iOS expand UITableViewCell on click with AutoLayout

Question

I'm working on simple application and now I'm focused on expanding UITableViewCell after user tap that cell. It's iOS 8 app so I have set:

self.tableVIew.rowHeight = UITableViewAutomaticDimension
self.tableVIew.estimatedRowHeight = 50

cell constraints look like this:

If user tap cell this function is called:

func extend() {
        self.contentView.removeConstraint(self.bottomConstraint)

        let additionalView = UIView()
        additionalView.setTranslatesAutoresizingMaskIntoConstraints(false)
        additionalView.backgroundColor = UIColor.orangeColor()
        self.contentView.addSubview(additionalView)
        self.contentView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:[additionalView(50)]-5-|", options: nil, metrics: nil, views: ["additionalView" : additionalView]))
        self.contentView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|-5-[additionalView(100)]-5@999-|", options: nil, metrics: nil, views: ["additionalView" : additionalView]))
    }

self.bottomConstraint is costraint between green circle bottom and cell contentView bottom.

The question is: Why this solution works only if there is priority set in constraint:

V:|-5-[additionalView(100)]-5@999-|

?

Without explicit priority I got errors:

(
    "<NSLayoutConstraint:0x7a63f7a0 'UIView-Encapsulated-Layout-Height' V:[UITableViewCellContentView:0x7a737840(60)]>",
    "<NSLayoutConstraint:0x7a644f40 V:|-(5)-[UIView:0x7a63f2a0]   (Names: '|':UITableViewCellContentView:0x7a737840 )>",
    "<NSLayoutConstraint:0x7a644fb0 V:[UIView:0x7a63f2a0(100)]>",
    "<NSLayoutConstraint:0x7a644f70 V:[UIView:0x7a63f2a0]-(5)-|   (Names: '|':UITableViewCellContentView:0x7a737840 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x7a644f70 V:[UIView:0x7a63f2a0]-(5)-|   (Names: '|':UITableViewCellContentView:0x7a737840 )>

Answer 1

I have made a demo that gives you the exact behaviour as you want. here is the link: https://github.com/rushisangani/TableViewCellExpand

Please set constraint of your expand/ collapse views as described in demo.