contentView, attribute: .leading, relatedBy: .equal, toItem: mainScrollView, attribute: .leading, multiplier: 1, constant: 0)
mainScrollView.addConstraint(leadingConstraintContent)
//Add Top
let topConstraintContent = NSLayoutConstraint(item: contentView, attribute: .top, relatedBy: .equal, toItem: mainScrollView, attribute: .top, multiplier: 1, constant: 0)
mainScrollView.addConstraint(topConstraintContent)
//Add Bottom
let bottomConstraintContent = NSLayoutConstraint(item: contentView, attribute: .bottom, relatedBy: .equal, toItem: mainScrollView, attribute: .bottom, multiplier: 1, constant: 0)
mainScrollView.addConstraint(bottomConstraintContent)
The first ScrollView is inserted because I added a background color and I can see it, but I cant see the contentview added as background color blue.
Any help?
UPDATE
I have tried the following to and no success:
mainScrollView.translatesAutoresizingMaskIntoConstraints = false
let contentView = UIView()
contentView.translatesAutoresizingMaskIntoConstraints = false
mainScrollView.backgroundColor = .red
contentView.backgroundColor = .blue
self.view.addSubview(mainScrollView)
mainScrollView.addSubview(contentView)
let viewsDictionary = ["mainScrollView": mainScrollView, "contentView": contentView]
let mainScrollViewVerticalConstraint = NSLayoutConstraint.constraints(withVisualFormat: "V:|[mainScrollView]|", options: [], metrics: nil, views: viewsDictionary)
let mainScrollViewHorizontalConstraint = NSLayoutConstraint.constraints(withVisualFormat: "H:|[mainScrollView]|", options: [], metrics: nil, views: viewsDictionary)
self.view.addConstraints(mainScrollViewVerticalConstraint)
self.view.addConstraints(mainScrollViewHorizontalConstraint)
let contentViewVerticalConstraint = NSLayoutConstraint.constraints(withVisualFormat: "V:|[contentView]|", options: [], metrics: nil, views: viewsDictionary)
let contentViewHorizontalConstraint = NSLayoutConstraint.constraints(withVisualFormat: "H:|[contentView]|", options: [], metrics: nil, views: viewsDictionary)
mainScrollView.addConstraints(contentViewVerticalConstraint)
mainScrollView.addConstraints(contentViewHorizontalConstraint)

Answer 1

When adding sub views the important thing to remember is to pin them in a way to make it possible for Auto Layout to evaluate the height, this is because the scroll view determines the content size by determining the sum of heights sub views (in case of vertical).
So you should specify the height in VFL . eg.
let contentViewVerticalConstraint=NSLayoutConstraint.constraints(withVisualFormat: "V:|[contentView(300)]", options: [], metrics: nil, views: viewsDictionary)

Answer 2

Create a custom view using .xib file
Apply the constraints for the custom view child components in .xib (auto layout)
Create a custom view instant
MyView *myview = [[[NSBundle mainBundle] loadNibNamed:#"nibName" owner:nil options:nil] objectAtIndex:0];
Set its frame for screen width as
CGRect screenSize = [UIScreen mainScreen].bounds.size;
myView = CGRectMake(0.0, 0.0, screenSize.width, 100.0);
Add to parent view
[parentView addSubView:myView];